Data Communications And Computer Network C ONTE NTS A T A GLA NC E Chapter 1 1 Bibliography Index Introduction 2 CHAPTER 1 INTRODUCTION Networks A network is a set of devices (often referred to as nodes or station) connected by communication links. A node can be a computer, printer or any other device capable of sending and/or receiving data generated by other nodes on the network. Goals of Computer Networks Resource sharing All programs, remote equipments and especially data can available to anyone on the network. We can share printer, floppy drive, CD ROM drive, CD writer, hard disk etc. that are attached to the network. High reliability Files could be replicated on two or three machines, so if one of them is unavailable (due to some hardware breakdown), the other copies of the files could be used. On the other word, the presence of multiple machines means that if one of them is failed, the other machines can take over its work. This goal is most important, since hardware failure is one of the most common real-life problems. Saving Money 3 Microcomputers have a much better cost/performance ratio than large computers. That is why most of the system designers prefer to build client-server model consisting of microcomputers. In client-server model, data stored on one or more shared file server machines and others are called client machines. Typically there are many clients using a small number of servers. In this model, communication takes form of a request message from the client to the server asking for some job to be done. Then the server does the work and sends back the reply to the client. Client machine Server machine Communication Medium A computer network can provide a powerful communication medium among widely separated people. By using network one can easily communicate to another, who lives far apart. One user can send a mail or email to another and can get reply immediately. This technology makes it possible to have virtual meetings, which called videoconference, among far-apart people. Anyone can access to information system about the arts, business, cooking, government, health, history, hobbies, recreation, science, sports, travels and too others topics. Many people can pay their bills; manage their bank account, download software and also can shopping by online. Scalability To increase system performance as the workload grows, just adding more computers. In client-server model new clients and new servers can be added as needed. Where as in mainframe, when the system is full then it must be replaced by larger one, which is too expensive. Components of Network 4 1. 2. Networks consist of many components; they are divided into two groups: Hardware Components. Software Components. Hardware Components The basic hardware components of a network include three types of devices: 1. Transmission Media (Medium). Transmission media is a physical path by which a message transfers from sender to receiver. 2. Access Devices. A device that can accept transmitted data in the network and also can places data on the network. 3. Repeater. A Repeater is a device that accepts transmitted signals, amplifies them and puts them back on the network. The software components required in a network include the following: 1. Protocol. A protocol is asset of rules that controls data communication. 2. Device Driver. A device driver is a hardware-level program that controls a specific device. 3. Communication Software. Communication software makes the available network bandwidth actually usable. Elements of a communication system A typical communication system consists of an information source and a sink connected by a communication system that transfers messages from transmitter to receiver over a communication channel. As the raw message is unsuitable for direct transmission it is led as input to a transmitter. The transmitter performs necessary signal processing such as encoding and modulation, so that make it suitable for transmission. The function of the transmitter is to couple the message onto the transmission channel in a form that matches the transfer characteristics of the channel best. 5 Information Source Information Sink Transmitter Encoding & Modulation Channel Receiver Decoding & Demodulation Noise Block diagram of a typical communication system The communication channel is the path or medium for electrical or electromagnetic transmission between transmitter and receiver. The transmitting message recovered by receiver as output. The receiver performs demodulation and decoding. Encoder (or simply coder) and decoder usually come as a single unit called CODEC. During forward information flow (i.e. transmission) it acts as COder, while during reverse flow (i.e. reception) it performs the function of a DECoder and hence the name. For analog channel, mapping is required at the transmitter to convert digital signals into a suitable waveform by modulation and back-mapping is required at the receiver to reconvert the received waveform into digital data by demodulation. The respective modules are known as modulation and demodulation and it perform by MOdulator and DEModulator respectively which collectively called MODEM. Modem is the device responsible for allowing a digital signal to be carried over an analog channel. It performs modulation at the sending end and demodulation at the receiving end. 6 Information Source Data CODEC Information Sink Coder Decoder Modulator TRANSMITTER MODEM Demodulator RECEIVER Channel Noise A complete block diagram of a data communication system Classification of Networks According to Transmission technology, there are two types of networks 1. Broadcast Networks. 2. Point-to-Point Network. Broadcast Network Broadcast Networks have a single communication channel that is shared by all the computers on the network. When a computer sends any message then it received by all the others computers on the network. An address field within the massage specifies for whom it is sent. If the message is sent for itself, it processes the message; if the message is intended for some other computer, it is just ignored. Broadcast Networks can be divided into static and dynamic, depending on how the channel is allocated. In typical static allocation, time would be divided into discrete intervals and run a round robin algorithm to allow each computer broadcast only when its time slot comes up. Static allocation wastes capacity when a computer has nothing to transmit during its allocated slot. In dynamic channel allocation, methods for a common channel are centralized and decentralized. In centralized channel allocation method, there is a central entity which determines who transmit next. In decentralized channel 7 allocation method, each computer decides for itself whether or not to transmit; there is no central entity. Point-to-Point Network Point-to-Point Networks consist of many connections between individual pairs of computers. When a computer sends any message on this type of network then it may have to first visit one or more intermediate computers. Often, multiple routes, of different lengths are possible and therefore routing algorithms play an important role in this type of network. Network categories Network categories can be specified based on size, ownership, the distance it covers and its physical architecture. According to scale there are mainly three types of networks 1. Local Area Network (LAN). 2. Metropolitan Area Network (MAN). 3. Wide Area Network (WAN). Distance 10 m 100 m 1 km 10 km 100 km 1000 km Location Room Building Campus City Country Continent Example Local Area Network -do-doMetropolitan Area Network Wide Area Network -do- Local Area Network (LAN) A Local Area Network is usually privately owned and links in a single room, building or campus covering a small geographical area (up to few thousand meters). LANs connect workstations, peripherals, terminals and other devices on the network. LANs run at high speed, typically at 10 to 100Mbps (1 Mb equal to 1,000,000 bits) and at very low error. LANs Different types of topologies are possible for LANs, like bus topology, ring topology, star topology etc. LANs are distinguished from other types of networks by their size, transmission technology and their topology. However, since all the equipments are located within a single establishment, LANs 8 normally installed and maintained by the organization. Hence LANs are also referred as private data networks. (a) (b) (c) Some topologies (a) Bus, (b) Ring, (c) Star • • LAN has no repeater/amplifier. LANs can run at high speed and at very low error. • LANs are distinguished from other kinds of networks by their size, transmission technology and their topology. Metropolitan Area Network (MAN) A Metropolitan Area Network (MAN) is designed to extend over an entire city. MAN is basically a bigger version of a LAN and normally uses similar technology. It may be a single network such as a cable television network or it may be means of connecting a number of LANs into a large network so that resources may be shared LAN-to-LAN as well as device-to-device. The high speed links between LANs within a MAN are made possible by fiber-optic connections. Metropolitan Area Network uses a standard called DQDB (Distributed Queue Dual Bus). DQDB consists of two unidirectional buses (cables) to which all the computers are connected, as shown in following figure. Each bus has a device that initiated transmission activity, is called head-end. To transmit a message, a computer has to know whether the 9 destination is to the left of it or to the right of it. If the destination is to the right, the sender uses the upper bus A and if the destination is to the left sender uses the lower bus B. • • • • Bus A MAN does not containDirection of flow on bus A switching elements. MAN can support both data and voice. MAN can use at local cable television network or a group of nearby corporate offices or a city in private or public area. There is a broadcast medium to which all computers are attached. Computer 3 1 2 N Head end Wide Area Network (WAN) A Wide Area Network (WAN) provides long distance transmission of data, voice, image and video information over large geographic areas, often a country or a continent. Bus B Direction of flow on bus B A Wide Area Network contains a collection of computers, called as hosts, which are used for user programs. The hosts are connected by a communication subnet or subnet. The subnet carries messages from host to host. The subnet consists of two different components: 1. Transmission Lines. 2. Switching Elements. Transmission lines travel messages between hosts. The switching elements are expert computers used to connect two or more transmission lines. These computers may be called as router. • WAN contain switching elements. Direction of Data flow According to the direction of data flow communication between two devices; the data transmission modes can be three types: 10 1. Simplex. 2. Half-duplex. 3. Full-duplex. Simplex In simplex mode communication data can be transmitted in only one direction. That means the communication is unidirectional. Only one of the two devices on a link can transmit data, the other can only receive. Keyboards and monitors, mouse are examples of simplex devices. The keyboard and mouse can only sent input; the monitor can only accept output. Device Device Direction of data Simplex Half-Duplex In half-duplex mode communication data can be transmitted in both direction, but not at the same time. Each device can transmit and receive data; when one device is sending data, the other can only receive and when second device is sending then first one can only receive. In a half-duplex transmission, the entire capacity of a channel is taken over by whichever of the two devices is transmission at the time. Walkie-talkies and radios are examples of half-duplex devices. Device Direction of data at time 1 Device Direction of data at time 2 Half-duplex Duplex In full-duplex (also called duplex) mode communication, both stations can transmit and receive data simultaneously. In full-duplex mode, signals going in either direction share 11 the capacity of the channel. This sharing can occur in two ways: either the channel must contain two physically transmission paths, one for sending and the other for receiving; or the capacity of the channel is divided between signals traveling in both directions. The telephone network is an example of full-duplex communication. When two people are communicating by a telephone line, both can talk and listen at the same time. Device Direction of data at same time Device Full-duplex Data Transmission Modes The transmission of binary data across a link can be able in serial mode and parallel mode. In serial mode, 1 bit is sent with each clock tick; there are two types of serial transmission: synchronous and asynchronous. In parallel mode, multiple bits are sent with each clock tick. Serial Transmission In serial transmission data are transmitted one bit at a time over a single link. Since communication within devices is parallel, so in serial transmission, parallel word should be converted into serial bits at the sender end: this is known as parallel-to-serial conversion. On the other hand at receiver end, serial bits should be converted into parallel word: this is known as serial-to-parallel conversion. • • • Require only one communication channel. Used in low speed data transmission. Serial-to-parallel and parallel-to-serial conversion devices are required. Serial transmission can be two types: 1. Synchronous Transmission 2. Asynchronous Transmission Synchronous Transmission In synchronous transmission, bits are sent one after another without start or stop bits or gaps. It is the responsibility of the receiver to group the bits. 12 Direction of data flow 00010000 11110111 11110110 11111011 11011 Sender Receiver Synchronous transmission • • • Generally use in high-speed data transmission. This type of data transmissions occurs without gaps and starts or stop bit. Regrouping the bits into meaning bytes is the responsibility of the receiver. Asynchronous Transmission In synchronous transmission, we send one start bit (0) at the beginning, followed by a byte and one or two stop bits (1) at the end of each byte. There may be a gap between each byte. This is also known as framing. • • • Generally use in low speed data transmission. Send one start bit (0) at beginning of the byte and one or two stop bits (1) at end of each byte. There are variable–length gaps between each byte. Direction of data flow Stop bit Data 1 00010111 0 1 11111011 0 Sender Gaps between data units Asynchronous transmission 1 00010111 0 1 10011011 0 Receiver Start bit 13 Parallel Transmission In parallel transmission, a group of bits is sent simultaneously, with each bit on a separate line. The advantage of parallel transmission is high-speed data transmission. In parallel transmission required multiple wire to transmit data stream, what is why it is expensive, parallel transmission is typically used at short distance. In parallel transmission the entire stream is transmitted at one time. • Use n communication channel to send n bits at one time. • Parallel transmission is use at short distance. • Used in high-speed data transmission. • As there are required multiple lines, it is expensive. Sender 1 0 1 1 0 0 0 1 Receiver Eight lines are needed to send eight bits together. Parallel transmission Baseband and Broadband Transmission Bandwidth use refers to the ways of allocating the capacity of transmission media. The total capacity or bandwidth can be divided into channels. A channel is simply a portion of the bandwidth that can be used for transmitting data. The two ways of using the bandwidth of transmission media are following: 1. Broadband. 2. Baseband. Baseband Transmission These transmissions use the entire bandwidth for a single channel. In baseband transmission digital signal transmit at its original frequency without modulation. Baseband is commonly used for digital signaling, but it can also used for analog signal. Signals flow in the form of discrete pulses of electricity or light. Most Local Area Networks use baseband signaling. 14 • • • • • • • Baseband transmission use digital signal over a single frequency. Signals flows in the form of discrete pulses of electrical or light. The digital signal uses the complete bandwidth of a single channel. Baseband systems use repeater. No modem is required. Used for a short distance. Commonly use in Local Area Networks. Broadband These transmissions provide the ability to divide the entire media bandwidth into multiple channels. Since each channel can carry a different analog signal, broadband networks support multiple simultaneously conversations over a single transmission medium. Digital signals are used to modulate a carrier signal whose frequency must be within the bandwidth (BW) of the channel. While Baseband systems use repeater, broadband systems use amplifiers to regenerate analog signals at their original strength. Since broadband signal flow is unidirectional, there must be two paths for data flow in order for a signal to reach all devices. Used for long distance communication using switching or leased lines from public carriers. • • • • • • Broadband systems use analog signaling and a range of frequencies. The signals are continuous and discrete. Signal flows across the physical medium in the form of electromagnetic or optical waves. Broadband signal flow is unidirectional. Therefore there must be to paths for data flow in order for a signal to reach all devices. Broadband systems use amplifiers to regenerate analog signals at their original strength. Always used in WANs. Also used for some LANs based on cable TV technology. Connection of Networks A network is a two or more device connected together through links. Imagine any link as a line drawn between two points. For communication to occur, two devices must be connected in some way to the same like at the same time. There are two possible types of connections: 1. Point-to-point. 2. Multipoint or Multidrop. Point-to-point A point-to-point connection provides a dedicated link between two devices. In point-topoint connection, the capacity of the channel is reserved for the transmission between 15 those two devices. Point-to-point connections may use wire or cable but microwave or satellite links are also possible. Computers Link Point-to-point connection Multipoint A multipoint (also known as Multidrop) connection is one in which more than two devices share a link. In multipoint connection, the capacity of the channel is shared. When several devices use the channel simultaneously, it is called spatially shared connection. When the devices take turns, it is called timeshare connection. Computers Link Multipoint connection Network Architecture Most of the networks are organized as a series of layers or levels. The number of layers, the name of each layer, the contents of each layer and the function of each layer vary from network to network. A five-layer network is illustrated in following figure. A protocol is a set of rules and conventions that controls data communications. It represents an agreement between the communication devices. Without a protocol, two devices may be connected but not communicating themselves. A protocol defines what is communicated, how it is communicated and when it is communicated. The key elements of a protocol are syntax, semantics and timing. Syntax 16 Syntax refers to the structure or format of the data, meaning the order in which they are presented. Semantics Semantics refers to the meaning of each section of bits. How is a particular pattern to be interpreted and what action is to be taken on that interpretation? Timing Timing refers two characteristics: when data should be sent and how fast they can be sent. The active elements in each layer are called entities. An entity is anything capable of sending or receiving information. The entities must be agreed on a protocol. The entities of the corresponding layers on different hosts are called peers. The peers are communicating using protocol. The processes on each host that communicate at a given layer are called peer-to-peer processes. Communication between different hosts is a peer-to-peer process using protocols to a given layer. No data are directly transferred from layer n on one host to layer n on another host. In fact, each layer passes data and control information to its below layer, until the lowest layer is reached. Below layer 1 is the physical medium through actual communication occurs. The passing data and network information down through the layers of the sending device and back up through the layers of the receiving device is made possible by an interface in between each pair of adjacent layers. Each interface defines what information and services a layer must provide for the layer above it. A set of layers and protocols is called network architecture. 17 Host 1 Layer 5 Layer 5 protocol Host 2 Layer 5 Layer 4 protocol Layer 4 Layer 3 protocol Layer 3 Layer 2 protocol Layer 2 Layer 1 protocol Layer 1 Layer 1 Layer 2 Layer 3 Layer 4 Layer 4/5 interface Layer 3/4 interface Layer 2/3 interface Layer 1/2 interface Physical Medium The communication of the top layer of the five-layer network is as follows: • A message M is produced by an application process running in layer 5 and given to layer 4 for transmission. • Layer 4 appends a header in front of the message to identify the message and passes it to layer 3. The header includes control information, such as sequence numbers to allow layer 4 on the destination host to deliver message in the right order (if the lower layers do not maintain sequence). Headers may contain sizes, times and other control fields. • Layer 3 splits up the incoming messages into smaller units, called packets and puts a layer 3 header to each packet. • Layer 3 decides which of the outgoing lines to use and passes the packets to layer 2. • Layer 2 adds a header to the front of each packet and also adds a trailer to the end of each packet. Then layer 2 gives to the resulting unit to layer 1 for physical transmission. • At receiver host the message moves upward, from layer to layer, with headers being stripped off as it progresses. None of the headers for layers below are passes up to layer n. 18 Layer 5 4 M Layer 5 protocol Layer 4 protocol H4 M Layer 3 protocol H4 M M 3 H3 H4 M1 H3 M2 H3 H4 M1 H3 M2 2 H2 H3 H4 M1 T 2 Layer 2 protocol H2 H3 M2 T2 H2 H3 H4 M1 T 2 H2 H3 M2 T2 1 Source Host Destination Host Interfaces and Services The active elements in each layer are called entities. An entity can be a software entity or a hardware entity. For example a process is a software entity where as an intelligent I/O chip is a hardware entity. Entities in the same layer on different hosts are called peer entities. The entities in layer n implement a service that is used by layer n+1. In this case layer n is called the service provider and layer n+1 is called the service user. Layer n may use the services of layer n-1 in order to provide its service. Layer N+1 ICI IDU SDU Interface Layer N ICI 19 SDU SDU Header N-PDU SAP Layer N+1 ICI IDU SDU Interface Layer N ICI SDU SDU Header N-PDU SAP - Service Access Point IDU - Interface Data Unit SDU – Service Data Unit PDU – Protocol Data Unit ICI – Interface Control Information SAP OSI Reference Model The OSI model is based on a proposal developed by International Standards Organization as a first step towards international standardization of the protocols used in the various layers. The model is called the ISO OSI (International Standard Organization Open Systems Interconnection) Reference Model; it deals with connecting open systems-that is systems that are open for communication with other systems. It is a seven-layer model. Its main objectives are to: 1. Allow manufactures of different systems to interconnect equipment through standard interfaces. 2. Allow software and hardware to integrate well and be portable on different systems. The principles of OSI model are as follows: 1. A layer should be formed where a different level of abstraction is required. 2. Each layer should execute a well-defined function. 3. The function of each layer should be chosen with an eye toward defining internationally standardized protocols. 4. The layer boundaries should be chosen to minimize the information flow across the interfaces. 5. The layers should be large enough that different functions need not be put 20 together in the same layer out of requirement, and small enough that architecture does not become cumbersome. Layer 7 Application Interface Presentation Interface Session Application protocol Application Presentation protocol Session protocol Session Transport protocol Name of unit exchanged APDU 6 Presentation PPDU 5 4 SPDU Transport Communication subnet boundary Transport TPDU Packet 3 Network Network Network Network 2 Data link Data link Data link Data link Frame Bit 1 Physical Host 1 Physical Router Physical Router Physical Host 2 Internal Subnet Protocol Physical layer host-router Protocol Data link layer host-router Protocol Network layer host-router Protocol The1. Physical Layer. OSI reference model are seven layers of ISI 2. Data Link Layer. 3. Network Layer. 4. Transport Layer. 5. Session Layer. 21 6. Presentation Layer. 7. Application layer. Physical Layer Physical layer is the bottom layer of the OSI Reference Model. The physical layer is responsible for transmission of the bit stream. It accepts frames of data from data link layer and transmits over a communication channel, one bit at a time. This layer is also responsible for the reception of incoming bit stream. These streams are then passes on the data link layer for reframing. For transmission physical layer must perform the following tasks: • Convert framed data from data link layer to a binary stream. • Transmit data as a binary stream; that is one bit at a time. For reception physical layer must perform the following tasks: • Accept appropriately addressed streams. • The binary stream pass up to the data link layer for reform into frames. The physical layer is taking cared to transmitting raw bits over a communication channel. It makes sure that when a transmitter sends a 1 bit then it is received by the receiver as a 1 bit, not as a 0 bit. Physical layer defines electrical and mechanical specifications of cables, connectors and signaling options that physically link two nodes on a network. The main tasks of the physical layer are to provide: • The transmission rate is defined by physical layer. That means this layer defines the number of bits sent each second (Data rate). • The physical layer defines the type of representation; how 0s and 1s are changed into signals-electrical or optical (Representation of bits). • The physical layer defines the characteristics of the interface between devices and the transmission media. It also defines the type of transmission medium. • The sender and receiver must use the same bit rate and their clocks must be synchronized at the bit level. • The physical layer also defines line configuration, physical topology, transmission mode. Data Link Layer Data link layer is the second layer of the OSI Reference Model. The data link layer is responsible for reassembling any binary streams received from the physical layer back into frames. The data link layer is also responsible for detecting and correcting any and all errors. The main tasks of the data link layer are to provide: 22 • Transferring data from the sending network layer to the receiving network layer (servicing provided to the network layer). • Data link layer implements an addressing system to handles the addressing problem locally. This layer adds a header to the frame to defining the sender and receiver of the frame (Physical addressing). • Accept the bit stream from physical layer and packing into frames (Framing). • Error detection and error correction of an erroneous frame (Error control). • No data send faster than the receiver can handle the traffic (Flow control). • When two or more devices are connected to the same link then it is necessary to determine which device has control over the link at any given time (Access control). The main task data link layer is to provide error free transmission. The sender break the input data into data frames, transmit the frames sequentially and process the acknowledge frames sent back by receiver. If there is an erroneous frame then data link layer protocol can retransmit the frame. Network Layer The network layer is the third layer of the OSI Reference Model. The network layer is taking cared to controlling the operation of the subnet. This layer is responsible for establishing the route to be used between source and destination computers and take responsible for the source to destination delivery of a packet possibly across multiple networks. The network layer ensures that each packet gets from the original source to the final destination. The main tasks of the network layer are to provide: • The connecting devices (called routers or switches) route or switch the packets from the source host to the destination (Routing). • When too many packets are present in the subnet at the same time, performance degrades. This situation is called congestion. This layer control of such congestion (Congestion control). • The network layer implements an addressing system to help distinguish the source and destination systems. This layer adds a header to the packet coming from the transport layer and includes the logical addresses of the sender and receiver (Logical addressing). Transport Layer The transport layer is the forth layer of the OSI Reference Model. The basic function of the transport layer is to accept data from session layer and split up into smaller packets then pass to the network layer; ensure that all of them arrive correctly at the other end. The transport layer is responsible for end-to-end integrity of transmissions. The transport layer can detect packets that are discarded by routers and automatically generate a retransmission request. 23 The main tasks of the transport layer: • The transport layer header must include port address. The network layer gets each packet to the correct computer; the transport layer gets the entire message to the correct process on that computer (Port addressing). • A message is divided into transmittable segments, each segment containing a sequence number. These numbers enable this layer to reassemble the message correctly (Segmentation and reassembly). • The transport layer can be either connectionless or connection-oriented. A connection-oriented transport layer makes a connection with the transport layer at the destination computer and then delivering the packets. A connectionless transport layer delivering each segment as a packet to the transport layer at the destination computer (Connection control). • This layer is responsible for flow control, which is performed end to end rather than single link (Flow control). • This layer is responsible for error control, which is performed end to end rather than single link (Error control). The sending transport layer ensures that the entire message arrives at the receiving transport layer without an error. For errors it generates retransmission request. Session Layer The session layer is fifth layer of the OSI Reference Model. The function of session layer is to manage the flow of communications during a connection between two computer systems. It determines whether communications can be unidirectional or bidirectional. It also ensures that one request is completed before a new one is accepted. The main tasks of the session layer are to provide: • Session Establishment • Session Release • Data Exchange • Expedited Exchange One of the services of the session layer is token management. It is ensure that both sides do not attempt the same operation at the same time. There is a central entity called as token which determines who transmit next. This token passes in round-robin fashion from side to side and provides the transmission right among one of them. Suppose a side has data for transmission, it can transmit the data only at the time when it gets the token and after transmission it released the token. Another session service is synchronization. When a computer trying to do a two-hour file transfer to another computer with a one-hour mean time between crashes then after each transfer is abort, the total transfer would have to start over again. And probably fail again the next time as well. To solve the problem, the session layer provides a way to insert checkpoints into the data stream, so that after a crash, only the data transferred after the last checkpoint have to be retransmitted. 24 Presentation Layer The presentation layer is layer 6 of the OSI Reference Model. The presentation layer handle the syntax and semantics of the information exchanged between two systems. This layer is designed for data translation, encryption, decryption and compression. The presentation layer is responsible for managing the way data is encoding. Not every computer system uses the same data-coding scheme, As different computers have different codes for presenting character strings (e.g. ASCII and Unicode), integers (e.g. one’s complement and two’s complement) and so on, the presentation layer make it possible to for computers with different representation to communicate. At the sending computer, this layer translates data from a format sent by application layer into a commonly recognized intermediary format. At the receiving computer, the presentation layer translates the intermediary format into a format useful to that computer’s application layer. The presentation layer is also responsible for protocol conversion, translating the data, encrypting the data, changing or converting the character set, and expanding graphics commands. This layer also manages data compression to reduce the number of bits that need to be transmitted. A utility known as the redirector operates at this layer. The purpose of the redirector is to redirect input/output operations to resource on a server. The main tasks of the presentation layer are to provide: • Translate data to a format that the receiving node can understand. For example from EBCDIC to ASCII (Translation). • Performs data encryption. Encryption means that the sender transforms the original information to another form. Decryption transforms the message back to its original form. • Performs data compression. Data compression reduces the size of the information. It is important in the transmission of multimedia. Application layer The application layer is the top layer of the OSI Reference Model. Application layer provides the interface between applications and network’s services. Application layer supports functions that control and supervise OSI application processes such as start/maintain/stop application; allocate/de-allocate OSI resources, accounting, check point and recovering. It also supports remote job execution, file transfer protocol, message transfer and virtual terminal. The main tasks of the application layer: • Allows a user to access files in a remote host, to retrieve files from a remote host for use in the local computer and manage or control files in a remote host locally (file transfer and access). • Allows a user to log into a remote computer and access the different resources of that computer (remote log-in). • Mail services (e-mail forwarding and storing) and accessing the World Wide 25 Web. • Directory services provide distributed database sources and access for global information about various objects and services. CHAPTER 2 COMMUNICATION Signal Electromagnetic waves propagated along a transmission medium are called signals. Signals can be two types i) Analog signal and ii) Digital signal. Analog and Digital Signals An analog signal has infinitely many levels of intensity over a period of time. As the wave moves from value A to value B, it passes through and includes an infinite number of values along its path. On the other hand, a digital signal can have only a limited number of defined values, often as 1 or 0. The digital signal is a discrete signal with a limited number of values. 26 Periodic and Aperiodic Signals A periodic signal complete a pattern within a measurable time frame called a period and repeats that pattern over subsequent identical periods. The completion of one full pattern is called cycle. An aperiodic or nonperiodic signal changes without exhibiting a pattern or cycle that repeats over time. Analog and digital signals both ban be periodic or aperiodic. In data communication we typically use periodic analog signals and aperiodic digital signals to sent data from one point to another. Analog Signal Analog signal can be classified as i. Simple analog signal. ii. Composite analog signal. A simple analog signal is a sine-wave, cannot be decomposed into simpler signals. A composite analog signal is composed of multiple sine-waves. Sine Wave The sine wave is the most fundamental form of a periodic analog signal. Mathematically describe a sine wave is s (t) =A sin (2π ft+φ ) where s is the instantaneous amplitude A is the peak amplitude, f is the frequency, φ is the phase. Characteristics of sine wave i. ii. iii. Amplitude. Frequency. Phase. Peak amplitude The peak amplitude of a signal represents the absolute value of its highest intensity, proportional to the energy its carries. For electrical signals, peak amplitude is normally measures in volts. 27 Amplitude 1s t T Period and Frequency Period refers to the amount of time, in seconds, a signal needs to complete one cycle. Frequency refers the number of periods in one second. Period is the inverse of frequency and frequency is the inverse of the period. f=1/T and T=1/f Period is formally expresses in seconds and frequency is expressed in hertz (Hz). Phase Phase describe the position of the waveform relative to time zero. Phase is measured in degree or radians. The Maximum Data Rate of a Channel Noiseless Channel: Nyquist Data Rate H. Nyquist proved that if an arbitrary signal has been run through a low-pass filter of bandwidth H, the filtered signal can be completely reconstructed by making only 2H samples per second. If the signal consists of V discrete levels (number of signal levels used to represent data), for a noiseless channel of H bandwidth, Nyquist’s theorem states: Maximum data rate = 2 H log2V bits/sec Example: A noiseless 5-kHz channel is transmitting a signal with two signal levels. The maximum bit rate can be calculated as: Maximum data rate = 2 × 5000 × log22 = 10000 bps. We can say that the channel cannot transmit two-level signals only at a rate 10,000 bps and not exceeding this rate. 28 Noisy Channel: Shannon Data Rate The amount of thermal noise is measured by the ratio of the signal power to the noise power, called the signal-to-noise ratio. For a noisy channel, the signal power denoted by S, the noise power by N and the signal-to-noise ratio is S/N (or SNR). Typically, the ratio is present in quantity 10 log10 S/N. These units are called decibels (dB). An S/N ratio of 10 is 10 dB, a ratio of 100 is 20dB, and a ratio of 1000 is 30dB and so on. Shannon’s the maximum data rate of a noisy channel whose bandwidth is H Hz and whose signal-to-noise ratio is S/N is given by Maximum data rate = H log2 (1+S/N) Example: A 5-kHz channel is transmitting a signal with two signal levels and signal-to-noise ratio of 30 dB. The maximum bit rate can be calculated as: Maximum data rate = 5000 × log2 (1+1000) = 5000 × 9.967 = 49836 bps. We can say that the channel never transmit much more than 50,000 bps, no matter how many signal levels are used and no matter hoe often samples are taken. Line Coding Line coding is the process of converting binary data, a sequence of bits, to digital signals. For example, data, text, image, audio and video that are stored in computer memory are all sequence of bits. 01011101 Line Coding Pulse Rate The Pulse rate defines the number of pulse per second. If a pulse carries only 1 bit, the pulse rate and the bit rate are the same. If the pulse carries more than 1 bit, then the bit rate is greater than the pulse rate. Bit Rate = Pulse Rate x log 2 L where L is number of data levels of the signal. Line Coding Unipolar Polar Bipolar Unipolar 29 Unipolar encoding uses only one voltage level. Amplitude 0 1 0 1 1 0 1 Time Polar Polar encoding uses two voltage levels (positive and negative). Polar Differential Manchester NRZ NRZ-L NRZ-I RZ Manchester NRZ (No return to Zero) In NRZ encoding, the value of the signal is always either positive or negative. NRZ-L (NRZ- Level):- In NRZ-L the level of the signal is dependent upon the state of bit. A +voltage usually means the bit is 0 and –voltage means the bit is 1. NRZ-I (NRZ- invert):- In NRZ-I the signal is inverted if a 1 is encountered. The change of the voltage represents bit – 1 and no change represents bit- 0. Amplitude NRZ-L 0 1 0 0 1 1 1 0 Time NRZ-I Time 30 Return Zero (RZ) In Return Zero encoding uses three values +, - and 0 values. In RZ, the signal changes not between bits but during each bit. Actually 1 = positive to zero. 0 = negative to zero. A good digital signal must contain a provision for synchronization. The disadvantage of RZ encoding is that it requires two signal changes to encode 1 bit and therefore occupies more bandwidth. Value 0 1 0 0 1 1 1 0 Time Manchester Encoding In Manchester encoding, the transition at the middle of the bit is used for both synchronization and bit representation. Amplitude 0 1 0 0 1 1 1 0 Time Zero One Differential Manchester Encoding In differential Manchester encoding, the transition at the middle of the bit is used only for synchronization. The bit representation is defined by the inversion at the beginning of the bit. 0 1 0 0 1 1 1 0 31 Amplitude Time Presence of transition at the beginning of the time means zero Bipolar In Bipolar encoding use three levels: +, 0 and - . Amplitude 0 1 0 0 1 1 1 0 Time The 1s are positive and negative alternately Sampling Pulse Amplitude Modulation (PAM) One analog – to – digital conversion method is called pulse amplitude modulation. This technique takes an analog signal, samples it, and generates a series of pulse based on the result of the sampling. The term sampling means measuring the amplitude of the signal at equal intervals. PAM uses a technique called sampled and hold. At the given moment, the signal level is read, and then held briefly. The sampled value occurs only instantaneously in the actual waveform, but in generalized over a still short but measurable period in the PAM result. The PAM is more useful to other areas of engineering than it is to data communication. However PAM is the foundation of an important analog-to-digital conversion method called pulse code modulation (PCM). 32 Analog Signal PAM PAM Signal Pulse Code Modulation (PCM) PCM modifies the pulses created by PAM to create completely digital signal. To do so, PCM first quantizes the PAM pulses. Quantization is a method of assigning integral values in a specific range to sampled instances. The binary digits are then transformed to a digital signal by using one of the line coding techniques. PCM is actually made up four separate processes: PAM, quantization, binary encoding and Line coding. PCM is the sampling method used to digitize voice in T-line transmission in the North American telecommunication system. 33 0001100001000….. Binary data Quantization Analog Data Quantized data Line coding 127 PAM -87 Digital data Binary encoding Sampled analog data From analog signal to PCM digital code Sampling Rate: Nyquist Theorem According to the Nyquist theorem, the sampling rate must be at least 2 times the highest frequency. Sampling Rate (Nyquist Theorem) = 2 x fh where fh is the highest frequency Modulation of analog signals Modulation of an analog signal or analog-to-analog conversion is the representation of analog information by an analog signal. Modulation is needed if the medium has a band pass nature or if only band pass bandwidth is available to us. An example is radio. The government assigns a Baseband bandwidth to each radio station. The analog signal produced by each station is a low-pass signal, all the same range. To be able to listen to different station, the low-pass signals need to be shifted, each to a different range. Modulate is to mix a data signal onto a carrier and modify its characteristics for transmission in a communication network. 34 Analog/analog modulation AM FM PM Amplitude Modulation (AM) In AM transmission, the carrier signal is modulated so that its amplitude varies with the changing the amplitudes of the modulating signal. The frequency and the phase of the carrier remain the same. AM Bandwidth The Bandwidth of an AM signal is equal to twice the bandwidth of the modulating signal and covers a range centered on the carrier frequency .AM stations are allowed carrier frequencies anywhere between 530 and 1700 KHz (1.7MHz). However, each station’s carrier frequency must separate from those on either side of it by at least 10 KHz to avoid interference. If one station uses carrier frequency 1100 KHz, the next station’s carrier frequency can not be lower than 1110 KHz. BWt=2 x BWm where 35 • • • • Amplitude modulation is easy to implementation. It can be used both for analog and digital signal. It is affected by the noise signal that may add up with the original signal. The strength of the signal decrease with distance traveled. Frequency Modulation (FM) In FM transmission, the frequency of the carrier signal is modulated to follow the changing voltage level (amplitude) of the modulating signal. The peak amplitude and phase of the carrier signal remain constant, but as the amplitude of the information signal changes, the frequency of the carrier signal changes correspondingly. 36 FM Bandwidth The bandwidth of a FM signal is equal to 10 times the bandwidth of the modulating signal and, like AM bandwidths, covers a range centered on the carrier frequency. FM stations are allowed carrier frequencies any where between 88 and 108MHz. Station must be separated by at least 200 KHz to keep their bandwidth. BWt=10 x BWm where 37 • Frequency modulated wave is least effected by noise due to electrical disturbance. • Frequency signal has a wide spectrum of frequencies and therefore needs much higher bandwidth than amplitude modulation. • The number of FM signals is smaller than the number of AM signals one can transmit over a channel with a fixed total bandwidth. Modulation of Digital Data Digital/analog modulation ASK FSK QAM PSK Bit Rate: - Bit rate is the number of bits transmitted during 1 second. Baud rate: - Baud rate is the number of signal units per second. Relation between bit rate and baud rate: - The baud rate is less than or equal to the bit rate (baud rate<=bit rate). Bit rate equals the baud rate times the number of bits represented by each signal unit. The baud rate equals the bit rate divided by the number of bits represented by each signal unit. Example 1: An analog signal carries 4 bits in each signal unit. If 1000 signal units are sent per second, find the baud rate and the bit rate. Baud rate = number of signal units per second =1000 bauds per second. Bit rate = baud rate X number of bits per signal unit = 1000 X 4 = 4000 bps. Example 2: The bit rate of a signal is 3000. If each signal unit carries 6 bits, what is the baud rate? Baud rate = 3000 / 6 = 500 baud/s Amplitude shift key (ASK) In amplitude shift keying, the strength of the carrier signal is varied to represent binary 1 or 0 .Both frequency and phase of remain constant while the amplitude changes. Which voltage represents 1 and which represent 0 are left to the system designer. The peak 38 amplitude of the signal during bit duration is constant, and its value depends on the bit (0 or 1). Unfortunately, ASK transmission is highly susceptible to noise interference. Noise usually affects the amplitude; therefore, ASK is the modulation method most affected by noise. A popular ASK technique is called on/off keying (OOK). In OOK one of the bit values is represented by no voltage. The advantage is a reduction in the amount of energy required to transmit information. ASK. Band width of ASK: - Band width requirements of ASK are calculated the formula BW= (1+d) X Nbaud where BW= Bandwidth Nbaud = Baud rate d = it is a factor to related to modulation process (with minimum value 0) Relationship between baud rate and bandwidth in ASK 39 Frequency Shift Key (FSK) In frequency shift keying, the frequency of the carrier signal is varied to represent binary 1 or 0. The frequency of the signal, during bit duration is constant, and its value depends on the bit (0 or 1): both peak amplitude and phase remain constant. FSK avoids the most of the problems from noise. Because the receiver looking for specific frequency changes over a given number of periods. It can ignore the voltage spikes. Band Width of FSK: - FSK shifts between two carrier frequencies, it is easier to analyze as two coexisting frequencies .We can say that the FSK spectrum is a combination of two ASK spectra centered on fc0 and fc1. The band width required of FSK transmission is equal to the baud rate of the signal plus the frequency shift (difference between the two carrier frequencies) BW= fc1-fc0+Nbaud Relationship between baud rate and bandwidth in FSK 40 Phase Shift Key (PSK):- In phase shift keying, the phase of the carrier is varied to represent binary 1 or 0. Both peak amplitude and frequency remain constant as the phase changes. For example if , we start with phase of 0-degree to represent binary 0 ,then we can change the phase to 180-degree to send binary 1.the phase of the signal during each bit duration is constant , and its value depend on the bit (0 or 1) 2-PSK PSK is not susceptible to the noise degradation that affects ASK or to the bandwidth limitation of FSK. This means the smaller variations in the signal can be detected reliably by the receiver .therefore instead of utilizing only two variations of a signal, each representing 1 bit; we can use four variations and let each phase shift represent 2 bits. 4-PSK 41 Band Width of PSK: - The minimum band width required to PSK transmission is the same as that required for ASK transmission. Relationship between baud rate and bandwidth in PSK Multiplexing Multiplexing is the set of techniques that allows the simultaneous transmission of multiple signals across a single data link. In a multiplexing technique system technique, n lines share the bandwidth of one link. 42 Multiplexing Analog Digital FDM WDM TDM Frequency Division Multiplexing (FDM) FDM is an analog technique that can be applied when bandwidth of a link is greater than the combined bandwidths of the signals to be transmitted. In FDM, signals are generated by each sending device module different carrier frequencies. These modulated signals are then combined into a single composite signal that can be transported by the link. Carrier frequencies are separated by sufficient bandwidth to accommodate the modulated signal. Channels must be separated by strips of unused bandwidth (guard bands) to prevent signals from overlapping. FDM Multiplexing Process Below figure is a conceptual illustration of the multiplexing process. Each input devices generated a signal. Inside the multiplexer, these signals are modulated onto different carrier frequencies .The resulting modulated signals are then combined into a single composite signal that is sent out over a media link that has enough bandwidth accommodate it. 43 Demultiplexing Process In demultiplexer uses a series of filters to decompose the multiplexed signal into its constituent composite signals .The individual signals are then passed to demodulator and that separates them from their carriers and passes them to the waiting receivers. A very common application of FDM is AM and FM radio broadcasting. Wave Division Multiplexing (WDM) WDM is designed to use the high data rate capability of fiber-optic cable. The optical fiber data rate is higher than the data rate of metallic transmission cable. Using a fiberoptic cable for one single line wastes the available bandwidth. Multiplexing allows us to connect several lines into one. WDM is conceptually the same as FDM, except that the multiplexing and demultiplexing involve optical signals transmitted through fiber-optic channels. The idea is the same: we are combining different signals of different frequencies. However, the difference is that the frequencies are very high. Several input beams of light, each containing very narrow band of frequencies from different sources are combined by multiplexer to make a wider band of frequencies. At the receiver, the signals are separated by the demultiplexer. 44 WDM: The technology of the WDM is very complex, the idea is very simple. In WDM we want to combine the light sources into one single light at the multiplexer and do the reverse at the demultiplexer. Combining and splitting of the light sources are easily handled by prisms. Prisms in WDM multiplexing and demultiplexing One application of WDM is the SONET network in which multiple optical fiber lines are multiplexed and demultiplexed. Time Division Multiplexing TDM is digital process that allows several connections to share the high bandwidth of a link .Instead of sharing a portion of the bandwidth as in FDM, time is shared. Each connection occupies a portion of time in the link. 45 The data flow of each connection is divided into units. The size of the unit can be 1 bit or several bits. Combines one unit of each connection to make a frame; for n input connections a frame is arranged into a minimum of n time slots, each slot carrying one unit from each connection. 3T A3 3T A2 B2 C3 3T A1 B1 C1 M U L T I P L E X E R D E M U L T I P L E X E R A3 A2 B2 A1 B1 C1 Channel A3 C3 A2 B2 Frame2 A1 B1 C1 Frame1 C3 Frame3 Data are taken from each line every 3T seconds. Each frame is 3 time slots. Each time slot duration is T seconds. Time Division Multiplexing (TDM) 46 CHAPTER 3 TRANSMISSION MEDIA Transmission media is a physical path by which a message transfers from sender to receiver. Sender transmitted signals to receiver through transmission media. Transmission Media are directly controlled by physical layer. Transmission media can be divided into two categories: guided media and unguided media. Guided media include twisted-pair cable, coaxial cable and fiber-optic cable. Unguided media is wireless such as radio and lasers through the air. Transmission media Guided media Unguided media Transmission media Fiber-optic cable Twisted-pair cable Coaxial cable Air Guided Media Guided media are provided a channel from one device to another device, such as twistedpair cable, coaxial cable and fiber-optic cable. Twisted-pair and coaxial cable use metallic (copper) conductors that accept and transform signals in the form of electrical current. Where as optical fiber is a glass cable that accepts and transports signals in the form of light. Twisted Pair Cable A twisted pair consists of two insulated conductors (normally copper wires), usually about 1mm thick. The wires are twisted together in a helical form. Two parallel wires form a simple antenna, then the noise or crosstalk will added with transmitted signals; 47 which results in a difference at the receiver. In twisted pair copper wires are twisted together to reduce noise and crosstalk. In twisted pair, the number of twists per unit of length determines the quality of the cable; more twists means more quality. Twisted pair can be used for either analog or digital transmission. The bandwidth depends on the thickness of the copper wires and the distance traveled. Because of performance and low cost, twisted pair are widely used for many years. Twisted-pair cable Unshielded Twistedpair cable (UTP) Shielded Twistedpair cable (STP) The most common twisted pair cable used in communication is referred as unshielded twisted pair (UTP). The IBM has produced a version of twisted pair cable called shielded twisted pair (STP). Shielded twisted pair cable enclosed in a metal foil or braided-mesh shield that protects against electromagnetic interference. As twisted air cable encased by metal foil; it improved quality by preventing the penetration of noise or crosstalk. But it is bulkier and more expensive than unshielded twisted pair that is why it used seldom outside of IBM. The twisted pair cable is mostly used in telephone system and in local area networks. Twisted pair can transmit data several kilometers without any amplification. But for long distance repeaters are needed. The EIA (Electronic Industries Association) has developed standards to classify unshielded twisted pair cable into seven categories. Category 1 2 3 4 5 6 7 Bandwidth Very low <2 MHz 16 MHz 20 MHz 100 MHz 200 MHz 600 MHz Data Rate <100 Kbps 2 Mbps 10 Mbps 20 Mbps 100 Mbps 200 Mbps 600 Mbps Digital/Analog Analog Analog/digital Digital Digital Digital Digital Digital Use Telephone T-1 lines LANs LANs LANs LANs LANs 48 Categories are determined by cable quality, where as the category 1 is lowest and category 7 is highest quality. The most common UTP connector is RJ45 (RJ means Registered Jack). The RJ45 is a keyed connector that means the connector can be inserted in only one way. • Twisted-pair cable consists of two insulated copper wires twisted together. Twisting allows each ware to have approximately the same noise environment. • Twisted-pair cable is used in telephone lines for voice and data communication. Twisted-pair cable also used in Local Area Networks. • Twisted-pair cables are two types Shielded Twisted–Pair (STP) and Unshielded Twisted Pair (UTP). • Twisted-pair cable is less expensive, that is why twisted-pair cable used for long years. Advantage of Twisted-pair Cable 1. The twisted-pair cable can easily install and can easily maintain. 2. The twisted-pair cable can be used for both analog and digital data transmission. 3. The twisted-pair cable is less expensive of transmission for short distance. Disadvantage of Twisted-pair Cable 1. Coaxial Cable Coaxial cable (also known as coax) is another common transmission media in communication system. Coaxial cable carries signals of higher frequency ranges than twister-pair cable. A coaxial cable has a central core conductor wire (usually copper) as the core, surrounded by an insulating material. The insulator is encased by a cylindrical conductor of metal foil, braid or a combination of two. The outer conductor is covered in an insulating sheath. The whole cable is protected by a plastic cover. 49 Coaxial cable Baseband Coaxial cable Two kind of coaxial cable widely used: 1. Baseband Coaxial cable. 2. Broadband Coaxial cable. Broadband Coaxial cable Baseband coaxial cable, a 50 Ω cable, is commonly used in digital transmission. Baseband coaxial cable is used in some local area networks. Broadband coaxial cable, a 75 Ω cable, is commonly used in analog transmission. Broadband coaxial cable is used in cable Television network and can be used in telephone system. Typically broadband cable runs a long distance; therefore require analog amplifiers. But amplifiers can only transmit in one direction. So, a computer cannot transmit packet to another computer if an amplifier be positioned between them. To solve these problem two types of broadband systems have been developed: 1. Dual cable. 2. Single cable. Head-end Amplifiers Outbound cable Head-end Single cable. Low frequencies for inbound, high frequencies for outbound communication. Inbound cable (a) Computers (b) Broadband networks. (a)Dual cable. (b) Single cable. 50 In dual cable system, two identical cables run in parallel. All computers transmit on cable 1 and receive on cable 2.To transmit data, a computer sends the data through cable 1, which runs to a device called head-end at the root of the cable tree. The head-end than transmit the data to cable 2. In single cable system, different frequency bands are used for inbound and outbound communication. The low-frequency band is used for communication from the computers to the head-end, which then shifts the signal to the high-frequency and transmits to the computers. In subsplit system, frequencies from 5 to 30 MHz are used for inbound and frequencies from 40 to 300MHz are used for outbound communication. In midsplit system, frequencies from 5 to 116 MHz are used for inbound and frequencies from 168 to 300 MHz are used for outbound communication. Copper core Insulating material Outer conductor Insulating sheath Plastic cover A coaxial cable Category RG-59 RG-58 RG-11 RG-62 Impedance 75 Ω 50 Ω 50 Ω 93 Ω Use Cable Television Thin Ethernet Thick Ethernet ARCnet LANs, IBM 3270 application The most common type of connector for coaxial cable is the BNC (Bayone-NeillConcelman). Three popular BNC connectors are the BNC connector, the BNC T connector and the BNC terminator. The BNC connector is used in cable television to connect the end of the cable to the TV set. The BNC T connector is used in Ethernet networks to branch out a cable for connection to a computer or other devices. The BNC terminator is used at the end of the cable to prevent the reflection of the signal. • Coaxial cable has following layers (starting from the center): a metallic rodshaped inner conductor, an insulator covering the rod, a metallic outer conductor (shield), an insulator covering the shield and a plastic cover. • Coaxial cable has better shielding than twister cable and has excellent noise resistance. • Coaxial cable can carry signals of higher frequency ranges than twisted-pair cable. Bandwidth depends on the cable length. 51 • Coaxial cable is used in cable TV networks and traditional Ethernet LANs. • The attenuation is much higher in coaxial cables than the twisted-pair cable. Although the coaxial cable has a much higher bandwidth but the signal weakened rapidly and needs frequent use of repeater. Advantage of Coaxial Cable 1. Bandwidth is much higher than twisted-pair cable. Although bandwidth depends on the cable length. 2. Coaxial cable is well-shielded than twisted-pair cable and has excellent noise immunity. 3. Coaxial cable can be used for both analog and digital transmission. For analog, 75 ohm broadband coaxial cable is used and for digital, 50 ohm Baseband coaxial cable is used. 4. Coaxial cable can cross longer distance at higher data rate. 5. The coaxial cable is relatively inexpensive than optical fiber cable and easy to handle. Disadvantage of Coaxial Cable 1. The attenuation is much higher than twisted-pair cable so needs repeaters frequently. Fiber Optics A Fiber optics cable is made of very fine fibers of glass or plastic that accepts and transports signals in the form of light. They consist of a glass core, roughly fifty micrometers in diameter, surrounded by a glass "optical cladding" giving an outside diameter of about 120 micrometers. They make use of TIR (Total Internal Reflection) to confine light within the core of the fiber. Optical Fibers are optical waveguides. This means that wherever the fiber goes the light, which is confined to the core of the fiber, also goes. So optical fibers can be used to make light bend round corners. There are two types of bends macro-bend and micro-bend. An optical transmission system has three components: the light source (LED or laser), the transmission medium (Fiber-optics cable) and the detector (Photodiode). A pulse of light indicates a 1 bit and the absence of light indicates a 0 bit. The detector generates an electrical pulse when light falls on it. By attaching a light source to one end of an optical fiber and a detector to the other end, a unidirectional data transmission system that accepts an electrical signal, converts to light pulse and transmits it. At the receiving end the incoming signal reconverts to an electrical signal. The receiving end of an optical fiber consists of a photodiodes. The typical response time of a photodiode is 1nsec so data rates will be 1 Gbps. A pulse of light must carry enough energy to be detected. Repeaters are needed only about 30km on long lines. The repeater convert the incoming light to an electrical signal, regenerated full strength if it has weakened and retransmitted as light. There are two types of light sources for which fiber cables are available. These sources 52 lights are: 1. Light Emitting Diodes (LEDs). 2. Light Amplification bi stimulated Emission Radiation (Lasers). Item Data rate Mode Distance Lifetime Temperature Sensitivity Cost LED Low Multimode 3 km Long life Minor Low Semiconductor Laser High Multimode or single mode 30 km Short life Substantial Expensive A single fiber has a glass or plastic core at the center through which the light propagates. In single-mode fibers, the core is 8 to 10 microns in diameter. In multimode fibers, the core is 50 microns. The core is surrounded by a glass cladding with a lower index of refraction than the core, to keep all the light in the core. Its diameter is usually 125 microns. Although the cladding does not carry light, it is nevertheless an essential part of the fiber. The cladding is not just a mere covering. It keeps the value of the critical angle constant throughout the whole length of the fiber. For protection the cladding is covered in a thin plastic jacket. Its diameter is usually 250 microns. Fibers are usually grouped together in bundles protected by an outer sheath/jacket which is made of either PVC or Teflon. And it is provided for protection against moisture, abrasion, crushing and other environment dangers. In between the outer sheath and the plastic jacket are Kevlar strands to strengthen the cable. Kevlar is a strong material used in the fabrication of bulletproof vests. An optical fiber with its protection jacket may be typically 0.635 cm in diameter. Optical fibers are defined by the ratio of diameter of their core to the diameter of their cladding. (a) Sheath (b) Core Core (Glass) (a) Side view of a single fiber. (b) End view of a sheath with three fibers When the angle of indent is less than critical angle, the light is refracted and moves closer Sheath to the surface. If a light ray incident on the boundary at critical angle, the light bends Cladding Cladding along to the interface. If the angle is greater Jacket critical angle, the light ray reflected than the (Glass) Jacket internally. However, if the diameter of core is reduced than the light ray can only moves (Plastic) in a straight line, without reflection. 53 Less dense More dense I Less than critical angle, refraction Less dense More dense I equal to critical angle, refraction Less dense More dense I greater than critical angle, reflection Bending of a light ray Cladding Light Source Cladding Optical fiber Core Propagation Modes There are two types of modes for propagation of light along optical channel. 1. Multimode. 2. Single-Mode. Multimode In multimode fiber multiple rays from a light source (LED) moves through the core in different paths. In multimode fibers, the core is 50 to 100 microns in diameter and 125 microns cladding. Multimode can be in two forms: 1. Step-index. 2. Graded-index. 54 Cladding Cladding Multimode step-index Optical fiber Cladding Cladding Multimode graded-index Optical fiber Cladding Cladding Single-mode Optical fiber Multimode Step-index In multimode step-index propagation, the core density is constant and the light beam moves through this constant density in a straight line until it reaches the interface of the core and the cladding. The direction changes suddenly at the interface between the core and the cladding. The term step index refers to the suddenness of this change. Multimode Graded-index In multimode graded-index propagation, the core density decreased with distance from the center. Density is highest at the center of the core and lowest at the edge. As the index of refraction is related to density; this causes a curving of the light beams. The word index refers to the index of refraction. Single-mode In single-mode fiber as the core’s diameter reduce, the light can moves in a straight line. As the core density is lower than multimode fiber, the critical angle that is close enough to 900, which make the propagation of beams almost straight. In this case, propagation of different beams is almost identical and delays are negligible. All the beams arrive at the destination together and can be recombined with little distortion to the signal. In singlemode fibers, the core is 8 to 10 microns in diameter and 125 microns cladding. Singlemode fiber uses an Injection Laser Diode (ILD) as a light source. 55 Type 50/125 62.5/125 100/125 7/125 Core(micron) 50 62.5 100 7 Cladding(micron) 125 125 125 125 Mode Multimode Graded-index Multimode Graded-index Multimode Graded-index Single-mode • Fiber-optic cables are composed of glass or plastic inner core surrounding by cladding, all encased in an outside jacket. • Fiber-optic cables transmit data signal in the form of light. The signal is propagated along the inner core by reflection • Fiber-optic transmission is become popular due to its noise resistance, low attenuation and high bandwidth capabilities. • In optical fibers signal propagation can be multimode (multiple beams from a light source) or single-mode (essentially one beam from a light source). • In multimode step-index propagation, the core density is constant and the light beam changes direction suddenly at the interface between the core and the cladding. • In multimode graded-index propagation, the core density decreased with distance from the center. This causes a curving of the light beams. • Fiber-optic cable is used in backbone networks, cable TV networks, Telecommunication, and Fast Ethernet networks. • Mode of data transmission is half-duplex. • An optical transmission system has three components: the light source (LED or laser), the transmission medium (Fiber-optics cable) and the detector (Photodiode). • Used mainly for digital data. A pulse of light indicates a 1 bit and the absence of light indicates a 0 bit. • The detector generates an electrical pulse when light falls on it. • The repeater convert the incoming light to an electrical signal, regenerated full strength if it has weakened and retransmitted as light Advantage of Optical Fiber 1. 2. 3. 4. 5. 6. Fiber-optic cable carry signals with much less energy loss than twister cable or coaxial cable and with a much higher bandwidth. Hence the data rate data rate is also higher than other cables. Fiber-optic cables are much lighter and thinner than copper cables with the same bandwidth. This means that much less space is required in underground cabling ducts. Also they are easier for installation engineers to handle. Fiber-optic cables suffer less attenuation than other guided media because light beam traveling in the fiber. This means that fibers can carry more channels of information over longer distances and with fewer repeaters. Fiber-optic cables are not affected by electromagnetic interference, power surges or power failure or corrosive chemicals in the air. Fiber-optic cables cannot easily be tapped. It has more immunity to tapping than copper cables. As fibers are very good dielectric, isolation coating is not required. 56 7. 8. No electric connection is required between the sender and the receiver. Fiber-optic cables are much more reliable than other cables. It can better stand environment condition, such as pollution, radiation etc. Its life longer in compare to copper wire. Disadvantage of Optical Fiber 1. Fiber-optic cables and the interfaces are more expensive than other guided media. 2. Propagation of light is unidirectional. So two fibers are needed if we need bidirectional communication. 3. It is new technology, therefore only few trained mechanics are available. Optical fibers cannot be joined together as an easily as copper cable and requires additional training of personnel and expensive precision splicing and measurement equipment. Unguided Media Unguided media (usually air) transport electromagnetic waves without the use of a physical conductor. This type of communication is often referred to as wireless communication. As signals are broadcast through air; signals are available to anyone who has a device capable of receiving them. Unguided signals (wireless data) are transmitted from source to destination in different ways. They are ground propagation, sky propagation and line-of-sight propagation. In ground propagation, radio waves through the lowest portion of the atmosphere. In skypropagation, higher frequency radio waves radiate upward into ionosphere where they reflected back to the earth. In line-of-sight propagation, very high-frequency signals are transmitted in the straight lines directly from antenna to antenna. Wireless transmission can be classified into three groups: 1. Radio Waves. 2. Microwaves. 3. Infrared waves. Radio Waves 57 Normally electromagnetic waves having frequencies between 3 KHz and 1 GHz are called radio waves. Those radio waves that propagate in sky mode can travel long distance. That is why for long distance broadcasting radio waves are used such as AM radio. Radio waves are omni directional. Radio waves use omni directional antennas that are sent out signals in other directions. Base on the wavelength, strength and the purpose of transmission there are several antenna. Low and medium frequency radio waves can pass through building walls. Radio wave band is narrow, just 1 GHz. Therefore when this band divided into sub-bands are also narrow, as a result to a low data rate for digital communication. Radio waves are used for multicast communication such as AM and FM radio, television, cordless phones and paging Radio waves include the following types: 1. Short-wave 2. Very-high-frequency (VHF) television and FM radio. 3. Ultra-high-frequency (UHF) radio and television. Microwaves Electromagnetic waves having frequencies between 1 to 300 GHz are called microwaves. Microwaves are unidirectional; propagation is line of sight. Microwaves are used for cellular phone, satellite and wireless LAN communications. The parabolic dish antenna and horn antenna are used for transmission of microwaves. When an antenna transmits microwave they can be narrowly focused. That means the sending and receiving antennas need to be aligned. Since microwave travels in the straight line, so the towers with the mounted antennas need to be in direct sight of each other and tall enough or close enough together. If towers are too far apart then the towers are needed to be very tall. Repeaters are often needed for long-distance communication. The curvature of the earth and other blocking obstacles do not allow two short towers to communicate using microwaves. Antennas must be directional, facing each other. Very high frequency microwaves cannot pass through building walls. As a result receiver cannot get signal inside buildings. The microwave band is relatively wide almost 299 GHz. Therefore wider sub-bands can be assigned and high data rate is possible. Infrared Waves Electromagnetic waves having frequencies between 300 GHz and 400 THz are called infrared waves. Such a wide bandwidth can be used to transmit digital data with very high data rate. The wavelengths of infrared signal are from 1mm to 770nm. As the wavelengths of infrared signal are too small, cannot pass through building walls. Infrared waves are used for short distance communication such as those between a PC and a peripheral device such as keyboards, mice and printers. But in short range communication, these do not interfere with the use of another system in the next room. 58 Infrared signals are useless for long range communication. We cannot use infrared signals outside a building because the sun’s rays contain infrared waves that interfere with the communication. The remote controls used on televisions, VCRs and stereos all use infrared communication. Infrared waves also can used in wireless LANs. Band VLF LF MF HF VHF UHF SHF EHF Range 3-30 KHz 30-300 KHz 300 KHz - 3 MHz 3-30 MHz 30-300 MHz 300 MHz- 3GHz 3 -30GHz 30-300 GHz Propagation Ground Ground Sky Sky Sky and Line-of-sight Line-of-sight Line-of-sight Line-of-sight Application Long-range radio Radio AM radio Ship/aircraft communication VHF TV, FM radio UHF TV, cellular phones, Satellite communication Radar, satellite Guided media Guided media transmit data in the form of electrical current or light. Here the data is transmitted by metal or glass conductor. Through guided media signal can travel according physical path. This type of media is generally used at low frequency data transmission. This type of media is used for generally short distance transmission. Here the data transmission is private; only that the receiver can receive the data to which the cable is connected. In the case of guided media analog and digital transmission possible. It has lower bandwidth. It has higher bandwidth than guided media. Through this media signal can pass through In the case of unguided media external least undesired external interference. It is interference can cause serious effect to protected from as much as possible from the signal. external interference. Guided media include twisted-pair cable, Unguided media is wireless such as radio coaxial cable and fiber-optic cable and lasers through the air. It is a lower cost media such as twisted-pair The media is high media as satellite is or coaxial cable. used or several high capable transmitter and receiver are used. Unguided media Unguided media transmit data by electromagnetic wave through air. Here the data is transmitted without used of any physical conductor. Through unguided media signal can travel according to their propagation like ground propagation, sky propagation and line-of-sight propagation. This type of media is generally used for high frequency data transmission. This type of media is used for large distance transmission such as satellite. Here any one can receive transmitted data which have capable receiver to receive it. Only analog transmission happens. 59 Microwave Communication Microwave communication widely used for long-distance telephone communication, cellular telephones, television distribution, etc. • Microwave is relatively economical as compared to fiber optics systems. • Microwave systems provide high speed data transmission rates. There are two types of microwave data communication systems. These are: 1. Terrestrial Communication. 2. Satellite Communication. Terrestrial Communication Terrestrial microwave systems usually use directional parabolic antennas to send and receive signals in the lower range. Most terrestrial microwave systems produce signals in the low range usually at 4 to 6 GHz and 21 to 23 GHz. The signals are highly focused and physical path must be line-if sight. Relay towers are used to extend signals. Short distance systems can be relatively inexpensive but long distance systems can be very expensive. • Most terrestrial systems use low frequency ranges. • Bandwidth capacity depends on the frequency used. • Attenuation depends on frequency, power, antenna size and atmospheric conditions. Normally, over short distances, attenuation is not significant. Highfrequency microwaves are more affected by rain and fog. • Short-distance systems can be economical, but long distance systems can be very expensive. • Installation of terrestrial microwave systems is extremely difficult. Because the transmission is line-of-sight, antenna must be carefully aligned. • Data rates are from 1 to 10Mbps. Satellite Communication A satellite network uses a combination of nodes that provides communication between any points on the earth. Example of different nodes in the network is a satellite, an earth station, or an end-user terminal or a telephone. Satellite microwave systems transmit signals between directional parabolic antennas. Like terrestrial microwave systems, they use low frequency ranges usually at 4 to 6 GHz and 11 to 14 GHz and must be in line-ofsight. The main difference with terrestrial microwave systems is that satellite microwave systems can reach the most remote places on earth and communicate with mobile devices. A communication satellite is basically a big microwave repeater in the sky. It contains several transponders, which can receive from the earth station incoming weak signal, amplifies it into high power signal, and rebroadcasts at another frequency (to avoid 60 interference with the incoming signal) to the receiving earth station. A typical satellite has 12-20 transponders with a 36-50 MHz bandwidth. • Satellite microwave systems usually use low frequency ranges. • The cost of building and launching a satellite is extremely expensive. Although satellite communications are expensive, the cost of cable (fiber optics) to cover the same distance may be even more expensive. • Installation of satellites is extremely technical and difficult. The earth-based systems may require exact adjustments. • Attenuation depends on frequency, power, antenna size and atmospheric conditions. High-frequency microwaves are more affected by rain and fog. • Bandwidth capacity depends on the frequency used. • Data rates are from 1 to 10Mbps. VSATs (Very Small Aperture Terminals) are low-cost microstations used in satellite communication. These tiny terminals have 1 meter antennas and can put out about 1 watt power. In many VSAT systems, the microstations do not have enough power to communicate directly with another via satellite. Hub is a special ground station, with a large high-gain antenna which needed to relay traffic between VSATs. In this mode of operation, either the sender or the receiver has a large antenna and a powerful amplifier. There is a time delay of 540 m second between a transmitted and received signal for a VSAT system with a hub. An artificial satellite need a path in which it travels around the earth is called orbit. Based on the location of the orbit, satellite can be divided into three categories: 1. GEO (Geosynchronous Earth Orbit). GEO is at the equatorial plane and resolves in phase with the earth. 2. LEO (Low-Earth Orbit). LEO satellite provides direct universal voice and data communications for handheld terminals and also provides universal broadband internet access. 3. MEO (Medium earth Orbit). MEO satellite provides time and location information for vehicles and ships. GEO Satellite Sending and receiving antennas must be in line-of-sight. For short time communication, a satellite can move faster and slower than the earth rotation. But for relay (constant) communication; the satellite must be move at the same speed as the earth so that it seems to remain fixed above a certain spot. Such satellites are called geosynchronous. The rotation period of geosynchronous satellite is same as the earth (rotation period is 23 hours 56 minutes 4.09 seconds). Only one orbit can be geosynchronous. This orbit is at equatorial plane and is 35,786 km above the equator. To avoid interference geosynchronous satellites are spacing 2 degree in the 360 degree equatorial plane, as a result there can only be 180 geosynchronous communication satellites in the sky at once. To provide full global transmission, it takes minimum of three satellites to cover whole earth. Three satellites are spacing 120 degree from each 61 other in geosynchronous orbit. • It is capable of providing continuous and uninterrupted communication over the desired area. • There is a time delay of 250 to 300 m second between a transmitted and received signal. • Small areas near north and south poles are not covered in the communication range of the satellite. • A costly launch vehicle is required. MEO Satellite MEO satellites are positioned between the two Van Allen belts. MEO satellites are located at altitudes between 5000 and 15,000 km. and a rotation period of 6 hours. Global Positioning System (GPS) satellites are MEO satellites that provides time and location information for vehicles and ships. GPS has 24 satellites in six orbits, with each orbit hosting four satellites. A GPS receiver can tells the current position of a satellite and sends a signal to four satellites. It calculates your position on the earth. LEO Satellite LEO satellites are normally at altitude between 500 to 2000 km. LEO satellites have polar orbits. The satellite has a speed of 20,000 to 25,000 km/h with a rotation period of 90 to 120 min. Iridium is designed to provide direct world wide voice and data communication using handheld terminals, a service similar to cellular telephony but on a global scale. The Iridium System has 66 satellites in six LEO orbits; each at an altitude of 750 km. Communication between two distant users requires relaying between several satellites. Globalstar system has 48 satellites in six polar orbits with each orbit hosting eight satellites. Communication between two distant users requires both satellites and earth stations, which means that ground stations can create more powerful signals. Teledesic satellites are LEO satellites that will provide universal broadband Internet access. Teledesic has 288 satellites in 12 LEO orbits with each orbit hosting 24 satellites. The orbits are at an altitude of 1350 km. Two frequencies are designed for each satellite to send and receive. Transmission from the earth to the satellite is called uplink. Transmission from the satellite to the earth is called downlink. Frequency bands for satellite communication are given in the following table. Band L S C Ku Bandwidth MHz 15 70 500 500 Uplink (GHz) 1.6 2.2 6 14 Downlink(GHz) 1.5 1.9 4 11 Problems Terrestrial interference Rain 62 Ka 3500 30 20 Rain; Expensive Transmission Impairments After signal travels through transmission media, which are not perfect, the received signal is not same as the transmission signal. The imperfection causes impairment in the signal. Three types of impairment usually occur in transmission line: 1. Attenuation 2. Distortion 3. Noise Attenuation Attenuation means loss of energy. When a signal travels through a medium, it losses some of its energy. So that it can overcome the resistance of the medium. Some of the electrical energy in the signal is converted to head. The loss is expressed in decibels (dB) per kilometer. The decibel is negative is if a signal is attenuated and positive if a signal is amplified. The amount of energy loss depends on the frequency. To compensate for frequency-dependent attenuation, amplifiers are used to amplify the signal. But it can never restore the signal to its original shape. Distortion Distortion means that the signal changes it forms or shape. Distortion occurs in a composite signal, made of different frequencies. It is caused by the fact that different Fourier components travel at different speeds. For digital data, fast components from one bit may catch up and overtake slow components from the bit ahead, mixing the two bits and increasing the probability of incorrect reception. Noise Noise is an unwanted energy from sources other than the transmitter. Several types of noise may corrupt the signal. Thermal Noise: Thermal noise is caused by random motion of electrons in a wire, which 63 creates an extra signal not originally sent by the transmitter. The thermal noise also known as white noise. That means frequencies corresponding to all colors of the light spectrum are present in equal amount in the thermal noise. Cross talk: Cross talk is caused by inductive coupling between two wires that are close to each other. One wire acts as a sending antenna and the other acts as a receiving antenna. Impulse Noise: Impulse noise caused by spikes (a signal with high energy in a very short period of time) on the power line, lighting or other causes. For digital data, impulse noise can wipe out one or more bits. Induced Noise: Induced noise come from the source such that motor and appliances. These devices act as a sending antenna and transmission media acts as a receiving antenna. 64 CHAPTER 4 NETWORK STRUCTURES Network Topology Topology is the structure of a network including physical arrangement of devices. Two or more devices connect to a link; two or more links form a topology. The topology of a network is the geometric representation of the relationship of all the links and linking devices (typically called as nodes) to one another. There are four basic topologies possible: bus, ring, star and mesh. Some other possible topologies are tree, intersecting rings and irregular. Bus Topology In bus topology all nodes are connected by a single long bus cable which acts as backbone. Nodes are connected to the bus cable by drop lines and taps. • • • • • The bus topology is simple, reliable in small networks, easy to use and also easy to understand. In bus topology least amount of cable is required. So it is less expensive than other topology. We can easily extent a bus by joining two cables with a connector and allow more computers to join the network. Many computers within a bus can slow down the performance. A cable break or loose connector or any malfunctioning computer anywhere between two computers can cause them not no able to communicate with each other and may be break down the whole network. Ring Topology In ring topology all nodes are arranged in a circle. Data travels around the ring in one direction, with each node on the ring acting as a repeater. Each node passes information to its next node, until it arrives at intended destination. Ring networks typically use a token passing protocol. • In ring topology performance is faster than bus topology. • Installation and reconfigure is easy because each device is linked to its immediate neighbors (either physically or logically). 65 • • • Signal loss problem is not subject in ring topology. Failure of one node on the ring can affect the whole network. Adding or removing nodes disrupts the network. Star Topology In star topology all nodes are attached to a central controller device usually called as hub. The all nodes are only point-to-point like only to the central device. The nodes are not directly liked to one another. • • • • • • A star topology is less expensive than a mesh topology. Installation and reconfigure is easy because each device needs only one link and one I/O port to connect. We can easily add new computers to a star network by run a new line from the computer to central position and plug in into the hub. Single computer failure is not affect the network. The hub can detect a network fault, isolate the offending computer or network cable and allow the rest of the network to continue operating. If the central hub fails then the whole network fails to operate. In star topology more cable is require than other network topologies (such as ring or bus). Although a star requires far less cable than a mesh topology. Mesh topology In a mesh topology each node has a dedicated point-to-point link to every other device. The term dedicated means that the link carries traffic only between the two devices it connects. A fully connected mesh network has n(n-1)/2 physical channels to link n devices; every device on the network must have n-1 I/O ports. • Privacy or security is more than other topologies. When data travels along a dedicated path, only the intended recipient notices it. • The dedicated links eliminating traffic problems. Each dedicated links guarantees that each connection can carry its own data load. • Single computer failure or one link unusable is not affecting the network. • Point-to-point links make fault identification and fault isolation easy. Traffic can be routed to avoid links with suspected problems. • In mesh topology more cable is require and more I/O ports are required. • Installation and reconnection are difficult because every device must be connected to every other device. • The hardware required to connect each link (I/O port and cable) can be expensive. • Wiring can be greater than the available space can accommodate. Controlled Access 66 In controlled access, the stations consult one another to find which station has the right to send. A station cannot send unless it has been authorized by other stations. Poll and select are popular controlled access methods. Polling One of the stations is designation as a primary station and other stations are secondary stations. Data transfer must be made through the primary station even when the final destination is a secondary station. The primary station controls the links. If the primary station is neither sending nor receiving data, it knows the link is available. If the primary station wants to receive data, it asks the secondary stations if they have anything to send; this method is called polling. If the primary station wants to send data, it tells the secondary stations to get ready to receive data; this method is called selecting. Select The select method is used whenever the primary station has anything to send. Primary station must alert the secondary station to the upcoming transmission and wait for an acknowledgment of the secondary station (as ready status). So before sending data, the primary station creates and transmits a select (SEL) frame, one field of which includes the address of the intended secondary station. After receiving acknowledgment the primary station send data to the destination secondary station. When secondary station has received data it sends acknowledgment to the primary station. Poll The poll method is used whenever the primary station has anything to send. When the primary is ready to receive data, it must ask (poll) each secondary station in turn if it has anything to send. When the secondary station is received, it responds either with a NAK frame if it has nothing to send or with data frame if it does. If the response is negative (a NAK frame), the primary station then polls the next secondary in the manner until it finds one with data to send. When the response is positive (a data frame), the primary station reads the frame and returns an acknowledgment (ACK frame) to the secondary station. Channel Sharing Techniques It is better to sharing a communication like among many users rather than dedicating it to a single user. Channel sharing technique leads a much better utilization of the channel bandwidth and also reduces the cost of networking. In computer networking, well-known channel sharing techniques are following: 1. Frequency Division Multiplexing (FDM). 2. Wave Division Multiplexing (WDM). 3. Time Division Multiplexing (TDM). 67 Switching There are the following approaches for providing reliable and efficient transport of messages across a point-to-point subnet. Circuit Switching A switching technology that establishes an electrical connection between stations using a dedicated path is called circuit switching. Circuit switching creates a direct physical connection between two devices such as computer or phones. When a call has been set up, a call request signal must propagate from caller source host to destination host and call accept signal must come back from destination host to the caller host. Once a call has been set up connection between both ends exists and it will continue until the exchange of messages is over and a disconnect request signal is issued by either host. A circuit switch is a device with n inputs and m outputs that creates a temporary connection between an input link and output link. The number of inputs does not have to match the number of outputs. Physical connection Set up when call is made Switching office • • • • • • • • An end-to-end dedicated physical path need to set up before any data can be sent. Once a call has been set up, a connection between both ends exists and it will continue until the exchange of messages is over and a disconnect request signal is issued by either host. Circuit switching provides continuous-time end-to-end transport of messages. The charge is based on distance and time. Available bandwidth is fixed and unused bandwidth on an allocated circuit is wasted. Each data follows the same route. Circuit switching has only suffered circuit set-up delay. Circuit switching is completely transparent. The sender and receiver can use any bit rate, format, or framing method. Carrier does not know or care about these basic parameters. 68 • Telephonic communication is an example of circuit switching. Circuit switching can use in two technologies: the space-division switch and timedivision switch. Call request signal Time Propagation delay Pkt Msg Pkt Pkt Time spent hunting for an outgoing trunk Call accept signal Pkt Msg Queueing delay Pkt Pkt Pkt Pkt Msg Pkt AB trunk Dat a BC CA trunk trunk A B (a) C D A B C (b) D A B (c) C D Timing of events in (a) circuit switching, (b) message switching, (c) packet switching. Message Switching A switching technology that messages to be transmitted between stations are all sent to a central station, which gathers them and routes them to the appropriate destination. When a sender has a block of data to be sent, it is stored in the first switching office (i.e. router) and then forwarded later, one hop at a time. Each block is received entirely, inspected for error and then retransmitted. Message switching can use in store-and-forward network. • The message is transport across the subnet from the source to destination, one hop at 69 • • • • a time. Message has switching suffered propagation delay and queuing delay. Message switching provides discrete-time hop-by-hop transport of messages. In message switching there is no upper limit on the block size. Simultaneously availability of the sender and receiver is not required in the message switching. Packet Switching In this switching technology the messages to be transmitted in the form of packets and placed into channels. Long massages are broken into smaller units called packets. Packets are formed by adding information to the beginning (which is called header) and adding information at the end (which is called trailer) of each group of user data. The packet header contains source and destination addresses, control bits, packet number and as well as different types of information such as billing and accounting information. The trailer contains error checking information, such as CRC (which is used to determine whether the packet has been corrupted in transmission) and end-of-message indicator. In packet switching the packets are transmitted by store-and-forward technique but provide a big improvement in performance. Packet switching network place a tight upper limit on block size (not more than a few kilo bytes). And also reduce the store-andforward delay because transmission of each packet as soon as that packet arrives, without waiting for its successor packets to arrive. The packets are routes using the source and destination addresses. Header User Data Trailer Header Contents: Beginning-of-Message Indicator. Source Address. Destination Address. Description of Data to follow (e.g. User Info, Control Info). Packet Sequence Number. Routing Information. Billing Information. Trailer Contents: Error-Checking Code. End-of-Message Indicator. • • In packet switching long massages are broken into smaller units called packets. Packet switching network place a tight upper limit on block size (not more than a few kilo bytes). 70 • • • • • Packet switching may utilize the bandwidth on an allocated circuit because circuits are never dedicated. Carrier determines the basic parameters, such as bit rate, format or framing method. Header is adding at beginning of each user data and trailer adding at the end. Packets from the same message may travel along several different routes. An example of packet switching is an airline reservation system. Circuit switching Yes Static Yes No Time and distance At setup time Yes Yes No Packet switching No Dynamic No Yes packet On every packet No No Yes Item Use dedicated path Bandwidth available Call setup required Use store-and-forward transmission Charging based on Congestion can occur Data follows the same route Wasted bandwidth Carrier determine the basic parameters (i.e. bit rate, format or frame method)) 71 CHAPTER 5 FLOW CONTROL AND ERROR CONTROL Automatic Repeat Request The data link controls (DLCs) generally use the concept of Automatic Repeat reQuest (ARQ). At the receiving end DLC module detect erroneous frames and then send a request to the transmitting DLC module to retransmission the incorrect frames. Retransmission of data necessitate due to in three cases: damaged frame, lost frame and lost acknowledgement. Damaged Frame A recognizable frame does arrive, but some of the bits have been altered during transmission. That means receiving frame has some error. Lost Frames A frame fails to arrive at the other side. For example, a noise burst may damage a frame to the extent that the receiver is not aware that a frame has been transmitted. Lost acknowledgement An acknowledgement fails to arrive at the source. The sender is not aware that acknowledgement has been transmitted from the receiver. The purpose of ARQ is to turn an unreliable data transmission to reliable one. There are two categories of ARQ: 1. Stop-and-wait ARQ 2. Continuous ARQ Stop-and-wait ARQ Stop-and-wait ARQ is based on stop-and-wait flow control technique. The stop-and-wait process allows the transmitter DLC station to transmit a block of data and then wait for the acknowledgement (ACK) signal from receiver station, which indicates correct reception. No other data frames can be sent until the receiver’s reply (ACK) arrives at the source station. There is chance that a frame that arrives at the destination (receiver) is 72 damaged. The receiver detects this error by using error detection technique. If the receiver detects an error, it simply discards corrupted frames and it sends a negative acknowledgement (NAK). The sender waits for acknow, and the message is then retransmitted. This type of protocol is most often found in simplex and half-duplex communication. The ACK/NAK frame is a small feedback frame from receiver to sender. Little dummy frames are used to carry ACK/NAK response. The acknowledgement is attached to the outgoing data frame from receiver to sender (i.e. full duplex operation) using ack field onto the header of the frames in the opposite direction. The technique of temporarily delaying outgoing acknowledgements so that they can be hooked onto the next outgoing data frame is known is piggybacking. In affect, the acknowledgement gets a free ride on the next outgoing data frame. The advantages of piggybacking are • Better use of available channel bandwidth. • Less traffic due to absence of dummy frames. • Less frame arrival interrupts to DLC software/hardware. • Perhaps fewer buffer requirement in the receiver. Continuous ARQ In the continuous ARQ frames transmit continuously. The transmitter contains buffer storage and both transmitter and receiver monitor frame counts. If the receiver detects an erroneous frame then NAK is sent to the transmitter with defective frame number N. when the transmitter gets the NAK message then it can retransmission in either of the following ways: 1. Go-Back-N 2. Selective Repeat Go-Back-N In Go-Back-N ARQ transmitter retransmission all frames starting from N. That means whenever transmitter received a NAK message, it simply goes back to frame N and resume transmission as before. Every time an NAK is received, the transmitter repositions the frame pointer back to frame N. The number of frames, which must be retransmitted, is at least one and often more, as all the frames from the erroneous frame are transmitted by the sender. The receiver simply discards all subsequent frames, sending no acknowledgements for the discarded frames. Go-Back-N ARQ is the most widely used type of ARQ protocol. Selective Repeat In Selective Repeat ARQ transmitter retransmission only the defective frame N and not the subsequent frames. The number of frames, which must be retransmitted, is always one, it being the frame containing error. The receiver buffered all correct frames following the erroneous frame. When transmitter receive a NAK message, it just retransmission the defective frame, not all its successors. If the second try succeeds, the receiver will rearrange the frames in sequence. Selective repeat is more efficient than go73 back-N, if less number of errors occur. But in this approach can require large amount of buffer memory. Both require the use of a full-duplex link. In comparison with stop-and-wait protocol, link efficiency is improved overall by both implementations because line idle and line turnaround times eliminated. Sender 0 1 2 3 4 5 6 7 3 4 5 6 7 ACK0 ACK1 ACK2 NAK3 ACK3 Receiver 0 1 2 Error E D D D D 3 4 Discarded Go-Back-N strategy Sender 0 1 2 3 4 5 6 3 7 8 9 10 11 ACK0 ACK1 ACK2 NAK3 ACK4 ACK5 ACK6 ACK3 ACK7 Receiver 0 1 2 Error E 4 5 6 3 7 8 Buffered Selective repeat strategy Sliding Window Protocols 74 When the channel is not error-free and/or the receiver has limited buffer spaces, flow of fames from sender to receiver must be controlled to prevent buffer overflow and/or duplication of frames at the receiver. A data link between a sender A and a receiver B is said to be window flow controlled, if there is an upper bound on the number of frames that can be transmitted by A but not yet been acknowledged by B. this upper bound (a positive integer) is called the window size or simply the window. The number of outstanding (i.e. unacknowledged) frames at any instant should not exceed the window size. Acknowledgements are either contained in special frames or are piggybacked on regular data frames in the reverse direction. The sliding window technique follow either go-back-N or selective repeat. The semantics of an n-bit sliding window protocol: • Each transmitting frame contains a sequence number between 0 to (2n-1). For stop-and-wait sliding window protocol, n=1, so that only two sequence numbers, (i.e. 0 and 1) are possible. A common value of n is 3, which allow eight frames to be transmitted without any acknowledgement. • At any point of time, the sender or receiver maintains a list of consecutive sequence numbers corresponding to frames it is permitted to send or receiver. • Initially, no frames are outstanding, so the lower and upper edges of the sender’s window are equal. • The frames are said to fall within the opening of the sending window. The receiver also maintains receiving window corresponding to the set of frames it is permitted to accept. The sending and receiving window sizes need not be identical. • The open portion of the sending window represents frames transmitted but as yet not acknowledged. • When a new frame arrives, it is assigned the next highest sequence number and upper edge of the window is advanced by one and provided the window is not fully closed. • When an acknowledgement comes in, the lower edge of the window is advanced by one. In this way the window continuously maintains a list of unacknowledged frames. • The open portion of the receiving window corresponds to the frames that the receiver is waiting to accept. When a frame is received, whose sequence number is equal to the lower edge of the open portion, its acknowledgement is generated and the window is rotated by one. • The sender window always remains at its initial size. Since any frame within the open portion of sending window may have to be retransmitted, the sender must keep all these frames in its buffer. In example window size is 1 with a 2-bit sequence number. The corresponding window operations are shown in the following figure. • Initially the lower and upper edges of the sender’s window are equal. The receiving window is open to accept the frame numbered 0. • Sending window is open and has transmitted frame 0 to the receiver. The receiving window still pen to receive frame 0. 75 • After frame 0 is received, the receiving window is rotated by one to be ready to receive the next frame. The acknowledgement is issued by receiver before the window is rotated. Sending window open to receive acknowledgement of frame 0. • The number of the acknowledgement frame indicates the number of the last frame received correctly. If this number matches with the number of the outstanding frame in the sender’s buffer, the frame is taken to be received properly by the receiver and the sender takes up the next frame for further transmission. If there is a mismatch, the sender retransmits the frame from the buffer. 76 Sender 3 0 3 Receiver 0 (a) Initial Setting 2 1 2 1 Window open to send frame 0 3 0 Window open to received frame 0 3 0 (b) After frame 0 is sent 2 1 Window has send frame 0 2 1 Window open to received frame 0 3 0 3 0 (c) After frame 0 is received and ACK 0 is sent 2 1 Window open receive ACK 0 2 1 Window rotated to accept frame 1 as ACK 0 sent 3 0 (d) After ACK0 is received 3 0 2 1 2 1 Window open to send frame 1 Window open to received frame 1 An example of sliding window protocol. Window size is 1 with a 2-bit sequence. number. 77 Error detection and Correction When data units transfer from one device to another device, the data units can become corrupted. Networks must be able to transfer data with complete accuracy. That is why a reliable networking system must have a mechanism for detecting and correcting those errors. Type of errors Errors are divided into two types: 1. Single-Bit Error. 2. Burst Error. Single-Bit Error Single-bit error means that only one bit of given data unit (for example byte, character, frame or packet) is changed from one to zero or zero to one. Suppose that a sender transmit group of 8 bits of ASCII characters. In following example, 01010001 (ASCII Q) was sent but 01000001 (ASCII A) was received at other end. Single-bit errors most likely occur in parallel transmission, but rare in serial transmission. One of the reasons is that one of the 8 wires is may be noisy. Suppose a sender transfer data at 1 Mbps. That means each bit lasts only 1/1,000,000 second or 1 µ s. If a noise lasts only 1µ s (normally noise lasts much longer than µ s) then it can affect only one bit. Sender Receiver Burst Error A burst error means that two or more bits in the data unit have changed from one to zero or from zero to one. In the following example, 01011101 01000011 was sent but 01000100 01000011 was Sender Receiver receive. Burst errors may not occur in consecutive bits. The length of burst error is 01000001 calculated 01010001 from first corrupted bit to the last corrupted bit. Some bits between them may not be corrupted. In this example, three bits has been changed from one to zero but the length of burst error is 5. 01011101 01000011 01000100 01000011 78 Burst errors most likely occur in serial transmission. Because data transfer in serial transmission is at slow speed. Suppose a sender transfer data at 1 Kbps. That means each bit lasts only 1/1000 second. If a noise lasts 1/100 second then it can affect 10 bits. Given any two codewords W1 and W2, the number of bit position in which the codewords differ is called the Hamming distance, dw1w2 between them. It is possible to determine how many bits differ, just Exclusive OR the two codeword and count the number of 1 bits in the result. It is given by dw1w2 = W1 ⊕ W2 It is significance that if two codewords are a Hamming distance d apart, it will require d single-bit errors to convert one into the other. Error Detection Three common types of error detection methods in data communications are follows: 1. 2. 3. Parity check. Cyclic redundancy check. Checksum. Parity Check The most common and least expensive method for error detection is the parity check. A parity bit is added to each data unit so that total number of 1s in the data unit becomes even or odd. For example, we want to transmit the data unit 1110111; total number of 1s in the data unit is 6, an even number. Before transmission we pass the data unit through a parity generator which counts the number of 1s and appends a parity bit at the end of the data unit. Most system uses evenparity checking but some system may use odd-parity. The even-parity checking function counts the number of 1s in the data unit; if the number is odd then it appends a parity bit 1 at the end of the data unit otherwise appends a parity bit 0. On other hand in odd-parity checking function counts the number of 1s in the data unit; if the number is even then it appends a parity bit 1 at the end of the data unit otherwise appends a parity bit 0. Simple parity check can detect all single-bit errors. It can detect burst error only if the total number of errors in each data unit is odd. Two-dimensional parity check • In two-dimensional parity check, a block of bits is arranged in a table. For example, following data unit divided in four rows and seven columns. • Calculate the parity bit for each data unit and create a new column. Note that eight column is calculated based on parity bit of each data unit. This is known as LRC (longitudinal redundancy check). 79 • Calculate the parity bit for each column and create a new row. Note that fifth row is calculated based on parity bit of each column. This is known as VRC (vertical redundancy check). Original block of data: 1 0 0 1 0 0 1 1 1 1 1 0 1 0 0 1011011 1 1 1 0 1 0101001 0 0 0 1 1 01010011 1 0 0 1 0 01110010 0111001 1 1 1 1 0 11001111 1100111 1 1 0 1 1 01011001 Transmitted block of data: 10110111 Receiver checks the parity bits, if some of the bits do not follow the even-parity rule and the whole block is rejected and negative acknowledgement sent to the sender and this block of data must be retransmitted by the sender. Otherwise the block of data is accepted. Cyclic Redundancy Check (CRC) One of the most common and widely used error-detection methods for synchronous data transmission is Cyclic Redundancy Check (CRC). CRC developed by IBM, uses CRC-16 as specific application of the CRC method. Mathematically, a bit string of length n is represented in powers of x such as x n-1 + xn-2 + …+ x2 + x1 + x0. As an example, 1011 has 4 bits and represents a 4-term polynomial with coefficients 1, 0, 1 and 1; the binary representation of x 3 + x + 1 is 1011, where missing terms in the polynomial are represented by 0’s. Polynomial arithmetic is done modulo-2. If we represent the data bits as dk-1, dk-2…, d1, d0, the data polynomial is given by d(x) =dk-1xk-1 + dk-2xk-2 + …+ d1x+d0 Polynomial arithmetic is done module 2, according to the rules of arithmetic field theory. There are no carries for addition or borrows for subtraction. Both addition and subtraction are identical to EXCLUSIVE OR (XOR). For example: 11001011 +10011001 01010010 11001011 - 10011001 01010010 In Cyclic Redundancy Check, the sender and receiver must be agreeing upon a generator polynomial G (x). Let the degree of frame M (x) is m, which must be longer than generator polynomial. The steps are as follows: 80 1. Let the degree of G (x) is r. Append r zeros at the right-end of the frame. Now the frame contains m+r bits and the polynomial becomes xrM(x). 2. Divide xrM(x) by generator polynomial G(x) using modulo 2 division. 3. Subtract the remainder (which is less to equal to r bits) from xrM(x) using modulo 2 subtraction. 4. The resultant frame T (x) is to be transmitted. Data 00...0 n bits Data Data CRC Divisor If zero accept, nonzero discard Remainder CRC n bits n+1 bits Divisor CRC Sender n+1 bits Remainder n bits Receiver Now T (x) is divisible by G (x). When the receiver gets the transmitted frame and tries to divide by G (x). If there is a remainder then there has been a transmission error, otherwise the frame is error-free. Frame: -1101011001 Generator: - 1 0 0 1 1 After adding 4 zero bits: - 1 1 0 1 0 1 1 0 0 1 0 0 0 0 10011 11010110010000 10011 10011 10011 10010 10011 1000 Remainder: - 1 0 0 0 Transmitted frame: - 1 1 0 1 0 1 1 0 0 1 1 0 0 0 If there are no errors, the receiver receives T (x) intact. The received frame is divided by G(x). 110000100 81 10011 11010110011000 10011 10011 10011 10011 10011 000 110000100 Since there is no remainder, it is assumed that there have been no errors. There are different types of generator polynomials, which are used for this purpose. Both the high and low order bits of G (x) must be one. A polynomial should be selected to have following properties: • It should not divisible by x. It guarantees that all burst errors of a length equal to the degree of polynomial are detected. • It should be divisible by x + 1. It guarantees that all burst errors affecting an odd number of bits are detected. Some standard polynomials are following: CRC-8 = x8 + x2 + x + 1 CRC-10 =x10 + x9 + x5 + x 4 + x2 + 1 CRC-12 = x12 + x11+ x3 + x2 + x + 1 CRC-16 = x16 + x15+ x2+1 CRC-CCITT / ITU-16 = x16 + x12 + x5 + 1 ITU-32 = x32 + x26 + x23 + x22 + x16 + x12 + x11 + x10 + x8 + x7 + x5 + x4 + x2 + x + 1 Checksum In sender, the checksum generator divides the data unit into equal segments of n bits (typically 16). These segments are added using ones complement to get the sum in such a way that the total is also n bits long. The sum is then complemented and appended to the end of the original data unit as redundancy bits, called the checksum field. This extended data unit (data unit with checksum) is transmitted across the network. So, if the sum of 82 the data segments is S then the checksum will be –S. • • • • Checksum generator follows these steps: The data unit is divided into equal segments of n bits. All the sections are added using ones complement to get the sum. The sum is complemented which called as checksum. The checksum append with data unit and sent with data. Sender Segment 1 Segment 2 Segment k Checksum n bits n bits n bits All 0s Receiver Segment 1 Segment 2 Segment k Checksum n bits n bits n bits n bits Sum n bits Complement n bits Checksum (b) Checksum Packet Sum n bits Complement n bits Result If the result is 0, accept, otherwise, discard In receiver, the checksum receiver divides the extended data unit into equal segments of n bits. All the segments are added using ones complement to get the sum and sum is then complemented. If the extended data unit is intact, the result should be zero. Otherwise the data unit contains an error and receiver rejects it. Checksum checker follows these steps: • The data unit is divided into equal segments of n bits. • All the sections are added using ones complement to get the sum. • The sum is complemented. • If the result is zero, the data are accepted; otherwise the data unit contains an error so they are rejected. Error Correction 83 Error correction can be handles in several methods. The two most common are error correction by retransmission (which referred as backward error correction) and forward error correction. Forward error corrections are used in some situations where retransmission is impractical. Examples are broadcast situations in which there are multiple receivers for one transmission and space probes, which essentially use simplex transmission. These find more use in applications other than data communication, such as computer memory. Forward Error Correction Single-bit error can be detected by the addition of a redundancy bit (parity bit). The additional bit can detect single-bit errors, because it distinguishes between only two states: error or not error. These two states are sufficient for error detection. In the case of single-bit error correction in a 7-bit ASCII character, the error correction code must determine which of the 7 bits has changed. In this case there are eight different states: no error, error in position 1, error in position 2, and so on, up to error in position 7. To show all eight states (000 to 111), it requires 3-bit redundancy code. Seven bits of ASCII character plus 3 bits of redundancy equals to 10 bits. Three bits can identify only eight possibilities. But what happen if an error occurs in redundancy bits? To calculate the number of redundancy bits r required to correct a given number of data bits m, it is need to find a relationship between r and m. With m data bits and r redundancy bits, the total resulting code is (m + r). If the transmitting unit has (m + r) bits, then r must be able to indicate at least (m + r + 1) different states. One state is no error and other (m + r) states mean error in each of the (m + r) positions. Therefore, r bits must be discover (m + r + 1) states. And r bits can indicates 2r different states. So, 2r must be equal to or greater than (m + r + 1). 2r>= m + r + 1 For example, if the value of m is 7, the smallest r value that can satisfy this equation is 4: 24>= 7 + 4 + 1 Following table shows possible m values and the corresponding r values. Number of Data Bits (m) 1 2-4 5-7 Number of Redundancy Bits (r) 2 3 4 Total Bits (m+r) 3 5-7 9-11 Single-bit Error correction by Hamming code Hamming code can correct single bit error. To correct the single bit error it follows these steps: • The bits of the codeword are numbered consecutively, starting with bit1 at the right end. The bits that are power of 2 (1, 2, 4, 8, 16, etc.) are check bits or redundancy bits(r). The rest bits (3, 5, 6, 7, 9, etc.) are filled up with the m data bits. • For example, a 7 bit ASCII character requires 4 redundancy bits that can placed in positions 1, 2, 4 and 8 (that are powers of 2), these bits are refer as r1, r2, r4, r8. The 84 data bits are found in bit positions 3, 5, 6, 7, 9, 10, 11 and 12. Therefore, the original codeword encoded as 11 bits codeword using a Hamming code. 12 d 11 d 10 d 9 d 8 r8 7 d 6 d 5 D 4 r4 3 d 2 r2 1 r1 Layout of data and redundancy bits (8+4=12): Bit position 1 2 3 4 5 6 7 8 9 10 11 12 Position number 0001 0010 0011 0100 0101 0110 0111 1000 1001 1010 1011 1100 Redundancy bit r1 r2 r4 d2 d3 d4 r8 d5 d6 d7 d8 Data bit d1 In the Hamming code, each redundancy bit is the parity bit; for one combination of data bits as given below: r1: bits 1, 3, 5, 7, 9 and 11 r2: bits 2, 3, 6, 7, 10 and 11 r4: bits 4, 5, 6, 7 and 12 r8: bits 8, 9, 10, 11 and 12 • Each redundancy bit operates on each data bit position whose position number contains a ‘1’ in the corresponding column position. 11 1 11 1 11 1 10 1 10 1 10 1 9 0 9 0 9 0 8 r8 8 r8 8 r8 7 1 7 1 7 1 6 0 6 0 6 0 5 0 5 0 5 0 4 r4 4 r4 4 r4 3 1 3 1 3 1 2 r2 2 r2 2 0 1 r1 1 1 1 1 85 Redundancy bits calculation 11 1 11 1 10 1 10 1 9 0 9 0 8 r8 8 0 7 1 7 1 6 0 6 0 5 0 5 0 4 1 4 1 3 1 3 1 2 0 2 0 1 1 1 1 Calculating redundancy bits: • Placed the data bits in its original position in the 11-bit codeword. Data bit positions are 3, 5, 6, 7, 9, 10 and 11. • Calculate the redundancy bits with even parities for the various bit combination. The parity value for each combination is the value of the corresponding r bit. • The total 11-bit unit sends to the receiver. 11 1 11 1 11 1 11 1 r8 r4 10 0 10 0 10 0 10 0 r2 r1 9 0 9 0 9 0 9 0 8 0 8 0 8 0 8 0 7 1 7 1 7 1 7 1 6 0 6 0 6 0 6 0 5 0 5 0 5 0 5 0 4 1 4 1 4 1 4 1 3 1 3 1 3 1 3 1 2 0 2 0 2 0 2 0 1 1 1 1 1 1 1 1 The bit in position 10 is in error 86 1 0 1 0 10 Error detection using Hamming code Error Detection and Correction: • Suppose, by the time the above transmission is received, the number 10 bit has been changed from 1 to 0. • The receiver gets the transmission and recalculates four redundancy bits, using same sets of combination with even parities. • Assembles the four redundancy bits into a binary number in order of r position (r8, r4, r2, r1). • The binary number is the precise position of the bit in error. In example this binary number is 1010 (10 in decimal). • After the error detection, the receiver can easily invert its value and corrects the error. In example 10th bit invert from 0 to 1 and accept the changed data unit. Note: This correction easily be implemented Burst Errortechnique canby Hamming code in hardware and the code is corrected before the receiver knows about it. Hamming codes can only correct single bit errors; it cannot correct a burst error. But it is possible to correct burst error by applying Hamming code. Error Detection and Correction: • Sequences of k consecutive codewords are arranged as a matrix, one codeword in each row. • The data should be transmitted one column at a time starting with the leftmost column. When the first column has been sent, the second column is sent and so on. • At receiver when the frame arrives, the matrix is reconstructed, one column at a time. • If the burst error of length k or less occurs, at most 1 bit in each of the k codewords will be affected. The Hamming code can correct the corrupted bit in each codeword. So the entire block of data can be automatically corrected. 1 0 0 1 1 1 0 0 1 0 1 In example six data units are sent where each data unit is a 7 bit ASCII character with Hamming code redundant bits. These data units are arranged as a matrix. 87 1 1 0 0 1 0 0 1 1 0 1 1 1 1 1 1 0 0 1 1 0 0 1 1 1 1 1 0 0 0 0 1 1 1 0 1 0 1 0 1 1 1 1 1 0 1 1 1 1 0 0 1 1 1 1 Data before being sent 1 1 1 1 1 0 0 1 1 1 0 1 0 0 1 1 1 1 1 0 1 1 0 1 1 1 1 1 1 1 1 0 0 0 0 0 0 0 0 0 1 0 0 1 1 0 1 1 1 1 1 0 1 1 0 0 0 1 1 1 1 1 0 1 1 1 Data in transition The data unit transmitted one column at a time starting with the leftmost column. When the first column has been sent, the second column is sent and so on. The bits are corrupted by a burst error are shown in red colors. 1 0 0 1 1 1 0 0 1 0 1 1 1 0 0 1 0 0 1 1 0 1 1 1 1 1 1 0 0 1 1 0 0 1 1 1 1 1 0 0 0 0 1 1 1 0 1 0 1 0 1 1 1 1 1 0 1 1 1 1 0 0 1 1 1 1 Received data Five consecutive bits are corrupted during transmission. After receiving, the data unit reorganized as a matrix. If the length of burst error is equal to or less than number of data unit, then at most 1 bit in each of the k data unit will be affected And apply Hamming code to correct the corrupted bits. Therefore the whole block of data can be corrected. 88 CHAPTER 6 LOCAL AREA NETWORK An interconnection of autonomous computers geographically spread within a small area (a few kilometers in diameter) is called a Local Area Network or LAN in short. IEEE 802 family of Standards The 802 family of Standards was developed by the IEEE (Institute of Electrical and Electronics Engineers) to enable equipments of different manufactures to communicate 89 between themselves over LANs and MANs. The IEEE 802 family of standards has several individual standards already developed and accepted and some more standards are under developed. The developed standards within the IEEE 802 Standards Family have also adopted by other standards bodies like ISO and ANSI. The components parts of the IEEE family of standards are shown below. IEEE family Standards Descriptions 802.1 Relationship of 802.X standards to ISO model, higher layer protocols, internetworking, network management and control, etc. 802.2 LLC architecture and protocol 802.3 CSMA/CD bus architecture and access protocol 802.4 Token passing bus architecture and access protocol 802.5 Token passing ring architecture and access protocol 802.6 MAN architectures and access protocol 802.7 Broadcast transmission 802.8 Optical fiber based LANs 802.9 Integration of LANs with PABX technology 802.10 Interoperable LAN security also known as SILS 802.11 Wireless LAN MAC and physical layer specification 802.12 100 Mbps demand priority access method physical layer and repeater, also known as 100VG-AnyLAN. The first six components, which together represent the organization of the IEEE, project 802 as the next three components represent later developments dealing with specialized topics related to LAN technology. The component part 802.2 is common to component parts 802.3 through 802.6, so that 802.2 Standards together with one of the latter four Standards describe a particular type of LAN architecture. 802 Architecture The IEEE 802 committee recommended three-layer communication architecture for LANs where the layers are respectively named Physical, Media Access Control (MAC) and Logical Link Control (LLC). The three layers may be viewed as the functional replacement of the lowest two layers (Physical and Data Link) of the OSI model. The physical layer in the IEEE 802 LAN standards performs similar functions as the physical layer in the OSI model, the functions being encoding/decoding of bits, transmission/reception of electrical signals, synchronization (generation/removal of the preamble signal), etc. The LLC and the MAC layers in 802 together perform the basic function of the OSI data link layer. The LLC layer, the higher of the two layers, provided a service to its higher layer for moving frames between two stations on the LAN. The function of the MAC layer is to allocate the multi-access channel between the randomly accessing stations so that each station can successfully transmit its frame without undue interference from the other stations. 90 Application Presentation Session Transport Network Data Link Physical Network LLC MAC Physical Lower layers in the OSI and the 802 models Ethernet LAN In 1976, Zerox Corporation built a Local Area Network named Ethernet, which connected 100 personal workstations on a 1 km cable and used a 2.94 Mbps data rate. The LAN employed an access protocol called Carrier Sense Multiple Access with Collision Detect (CSMA/CD), which really evolved from ALOHA protocol. Based on Ethernet LAN, Zerox Corporation, DEC and Intel Corporation collaborated to draw up a standard for a 10 Mbps Ethernet, submitted it to the IEEE and this really became the basis for the IEEE 802.3 standard on Ethernet. The access protocol in IEEE 802.3 standard is called 1-persistent CSMA/CD. The Ethernet frame contains eight fields: Preamble: The first field of the 802.3 frame contains 7 octets of alternating 0s and 1s that alert the receiving system to the incoming frame and enable it to synchronize its input timing. The pattern provides only an alert and a timing pulse. The 56-bit pattern allows the stations to miss some bits at the beginning of the frame. The preamble is actually added at the physical layer and is not part of the frame. Start frame delimiter (SFD): This field (10101011) signals the beginning of the frame. The SFD tells the stations that they have a last chance for synchronization. Destination address (DA): This field is 6 octets and contains the physical address of the destination station or stations to receive the packet. The address consisting of all 1 bits is reserved for broadcast. The higher order bit of the destination address is a 0 for ordinary addresses and 1 for group addresses. When a frame is sent to a group address, all the stations in the group receive it. Sending to a group of stations is called multicast. Source address (SA): This field is 6 octets and contains the physical address of the sender of the packet. Length/type: This field is defined as a length or type field. If the value of the field is less than 1518, it is a length field and defines the length of the data that follows. 91 On the other hand, if the value of the field is greater than 1536, it defines the type of the PDU packets that is encapsulated in the frame. Data: This field carries data encapsulated from the upper-layer protocols. It is a minimum of 46 and a maximum of 1500 bytes. Pad: If the data portion of a frame is less than 46 bytes, the pad field is used to fill out the frame to the minimum size. CRC: The last field contains the error detection information, in this case a CRC-32. 7-octet Preamble 1-octet Start delimiter 6-octet 6-octet Destination Source address address Length Data of Data, 0 to 1500 field octet Pad 0 to 46 octet CRC 4-octet The IEEE 802.3 data frame format 1 Persistent CSMA protocol works as follows. When a station has data to send, it first listens to the channel to see if anyone else is transmitting at that moment. If the channel is busy, the station continues sense the channel until it becomes unused. When the station detects an unused channel, it transmits a frame. If a collision occurs, the station waits a random amount of time and starts all over again. The protocol is called 1-persistant because the station transmits with probability of 1 whenever it finds the channel unused. The Ethernet protocol i.e. the 1-persistent CSMA/CD protocol, significantly improves upon the performance of the 1-persistent by adding the “collision detect” feature. In 1persistent CSMA/CD protocol the station continues sense the channel even while transmitting. When a transmitting station detects a collision it stops its transmission immediately, which minimized the channel capacity wastage. Five types of cabling are commonly used in Baseband 802.3 LANs. Name Cable Max. segment Nodes/segment Advantage 10Base5 Thick coax 500 m 100 Good for backbones 10Base2 Thin coax 200 m 30 Cheapest system 10Base-T Twisted pair 100m 1024 Easy maintenance 10Base-F Fiber optics 2000 m 1024 Best between buildings First type of cable is 10Base5; it means that it operates at 100 Mbps; using Baseband signaling and can support segments of up to 500 meters. Connections are generally made using vampire taps. Second cable type is 10Base2. BNC connectors are using to form Tjunction. Token Bus LAN 92 Token Bus LAN was standardized in the IEEE 802.4 specification. Physically, the token bus is a linear or tree-shaped cable onto which the stations are attached. Logically, the stations are organized in the ring. The cable forming the bus in 802.4 is a 75-ohm broadband coaxial cable, which is used for cable television. Station address Logical ring 37 11 13 17 Broadband coaxial cable 34 7 22 Direction of token movement 19 14 This station currently out of logical ring Architecture of a token bus LAN The LAN access by a special control frame called token, which is passed around the logical ring, formed by connecting the highest numbered station, the successive lower numbered station and back to the highest numbered stationed itself. Each station knows the address of its predecessors (upstream neighbour in the logical ring) and the successors (downstream neighbour). After receiving the token, a station sends its data frames and then passes the token to its successor. The token bus LAN uses a logical ring based upon a coaxial cable, which allows bidirectional propagation of electrical signals rather than a physical ring. The position of a station in the logical ring is determined by its address rather than its physical position to the cable. The token must contain the successor address and be passed to the successor. A station is never physically disconnected but is only logically disconnected during time of power-off, faulty operation, etc. this means that the insertion into or the deletion from the logical ring of any station can be carried out simply and no physical switching is necessary. The Token bus frame contains eight fields: Preamble: The first field of the 802.4 frame contains 1 octet of alternating 0s and 1s that alert the receiving system to the incoming frame and enable it to synchronize its input timing. The pattern provides only an alert and a timing pulse. Start frame delimiter (SFD): This field signals the beginning of the frame. It contains analog encoding of symbols other than 0s and 1s. Frame control: The frame control is used to distinguish data frames from control field it can also carry an indicator requiring the destination station to acknowledge correct or incorrect receipt of the frame. Destination address (DA): This field is 6 octets and contains the physical address of the destination station or stations to receive the packet. 93 Source address (SA): This field is 6 octets and contains the physical address of the sender of the packet. Data: The data field has variable length of data. It may be up to 8182 bytes long when 2 octet addresses are used and up to 8174 bytes long when 6 octet addresses are used. Checksum: The checksum is used to detect transmission errors. End frame delimiter (EFD): This field are used to marks the frame boundary. This field signals the ending of the frame. 1-octet Preamble 1-octet Start delimiter 1-octet Frame control 6-octet Destination address 6-octet Source address Variable 4-octet length of Checksum Data, up to 8182 octet 1-octet End delimiter The IEEE 802.4 data frame. Token Ring Token ring is a network system that uses a ring topology and a token channel access method. Token-passing LAN developed and supported by IBM. Token ring was standardized in the IEEE 802.5 specification. Token ring runs at 4 or 16 Mbps over a ring topology. Station Unidirectiona l ring Ring interface Architecture of a token ring In a token ring a small frame with special format, called token rotate around the ring in one consistent direction whenever all stations are idle (no data are being sent). If a station needs to send data, it waits for the token. The station receives the token from its nearest active upstream neighbour (NAUN) and sends one or more frames (as long as if has frames to send or the allocated time has not expired). And finally it released the token to its nearest active downstream neighbour (NADN). Each station is given an equal chance 94 to have the token and take control in order to pass data. This is called media access. 1-octet Start 1-octet Access 1-octet End delimiter control field delimiter (a) 1-octet Start delimiter 1-octet Access control 1-octet 6-octet 6-octet Variable 4-octet Frame Destination Source length Checksum control address address of Data, up to 4099 octet 1-octet 1-octet Ending Frame delimiter status (b) The IEEE 802.5 (a) token frame, (b) data frame. The token frame contains three fields: Start delimiter: This field signals the beginning of the frame. Access control field: Access control field of the token frame contains four fields: Priority field (3-bit), Token field (1-bit), Monitor field (1-bit) and Request Priority field (3-bit). End delimiter: This field signals the ending of the frame. The Token ring frame contains nine fields: Start frame delimiter (SFD): This field signals the beginning of the frame. Access control: The access control field contains the token bit and also the Monitor bits and Reservation bits. Frame control: The frame control is used to distinguish data frames from various possible control frames. This field indicates whether the variable length user data (FF=00) or a management message in the form of a MAC vector (FF=01) Destination address (DA): This field is 6 octets and contains the physical address of the destination station or stations to receive the packet. Source address (SA): This field is 6 octets and contains the physical address of the sender of the packet. Data: The data field has variable length of data. It may be as long as possible, provided that the frame can still be transmitted within the token-holding time. Checksum: The checksum is used to detect transmission errors. End frame delimiter (EFD): This field are used to marks the frame boundary. This field signals the ending of the frame. Frame status: The frame status field which is the acknowledgement field. It contains the A and C bits. Both the Address Recognized (A) and the Frame Copied (C) bits are reset to 0 at the time of transmission and are set to 1 by the receiver if it is able to recognize the address and copy the frame respectively. The frame retransmits only if A=1 and C=0. Three combinations are possible: 1. A=0 and C=0; destination not present or not powered up. 2. A=1 and C=0; destination present but frame not accepted. 95 3. A=1 and C=1; destination present and frame copied. Token Ring has following features: • Unlike Ethernet, Token Ring continues to operate reliably under heavy loads. • Build-in diagnostic, repair program and recovery mechanisms, such as beaconing and auto-reconfiguration, make the protocol more reliable. • Token Ring makes connecting a LAN to an IBM Mainframe easier. • Fault-tolerance features are provided through ring reconfiguration, called ringwrap. • Token Ring can be very difficult to troubleshooting and requires considerable proficiency. • Token Ring cards and equipment are very costly than Ethernet or ARCnet systems. FDDI FDDI (Fiber Distributed Data Interface) is a high-speed (100 Mbps) fiber optics token ring LAN over distances up to 200 km with up to 1000 stations connected. Today an FDDI network is also used as a MAN. FDDI uses fiber optics cables to implement very fast, reliable networks. FDDI uses multimode fiber, which is less expensive than single mode fiber. It also used LEDs as a light source rather than lasers. The FDDI uses dual ring topology. The cabling consists of two fiber rings, one transmitting clockwise and other transmitting counterclockwise. If either one break, the other can be used as a backup. If both break at the same point, the two rings can be joined into a single ring. Each station contains relays that can be used to join the two rings or bypass the station in the incident of station problem. There are two classes of stations, A and B, used in FDDI. Class A stations can be connected to both types of rings. The class B stations can be connected to only one type of ring and are cheaper. During installation, either class A or class B is selected depending upon the importance of fault tolerance. Class B node 4 Writing Concent rator Class B node 3 Primary ring Class A node 1 Secondary ring Class A node 2 FDDI class A and class B node connections 96 The basic FDDI protocols are closely modeled by the 802.5 protocols. If a station wants to transmit data, it must first capture the token. After getting the token, the station transmits a frame and removes it when it comes around again. There is one difference between FDDI and 802.5. In the case of 802.5, a station may not generate a new token until its frame has gone all the way around and comes back. In FDDI having 1000 stations and 200 km of fiber, the wastage of time while waiting for the frame is substantial. For this reason, a decision has been taken to allow a station to put a new token back on the ring as soon as it has finished transmitting its frames. In this situation, several frames must be present in the ring at the same time. (a) (b) The X.25 protocol mapped to the OSI model The FDDI frame contains nine fields: Preamble: The first field of the 802.5 frame contains 1 octet of alternating 0s and 1s that alert the receiving system to the incoming frame and enable it to synchronize its input timing. The pattern provides only an alert and a timing pulse. Start frame delimiter (SFD): This field signals the beginning of the frame. It contains analog encoding of symbols other than 0s and 1s. Frame control: The frame control field tells what kind of frame this is (data, control, etc.) Destination address (DA): This field is 6 octets and contains the physical address of the destination station or stations to receive the packet. Source address (SA): This field is 6 octets and contains the physical address of the sender of the packet. Data: The data field has variable length of data. It may be up to 8182 bytes long when 2 octet addresses are used and up to 8174 bytes long when 6 octet addresses are used. Checksum: The checksum is used to detect transmission errors. End frame delimiter (EFD): This field are used to marks the frame boundary. This field signals the ending of the frame. Frame status: The frame status byte holds acknowledgement bits, similar to those of 802.5. 97 8-octet Preamble 1-octet Start delimiter 1-octet Frame control 6-octet Destination address 6-octet Source address Variable 4-octet length of Checksum Data, up to 4478 octet 1-octet Ending delimiter 1-octet Frame status Wireless LAN Bluetooth Bluetooth is a wireless LAN technology designed to connect devices of different functions such as telephones, notebooks, personal computers, cameras, printers, and so on. Bluetooth is a network, which formed spontaneously. The devices on the network sometimes called gadgets, which connects each other and form a network called piconet Bluetooth defines two types of networks: piconet and scatternet. Piconet A Bluetooth network is called a piconet. A piconet can have up to eight stations, one of which is called the master. The rest stations are called slaves. All the slaves are synchronized their clocks and hopping sequence with the master. A piconet can have only one master station and maximum of seven slaves. But an additional eight slaves can be in the parked state. A slave station in the parked state is synchronized with the muster but cannot take part in communication until it is moved from the parked sate. As only eight stations can active in a piconet, this is why when an active station goes to the parked state then a station can be active from the parked state. Scatternet Piconets can be combined to form a scatternet. A slave station in one piconet can become the master in another piconet. This station can receive data from the master in the first piconet (as a slave) and deliver the data to the slaves in the second piconet (as a master). A station can be member of two piconets. 98 CHAPTER 7 INTERNET WORKING Electronic mail (Email) Email (Electronic mail) is most popular network services. Email is used for ending a single message that includes text, voice, video or graphics to one or more receivers. SMTP (Simple Mail Transfer Protocol) has to design to handle electronic mail in Internet. telnet The telnet protocol is used to establish an on-line connection to a remote system. It is used the same way as the telnet program. The telnet utility used to connect remote UNIX system from Windows system. Before the use of telnet there must have an account on the remote system. If you enter telnet command with the IP address you have to enter user name and password to gain the access to the remote system. As long as you are logged in, anything you type host terminal is sent to the remote system. Any files that you access or any commands that you run will always be on remote system. At the time of log out you can press <Ctrl+d>or type exit to log out and return to the local host. FTP (File Transfer Protocol) TCP/IP has a special command ftp to transfer files between two computers that is widely used in internet. The command can be used to transfer both binary and text files. The ftp command works in two steps. First, it makes a connection with the remote 99 system. This can be done in two ways: either by ftp with hostname (comp) or another ftp and later open command with hostname. After the connection has been established, it asks for the username and the password. Termination of ftp is done in two steps. At first disconnect the remote system by close command and then quit ftp by typing bye or quit. World Wide Web (WWW) The World Wide Web is an architectural framework for accessing linked documents spread out over thousands of computers all over the internet. The WWW has a unique combination of flexibility, portability and user-friendly features. The WWW project was initiated in 1989 by CERN, the European center for nuclear research. The WWW Internet Telephony Short Messaging Services (SMS) Internet Fax Video Conferencing: VoIP Voice over IP (VoIP) is a real-time interactive audio/video application. The idea is to use the Internet as telephone with video and some other capabilities. Instead of communicating over circuit-switching network, this application provides communication between two parties over the packet-switched Internet. Two protocols have to design to handle this type of communication: SIP and H.323. HTML (HyperText Markup Language) We pages are written in a language called HTML (HyperText Markup Language). HTML allows users to produce Web pages that include text, graphics and linkers/pointers to other Web pages. A proper Web page consists of a head and a body enclosed by <HTML> and </HTML> tags (formatting commands). Head The head is the first page of a Web page. The head contains the title of the page and other parameters that the browser will use. The head is bracketed by the <HEAD> and </HEAD> tags. Body The actual contents of a page are in the body, which includes the text and the tags. Whereas the text is the actual information contained in a page and the tag define the appearance of the document. The body is bracketed by the <BODY> and </BODY> tags. A tag is enclosed in two signs (< and >) and usually comes in pairs. The beginning tag starts with the name of the tag and the ending tag starts with a slash followed by the name 100 of the tag. The commands inside the tags are called directives. Tags can be in either lowercase or uppercase. That is <HEAD> and <head> means the same thing. A tag can have a list of attributes, each of which can be followed by an equals sign and a value associated with the attribute. The format of beginning tag: < TagName Attribute = Value Attribute = Value Attribute = Value ……> The format of ending tag: < /TagName > URL (Uniform Resource Locators) DHTML XML ASP Network programming concepts with Java/PHP Concepts of Web Site Design and Hosting 101 CHAPTER 8 NETWORK SECURITY Security provides four services: privacy, authentication, integrity and non-repudiation. Privacy Privacy has to do keeping information out of the hands of unauthorized persons. That means the sender and the receiver expects confidentiality. Authentication Authentication means that the sender needs to be sure of the sender’s identity and that an imposter has not sent the message. Integrity Non-repudiation Encryption/Decryption Encryption is a process which converting stored or transmitted data to a coded form in order to prevent it from being read by unauthorized persons. It is also application of a specific algorithm to alter the appearance of data, making it incomprehensible to those who are not authorized to see the information. Decryption is the reverse application of an encryption algorithm to encrypted data, in 102 that way restoring the data to its original, unencrypted state. Digital Signature An electronic message can be authenticated by a digital signature. Digital signatures are another way to assume the recipient of an electronic message that the message is coming from the right party. Also string of bits appended to a message that provides authentication and data integrity. APPENDIX -I Hubs Hub is a central controller device in a star topology that provides a common connection among the devices. Each device has a dedicated point-to-point link only to a central controller. If one device wants to send data to another, it sends the data to the hub, which then relays the data to the other connected device. A hub is actually a multiport repeater and as such it obeys the same rules as repeater. They operate at the OSI model Physical Layer. Hubs can also be used to create multiple levels of hierarchy (tree topology). Connecting Hubs together through ports creates Cascading Hubs. One master hub (level 1) is connected to many slave hubs (level 2). The slave hubs are masters to slave hubs (level 3) in a hierarchy tree. The maximum number of stations in a Cascaded Hub Network is limited to 128. • Hubs can use as a central unit from which to connect multiple nodes into one network. • It can permit large numbers of computers to be connected on single or multiple LANs. • It can reduce network congestion by centralizing network design. • Enable high speed data communication. • Hubs provide connections for several different media types (e.g. twister pair, coaxial, fiber optics). • Consolidate the network backbone. • Provide multi-protocol services, such as Ethernet-to-FDDI connectivity. • Enable centralized network management. Switches Switch is a device connecting multiple communication lines together. Switches have all but replaced bridges except for small application. Switches constantly monitor the traffic that comes across them and reroute their internal connections atomically to provide the most efficient operation for the network. 103 The switch normally has a buffer for each link to which it is connected. When it receives a packet, it stores the packet in the buffer of the receiving link and checks the address to find the outgoing link. If the outgoing link is idle, the switch sends the frame to the particular link. Switches are made based on two different strategies: store-and-forward. A store-andforward switch stores the frame in the input buffer until the whole packet has arrived. Gateway Gateway is a device that is used to interface two different incompatible network facilities. Gateways perform protocol conversion for all seven layers of the OSI model. A common use for a gateway is to connect the Internet to the telephone networks or to connect a LAN and a larger system, such as a mainframe computer or a large packet-switching network, whose communications protocols are different. A gateway reformats the data so that it will be acceptable to the system it is passing into by changing protocols and transmitting packets between two entirely different systems. Gateways handle messages, addresses and protocol conversions necessary to deliver a message from one network to different network. Gateways offer greatest flexibility in internetworking communications. A gateway is a combination of hardware and software with its own processor and memory used to perform protocol conversions. It is usually slower then a bridge or router because they need to perform such intensive conversion and that they can be expensive. And more complex design, implementation, maintenance and operation of a gateway. • The gateway determines where the packet is going and also converts the message from one packet to another or from one data code system to another. • A gateway is slower than router. Routers Routers are devices that connect two or more logically separate networks. They consist of a combination of hardware and software. The hardware may be a network server, a separate computer and the physical interfaces to the various networks in the internetworking. . The two most important parts of software in router are the operating system and the routing algorithms. Routers use logical and physical addressing to interconnect different physically and logically separate network segments. They organized a large network into logical network segments. Each sub-network is given a logical address, which allows the networks to be separate but can communicate each other and transmit data when necessary. Data is grouped into packets, each packet having a physical device address, has a logical network address. The router examines the data contained in every packet it receives for detailed information. Based on information, the router decides whether to block of packet from the rest of the network or transmit it. Router also attempts to send the packet by the most 104 efficient path through the network. Routers do this by using various routing protocols. It also uses one or more metrics to determine the optimal path along which network traffic should be forwarded. Routers operate at the network layer of the OSI model. Routers slow down network communications, so do not use them unnecessarily. • Routers perform a function very similar to that a bridge, but routers keep the networks separate. • Router processing slower than bridge processing, because they have to check both the device address and network address. • Routers are more intelligent than bridges because they use algorithms to determine the best path to send a packet to a network. • Routers work at the network layer of the OSI model. Whereas bridges operate in both the physical and the data link layers. • Routers do not alter the form of the packet as gateways. They retransmit the packets in its original form by store-and-forward services. • A router is slower than bridge but faster than gateway. Bridges Bridge is a device that supports LAN-to-LAN communications. Bridges handles traffic between two similar or different LANs. This device bridges two different network segments regardless of their topology or wiring. It memorized all the network addresses on the both sides of the segments and manage the flow of traffic between the LANs by reading the address of the every packet of data that it receives. The address is contained in the header of each network packet being transmitted. A bridge operates in both the physical and the data link layers. As a physical layer device, the bridge regenerates the signal it receives. As the data link layer device, it can check the physical (MAC) addresses (source and destination) containing in the frame. A bridge has filtering and forwarding capabilities. It can check the destination address of a frame and decide if the frame should be forwarded or dropped. If the frame is to be forwarded, the decision must specify the port. A bridge has a table that maps addressed to ports. 105 Bridge Segment A Segment B The bridge connecting two different types of networks A bridge operates in the following manner: 2. A bridge receives all signals from both segment A and B. 3. The bridge reads the addresses and filters all signals from A that are addresses to segment A, because they need not to cross the bridge. 4. Signals from segment A addressed to a computer on segment B are retransmitted to segment B. 5. The signals from segment B are treated in the same way. Difference between Router and Bridge: • Routers perform a function very similar to that a bridge, bur routers keep the networks separate. • Router processing slower than bridge processing, because they have to check both the device address and network address. • Routers are more intelligent than bridges because they use algorithms to determine the best path to send a packet to a network. • Routers work at the network layer of the OSI model. Whereas bridges operate in both the physical and the data link layers. Transceivers The transceiver is a transmitter and a receiver. It transmits signals over the medium; it receives signals over the medium; it also detects collisions. A transceiver can be external or internal. An external transceiver is installed close to the media and is connected via an AUI to the station. An internal transceiver is installed inside the station (on the interface card) and does not need an AUI cable. The transceiver or medium attachment unit (MAU) is medium-dependent. It creates the appropriate signal for each particular medium. There is a MAU for each type of medium 106 used in 10-Mbps Ethernet. The coaxial cable need its own type of MAU, the twisted-pair medium needs a twisted-pair MAU and fiber-optic cable need a fiber-optic MAU. Repeaters A repeater is a device that extends the distance a signal can travel by regenerating the signal. It operates only in the Physical Layer. Signals carry information within a network can travel a fixed distance before attenuation. A repeater receives a signal before it becomes too weak or corrupted, regenerates the original bit patterns. The repeater then transmits the regenerated signal. A repeater can extend the physical length of the network. The portions of the network separated by the repeater are called segments. Repeater cannot connect two LANs; it connects two segments of the same LAN. Repeater acts as a two-port node; when it receives a frame from any of the ports, it regenerates and forwards it to the other port. A repeater forwards every frame; it has no filtering capability. A repeater does not amplify the signal; it regenerates the signal. When it receives a weakened or corrupted signal, it creates a copy, bit for bit, at the original strength. A repeater can retransmit signals in both directions. X.25 Protocol X.25 is an ITU-T (International Telecommunications Union) standard that defines how a connection between a DTE and DCE is maintained for remote terminal access and computer communications in packet-switching networks. X.25 allows a variety of devices that are designated as data terminal equipment (DTE) to talk to the public data network (PDN). The PDN is designated as data communications equipment (DCE), as are devices as modems, packet switches and other ports. Hardware/software devices, such as terminals, hosts and routers that deliver data to or from a network I/O port are DTE. The X.25 protocol is a DTE-to-DCE synchronous interface. To begin communication, one DTE device calls another DTE to request a data exchange session. The DTE called can accept or refuse the connection. If the called DTE accepts the connection, the two systems begin full-duplex data transfer. Either side can terminate the connection at any time. X.25 is connection-oriented and supports both switched virtual circuits and permanent virtual circuit. A switched virtual circuit is created when a DTE sends a packet to the network requesting to make a call to a remote DTE. A permanent virtual circuit is used the same way but it is set up in advanced by agreement between the customer and carrier. It is always present and no call setup is required to use it. X.25 protocol is commonly used in wide area communications with multiple communicating devices. It is much simpler for interfacing equipment or networks. X.25 network is typically has very low 64 kbps data rate. This is rarely sufficient to support modern networking. X.25 is usually far less expensive than –Frame Relay and ATM. X.25 functions at the network layer. It normally interfaces with the protocol called LAPB (Link Access Procedures Balanced) at the data link layer, which in turn runs over X.21 or another physical layer CCITT protocol, such as X.21bis or V.32. 107 X.25 is a packet-switching protocol that defines the interface between a synchronous packet-switching host computer and analog dedicated circuits or dial-up switched circuits in the voice-grade public data network. It dominant features are: • Virtual circuit switching and dynamic virtual routing to transport self contained, self-addressed message packets. • Ability to use any available network channels or links. • Ability to use redundancy error checking at every node. Application Presentation Session Transport Network Data Link Physical (a) X.25 LAPB X.21 and others The X.25 protocol defines several levels of interface. At the physical level there is the electrical connection between DCE and DTE. This level uses the X.21 standard for fullduplex synchronous transmission. X.21 specifies the physical, electrical and procedural interface between the host and the network. The second level of X.25 defines the link access procedure. The link access procedure manages the link between the DTE and the DCE. It designed to deal with transmission errors on the telephone line between DTE and the DCE. The third level of the X.25 describes a packet-level procedure, which controls virtual call through a public data network. This permits the establishment of end-to-end virtual circuits analogous to vice telephone call. And it deals with addressing, flow control, delivery confirmation, interrupt etc. Finally, X.25 illuminates the function of the packet assembler/ disassembler (PAD). PAD or black box is installed to which terminals can connect. The PAD provides a connection to the network and translates the data to and from the terminal into packets that the X.25 network’s DCE can accept. 108 MODEM Modem is a device consisting of a modulator and a demodulator. It converts a digital signal into an analog signal and also converts an analog signal into a digital signal. For analog channel, mapping is required at the transmitter to convert digital signals into a suitable waveform by modulation and back-mapping is required at the receiver to reconvert the received waveform into digital data by demodulation. The respective modules are known as modulation and demodulation and it perform by MOdulator and DEModulator respectively which collectively called MODEM. Modem is the device responsible for allowing a digital signal to be carried over an analog channel. The communication can be bidirectional. It performs modulation at the sending end and demodulation at the receiving end. Modem used to connect the (digital) computer with the (analog) telephone line. Modem speed range from 300 bps to 56 kbps. Today, many of the most popular modems available are based on the V-series standards published by the ITU-T. ITU Recommendations V.21 V.22 V.22 bis (bis means second) V.27 terbo (terbo means third) V.29 V.32 V.32 bis V.34 Bit rate (bps) 300 1200 2400 4800 9600 9600 14400 28800 Modulation FSK PSK ASK/PSK PSK ASK/PSK ASK/PSK ASK/PSK ASK/PSK Analog modulation can occur in three ways: 1. Amplitude Modulation (AM). 2. Frequency Modulation (FM). 3. Phase Modulation (PM). Amplitude modulation Amplitude modulation varies the strength of the signal to determine whether a zero or a one bit is being transmitted. A signal with low amplitude represents a zero and signal that has high amplitude represents a one. Frequency modulation Frequency modulation uses a change in the number of times per second the sine wave repeats to indicate a zero or a one. The particular frequency of a sine wave being used to represent the zero and a frequency that is twice the original to represent the one. Phase modulation Phase modulation uses a change in the phase of the wave to indicate a zero or a one. The phase of a sine wave being used to create the zero and a phase shift being used to indicate the one. 109 Routing Algorithms The main function of the network layer is routing packets from the source host to destination host. When the source and destination are not on the same network, the routing algorithms decide which output line an incoming packet should be transmitted on. Routing algorithms can be grouped into two major classes: nonadaptive and adaptive. Nonadaptive algorithms do not base their routing decisions on measurements or current traffic and topology. When the network is booted the choice of route for each destination is computed in advance and created a static routing table. This table is not update automatically when there is a change in the network. The table must be manually altered by the administrator. This procedure is sometimes called static routing. Static routing is a packet switching technique, which is more likely to malfunction, and congestion but reduces s a network’s overhead by relieving the node of many computational responsibilities. Two types of static routing are available. These are fixed routing and flooding. Adaptive algorithms change their routing decisions on current traffic and changes in the network topology. When the network is booted the choice of route for each destination is stored in dynamic routing table. The dynamic routing table is updated periodically whenever there is a change in the network topology or in the current traffic. Congestion Congestion in a network may occur if the load on the network (the number of packets sent to the network) is greater than the capacity of the network (the number of packets a network can handle). Congestion is an important issue in packet-switching network. When too many packets are present in the subnet, the performance degrades. This situation is called congestion. 110 Packets delivered Maximum carrying capacity of subnet Congested Packets sent When too many packets are sent, the performance degrades When the number of packets transmitted into the subnet by the hosts is within capacity of the network, they are all delivered. The number of packets are delivered is proportional to the number of packets are sent. When the number of packets increases too far then the network no longer able to manage and they begin losing packets. At very high traffic performance collapses completely and almost no packets are delivered. Congestion control refers to the techniques to the control the congestion and keeps the load below the capacity. Congestion control involves two factors that measure the performance of a network: delay and throughput. Congestion control mechanisms can divide into two categories: open-loop congestion control (prevention) and closed-loop congestion control (removal). Open-Loop Congestion Control Open-loop congestion controls are applied to prevent congestion before it occurs. This type of congestion control is handled by either the source or the destination. Open-loop congestion controls are follows: Retransmission Strategy A good retransmission strategy can prevent congestion. This strategy must be designed to optimize efficient and at the same time prevent congestion. Windows Strategy 111 The size of window at the sender may also affect congestion. The selective repeat window is better than Go-Back-N window for congestion window. Acknowledgement Strategy The acknowledgement strategy imposed by the receiver may also affect congestion. If the receiver does not acknowledge every packet it receives, it may slow down the sender and help prevent congestion. Discarding Strategy A good discarding strategy by the router can prevent congestion and at the same time may not harm the integrity of the transmission. If the routers discard less sensitive packets when congestion is likely to happen, the integrity of transmission is still preserved and congestion is prevented. Closed-Loop Congestion Control Closed-loop congestion controls are applied to alleviate congestion after it occurred. Several type of congestion controls have been used by different protocols. Open-loop congestion controls are follows: Choke Point A choke point is a packet sent by a router to the source to inform it of congestion. Back Pressure When a router is congested, it can inform the upstream router to reduce the rate of outgoing packets. The action can be recursive all the way to the router before the source. This process is called back pressure. Implicit Signaling The source can detect an implicit signal concerning congestion and slow down its sending rate. For example, the delay in receiving an acknowledgement can be a signal that the network is congested. Explicit Signaling The routers that experience can send an explicit signal to inform the sender or the receiver of congestion. Explicit signaling can occur in either forward or the backward direction. Backward signaling set in a packet moving in the direction opposite to the congestion. This signaling can warn source that there is congestion and that it needs to slow down to avoid the discarding packets. Forward signaling set in a packet moving in the direction of the congestion. This signaling can warn the destination that there is congestion and that it needs to slow down the acknowledgements to improve the congestion. 112 APPENDIX -II An important concept in communications field is the use of decibel (dB) unit as the basis for a number of measurements. It is the primary measurement unit in telephony to compare signal powers or voltage ratios. To define, dB let us consider the circuit element shown in figure. This could be an amplifier or an attenuator or a filter or a transmission line. The input voltage V 1 delivers a power P1 to the element and the output power and the output voltage are P 2 and V2 respectively. The power gain G in dB unit for this element is defined as the ratio of two power levels, P1 and P2: G = 10 log10 (P2/P1) dB, where (P2/P1) is referred to as the absolute power gain. When P2>P1 the gain is positive that means the signal passes through the circuit element is amplified. Whereas if P 2<P1, the gain is negative that means the signal is attenuated (power loss in the circuit element). V1 P1 G P2 V2 A circuit element of gain G If the input and output impedances are constant R, decibels can also be used to represent the ratio of two voltages: G = 10 log10 ((V2/R) / (V1/R)) dB = 20 log10 (V2/V1) dB The decibel unit is not absolute unit; rather it is a ratio (dimensionless quantity). The use of decibels is particularly important when computing the overall gain of a cascaded series of amplification and attenuation elements. Thus using the formula log (AB) = log A + log B If G1, G2… Gn are the gain expressed in dB of cascaded circuit elements, then the overall gain of those elements is Gtotal = G1 + G2 +… + Gn = ∑Gi i =1 n Example 113 A signal travels a long distance from point 1 to point 4. The signal is attenuated -3 dB by the time it reached point 2. Between points 2 and 3, the signal is amplified 7 dB. Again, between points 3 and 4, the signal is attenuated -3dB. We can calculate decibel for the signal just by adding the decibel measurements between each set of points. In the case, the dB can be calculated as dB = -3 +7 -3 = +1 This means that the signal has gained power. TCP/IP (Transmission Control Protocol / Internet Protocol) Different features of TCP/IP • • • • • TCP/IP is independent of the network hardware. TCP/IP can recover failure; it is able to divert data immediately through other routers if one or more parts of network failed. TCP/IP provides the facility to connect new subnetworks without significant interference of services. TCP/IP is reliable to handling high error rate with facilities for full error control. TCP/IP is also reliable of transmission of files, remote login, and remote execution of commands. Every host in a network has two addresses: a hardwired MAC address and a logical IP address. TCP/IP uses both these addresses. MAC Address Every Ethernet network card has a 48-bit physical address hard-coded into the board by the hardware manufacturer. This address is unique all over the world and it is known as the MAC (Media Access Control) address or Ethernet address. The MAC address consists of a set of six colon-delimited hexadecimal numbers. The IEEE and other standard organizations have ensured the uniqueness of this address all over the world. This address is used by one of the layers of the TCP/IP protocol stack. The MAC address can be known by administrator’s ifconfig (Interface Configuration) command. A typical line from the command output could read like as: Eth0 Link encap: 10Mbps Ethernet HWaddr 00:00:E8:2E:47:0c The ifconfig command (in UNIX) sets the IP address, the subnet mask and the broadcast address for each interface. Its most basic function is assigning the IP address. IP Address 114 An IP address is the logical software address, which is also known as the Internet address because the numbering scheme of IP address also followed on the Internet. The IP address is a sequence of four dot-delimited decimal numbers. A typical address looks like as: 192.0.0.101 To use the IP configuration (WINIPCFG) utility (for Windows 98/ME) 1. Click Start, and then click Run. 2. In the Open box, type: winipcfg 3. To see address information for your network adapter(s), select an adapter from the list in Ethernet Adapter Information. Notes: The IP Configuration utility allows users or administrators to see the current IP address and other useful information about your network configuration. TCP/IP has only four layers and these four layers are: 1. Application Layer, representing the application 2. Transport Layer, which controls the reliability of transmission 3. Internet Layer, which takes care of addressing of the data packets 4. Network access Layer, which makes sure that IP addresses are finally converted to MAC address. Application Layer Transport Layer Internet Layer Network access Layer Protocol Stack TELNET FTP SMTP HTTP DNS SNMP NFS TCP UDP SCTP IP ARP RARP ICMP IGMP Layer Application Transport Internet Network access FTP: File Transfer Protocol IP: Internetworking Protocol TCP: Transmission Control Protocol 115 UDP: User Datagram Protocol ARP: Address Resolution Protocol RARP: Reverse Address Resolution Protocol ICMP: Internet Control Message Protocol IGMP: Internet Group Message Protocol SMTP: Simple Mail Transfer Protocol SCTP: Stream Control Transmission Protocol The Application Layer On top of the TCP/IP protocol architecture is the application layer. Data generates from the application layer and it transferred to the transport layer in the form of stream. The application layer does not add any header to this stream. This layer contains all higherlevel protocols like FTP, TELNET, SMTP, DNS and so onward. The virtual terminal protocol (TELNET) allows a user on one machine to log into a remote system and work there. The file transfer protocol (FTP) provides a technique to transfer data from one machine to another machine. SMTP is a specialized protocol that was developed for Electronic mail. Domain Name Service (DNS) is use for mapping host names onto their network addresses. HTTP is use for delivering web pages over the network. Some protocols (such as FTP, TELNET etc.) can be used only if the user has some knowledge of the network. Other protocols (like OSPF-Open Shortest Path First) run without the user even knowing that they exist. The Transport Layer The top of the internet layer is the Host-to-Host Transport Layer, usually called as transport layer. The transport layer defines two end-to-end protocols-TCP (Transfer Control Protocol) and UDP (User Datagram Protocol). The first one TCP is a reliable connection-oriented protocol that allows a byte stream generating on one machine to be transferred without error on any other machine in the network. At the source it divides the incoming stream from application layer into segments, encapsulating each a header and passes each one onto the internet layer. The header mainly contains a checksum and a sequence number to facilitate reassembly in the right order at the other end. At the destination it reassembles the received message into the output stream. TCP provides error detection and recovery facilities. TCP also handles flow control to make sure a fast sender cannot swamp a slow receiver with more messages than it can handle. TCP is used by most applications like ftp, telnet, rlogin, etc. TCP is a connection-oriented protocol. That means it exchanges control information with the remote system to verify that it is ready to receive data before any data is sent. When the handshake is successful, the systems are said to have established a connection and TCP then proceeds with data transfer. TCP provides reliability with a mechanism called Positive Acknowledgment with Retransmission (PAR). When the data segment is received errorless, the receiver sends a positive acknowledgement back to the sender. If the data segment is erroneous, the 116 receiver discards it. The sending TCP module re-transmits any segment for which no positive acknowledgement has been received within the timeout period. TCP is quite efficient to re-transmission simply the segment which caused the problem, rather than the entire data. These features of acknowledgements, timeout and retransmission make TCP reliable protocol. These facilities are not available with UDP. The second protocol in this layer is UDP, which provides low-overhead, connectionless datagram delivery service. UDP is an unreliable connectionless protocol. The term “unreliable” means that there are no techniques in this protocol for verifying that data reached other end of the network correctly. UDP widely used for client-server requestreply queries and such applications in which prompt delivery is more important than accurate delivery. UDP is used some applications like DNS, NFS, etc. The Internet Layer The internet layer also known as network layer. The network layer is control to transfer of IP packets to their proper destination. The internet layer defines two protocols-IP (Internet Protocol) and ICMP (Internet Control Message Protocol). The Internet has several control protocols used in the network layer, including ARP (Address Resolution Protocol), RARP (Reverse Address Resolution Protocol) and BOOTP (Bootstrap Protocol). IP is an unreliable connectionless protocol that receives a segment from transport layer; it first compares it with the maximum size (MTU, the Maximum Transfer Unit) that the next layer can handle. Although IP itself can handle a datagram consisting of 65,535 bytes, the network access layer datagrams hardly ever exceed 1500 bytes. After knowing the MTU of the data, IP may have to segments into packets or datagrams. The IP datagram header includes both the source and destination address and sequence number of the fragmented segments. At the receiving end IP checks the header information and when the header fails the integrity test it use the ICMP protocol to relay the error message to the sender. IP also has a role to play in routing the datagrams to their proper destination. IP is a connectionless protocol. That means it does not exchange control information (called a handshake) to establish an end-to-end connection before transmitting data. IP is called unreliable protocol because it contains no error detection and recovery code. It does not check whether the data was correctly received to the connected network or not. ICMP protocol is use to relay an error message to the sender and echoing requests. ICMP is also used by the ping command, which echoes a request to a host to test the connectivity of the network. The Network Access Layer The network access layer is the lower layer in the TCP/IP protocol hierarchy. The network layer is also known as link layer. The layer comprises the network interface card, 117 the protocols and the details of the physical media. It accepts datagrams from internet layer and encapsulates them after converting all IP addresses to MAC addresses. It finally sends outs a frame to the wire. ARP (Address Resolution Protocol) is the main protocol used by the layer. In IP header contains source and destination address. These are logical 32-bit addresses consisting of a sequence of four dot-delimited decimal numbers. But the network access layer can understand only the MAC address, the 48 bits physical address consisting of a sequence of six colon-delimited hexadecimal numbers. The network access layer has a translation facility, which converts all IP addresses to MAC addresses and vice versa. ARP handles this translation. ARP maintains a temporarily storage memory that contains latest MAC addresses; these are use for transmission without making a broadcasting with host each time. Internet Protocol The functions of Internet Protocol: • Defining the datagram, this is the basic unit of transmission in the Internet. • Defining the Internet addressing scheme. • Moving data between the Network Access Layer and the Transport Layer. • Routing datagrams to remote hosts. • Performing fragmentation and re-assembly of datagrams. 32 bits Source Port Sequence number Acknowledgement number TCP header length Reserved U A P R S F R C S S Y I G K H T N N Checksum Options (0 or 32 bit words) Data (optional) Window size Urgent pointer Destination Port 118 User Datagram Protocol (UDP) 32 bits Source port UDP length Destination port UDP checksum IP Addressing Every host and router on the Internet has an Internet address or IP address. An IP address is a 32 bit dot-delimited decimal numbers that uniquely and universally defines the connection of a host or a router in the Internet: no two machines have the same IP address. 119 1st octet Class A 0 Class B 10 Class C 110 Class D 1110 Class E 11110 2nd octet 3rd octet 4th octet Range of Host addresses 1.0.0.0 to 127.255.255.255 128.0.0.0 to 191.255.255.255 192.0.0.0 to 223.255.255.255 224.0.0.0 to 239.255.255.255 240.0.0.0 to 247.255.255.255 Classes with IP addressing There are five classes of IP addresses: Class A, B, C, D and E. Classes A, B and C differ in the number of hosts allowed per network. Class D is for multicasting and class E is reserved. IP addresses are designed with two levels of hierarchy. Every IP address consists of two portions- a network address/number (also known as netid) and a host address/number (also known as hostid). All nodes in the network have same network address, while the host address is unique to the host only. The type of network can easily understand by looking at the first octet of the address. The lowest IP address is 0.0.0.0 and the highest is 255.255.255.255. Network addresses are assigned by InterNIC (Internet Network Information Center) to avoid conflicts. Local system administrator can set host addresses. Class A address This address takes the form N.H.H.H where N is the network address and H is the host address. The value of N takes in between 1 and 128. The 24 bits are available for H: a class A network theoretically can handle up to 16,777,216 hosts (but actually can not handle). Class A address are allotted to very large corporations and universities but are no longer assigned now. Class A addresses range from 1.0.0.0 to 127.255.255.255 • • • • • The first bit of a Class A address is always 0. Class A is divided into 128 blocks. Total number of addresses in each block is 16,777,216. But many addresses are wasted. Class A addresses range from 1.0.0.0 to 127.255.255.255 Class A was designed for big-size organization. 120 Class B address This address takes the form N.N.H.H where first two octets are network address and has the values in between 128 to 191 in the first octet and any value for the second octet. Class B network theoretically can handle up to 64,516 hosts. Class B addresses range from 128.0.0.0 to 191.255.255.255 • • • • The first two bits of Class B address are 10. Class B is divided into 16,384 blocks. 16 blocks are reserved for private address. The remaining 16,368 blocks are assigned for mid-size organizations. Each block in Class B contains 65,536 addresses. But many addresses are wasted. Class B addresses range from 128.0.0.0 to 191.255.255.255 Class C address This address takes the form N.N.N.H where network address uses first three octets. The first octet varies from 192 to 223 and the second and third octets can take any values. This network can support 256 hosts, but in reality, it cal support up to 254 (the 0 and 255 are reserved). Class C addresses range from 192.0.0.0 to 223.255.255.255 • • • • • The first three bits of a Class C address are 110. Class C is divided into 2,097,152 blocks. 256 blocks are reserved for private addresses. The remaining 2,096,896 blocks are assigned for small organization. Class C addresses range from 192.0.0.0 to 223.255.255.255 Each block in Class C contains 256 addresses. But many addresses are wasted. The number of address in class C is smaller than the needs of most organizations. Class D address There is one block of class D addresses. These addresses are used for multicasting but have seen only limited usage. Multicasting addresses are used to address groups of computers all at one time. They identify a group of computers that share a common application, such as videoconference, as opposed to a group of computers that share a common network. A multicast address is a unique network address that directs packets with that destination address to predefined groups of IP addresses. ). Class D addresses range from 224.0.0.0 to 239.255.255.255 • The first four bits of Class D address are 1110. Class E address There is one block of class E addresses which are reserved by the InterNIC for its own research and for future use. It is designed for use as reserved addresses. Therefore, no Class E addresses have been released for use on the Internet. ). Class E addresses range from 240.0.0.0 to 247.255.255.255 • The first five bits of Class E address are 11110. 121 1st octet Class A Class B Class C Class D Class E Network 2nd octet 3rd octet Host Host 4th octet Range of Host addresses 0.0.0.0 to 127.255.255.255 128.0.0.0 to 191.255.255.255 Network Network Host 192.0.0.0 to 223.255.255.255 224.0.0.0 to 239.255.255.255 240.0.0.0 to 247.255.255.255 Multicast address Reserved for future use IP address format Reserved Addresses • Every network itself has an address. Since all hosts on a network have a common network number of the IP address. This is the first address in the block is used to identify the organization which is known as network address. This address is obtained by setting the host number of any node’s address to zeroes. So the organization is not allowed to use the first address of the block for any host in the network. Every network needs a separate broadcast address. Directed broadcast; this is the last address in the block is used to addresses all hosts on another network. This address is obtained by setting the host number of any node’s address to ones. Obviously this address can not be assigned to any host in the network. This allows to be sent broadcast packets to distant LANs anywhere in the network. Addresses the current host on the current network, such as during a DHCP (Dynamic Host Configuration Protocol) transaction before a workstation is assigned an IP address. This address is obtained by setting the all bits of the IP address to zeroes. Limited broadcast; addresses all the hosts on the current network. This address is obtained by setting the all bits of the IP address to ones. This allows broadcast packets on the local network. Addresses a specific host on the current network. This address is obtained by setting the network number of the IP address to zeroes. Internal host loop-back address. This address is obtained by setting first quad to 127. Packets sent to that address are not put onto network; they are processed locally and treated as incoming packets. This allows packets to be sent to the local network without the sender knowing its address. • • • • • 122 Subnet A subnet is simply a subdivision of the network address by taking some of the host number bits and using them as a subnet number. Sometimes, an organization needs to divide a network into several smaller groups: each group is a collection the hosts. For, example, a university may want to group its hosts according to department. However, the university has one network address, but needs several subnetwork addresses. The outsides the organization knows the network by its network address. Inside the organization each subnetwork is known by its subnetwork address. Subnetting divides one large network into several smaller networks. It adds an intermediate level of hierarchy in IP addressing and become three levels of hierarchy. To reach a host on the Internet, we first reach the network by using network address (netid). Then we reach the subnetwork by using subnetwork address (subnetid). Finally we can reach the host by using host address (hostid). Mask Since a network administrator knows the network address and the subnetwork addresses, but router does not know, the router outside the organization has a routing table with one column based on the network addresses. The router inside the organization has a routing table based on the subnetwork addresses. The 32-bit number mask is the key. The routers outside the organization use a default mask and the routers inside the organization use a subnet mask. Default Mask Default masking is a process that extracts the network address from an IP address. A default mask is a binary number that gives the network address when ANDed with an address in the block. Class A 11111111 00000000 B 11111111 00000000 C 11111111 00000000 Subnet Mask Subnet masking is a process that extracts the subnetwork address from an IP address. A subnet mask is a 32-bit binary numbering which the bits correspond to those of the IP address. Binary 00000000 11111111 11111111 Decimal 00000000 255.0.0.0 00000000 255.255.0.0 11111111 255.255.255. 0 Slash /8 /16 /24 123 255.255.0.0 Default Mask 11111111 11111111 00000000 16 255.255.252.0 Subnet Mask 11111111 11111111 111111 00 6 10 3 00000000 00000000 Default mask and Subnet mask of Class B. The number of subnets is determined by the number of extra 1s in subnet mask than the number of 1s in the corresponding default mask. If the number of extra 1s is n, the number of subnets is2n. If the number of subnets in N then the number of extra 1s in the subnet mask is log2N. In a network each router has a table listing some number of several network addresses (IP addresses) and some number of host addresses (IP addresses) on the current network. The first category tells how to locate to distant networks. The second category tells how to locate to local hosts. When an IP packet arrives at a router then its destination address is looked up in the routing table. If the packet is for distant network, it forwarded to the next router on the interface given in the table. If the packet is for local host on the current network, it sent directly to the destination. If the network is not present, the packet is forwarded to a default router with more extensive tables. However subnetting reduces router table space by creating a three-level hierarchy. For example, an organization uses a Class C address and it could split the 16-bit host number into a 6-bit subnet number and a 10-bit host number. This will allow 62 subnetwork (LANs where 0 and 63 are reserved), each with up to 1022 hosts (0 and 1023 are reserved). In this example, the first subnet might use IP addresses starting at 130.50.4.1 (in binary 10000010.00110010.00000100.00000001), the second subnet might start at 130.50.8.1 (in binary 10000010.00110010.00001000.00000001) and so on. If a packet addresses to 130.50.15.6 and arriving at a router is been AND with the subnet mask of figure to give the address 130.50.12.0. This address is looked up in the routing tables to find out how to get to hosts on subnet 3. 124 ISDN (Integrated Services Digital Network) ISDN (Integrated Services Digital Network) was developed by ITU-T. ISDN data services integrate voice, data and video information on to a single channel. There are a number of types ISDN data channels used for this purpose. ISDN is fully digital circuit-switching telephone system. The primary goal of ISDN system is the integration of voice and non-voice services. ISDN is a digital service that can provide a high bandwidth than standard telephone service, but unlike a leased line, it is not permanent. The main features if ISDN are as follows: 1) ISDN supports various services related to voice communications (e.g. telephone calls) and non-voice communications (e.g. digital data transfer). 2) ISDN supports both circuit switching and packet switching. 3) ISDN provides sophisticated service features, maintenance and network management functions. 4) ISDN has variety of configurations. ISDN can be implemented in a variety of configurations according to specific national situations. ISDN System Architecture All the ISDN installations must have a device called an NT1 (Network Termination 1) connecting to the ISDN exchange using the twister pair. The network terminating device has a connector into which a bus cable can be inserted. Up to eight ISDN telephones, ISDN fax machines, ISDN terminal, ISDN alarm and other devices can be connected to an NT1 by the cable. Network Termination 1 (NT1) Non-intelligent devices concerned with physical and electrical characteristics of the signals. They primarily perform OSI layer 1 functions such as synchronizing and timing. NT1 devices typically form the boundary between a user’s site and ISDN central office. The central office, in turn functions such as the telephone system’s central office, providing access to other sites. Network Termination 2 (NT2) Intelligent devices capable of performing functions specified in OSI layer 2 and 3 such as switching, concentration and multiplexing. A common NT2 device is a ISDN PBX (Private Branch eXchange). It can be used to connect a user’s equipment together or an NT1 to provide access to the ISDN central office. Network Termination 12 (NT12) NT12 is a combination of NT1 and NT2 in a single device. 125 Terminal Equipment 1 (TE1) Terminal Equipment 1 are ISDN devices such as an ISDN terminal, digital telephone or computer with an ISDN compatible interface. Terminal Equipment 2 (TE2) Non-ISDN devices including printers, PCs, analog telephones or anything that has a nonISDN interfaces such as RS-232 or X.21. Terminal Adapter (TA) These devices are designed to be used with TE2 equipment to convert their signals to an ISDN compatible format. The purpose to integrated non-ISDN devices into a ISDN network. ISDN Terminal ISDN Telephone T U Digital Bit pipe NT1 U ISDN Exchange ISDN system for domestic or small business purpose The NT1 performs various operations like network administration, local and remote loopback testing, maintenance and performance monitoring. To connect a new device to the bus, the device must be assigned a unique address. When a device is installed in the bus, it sends a request to NT1 for a unique address. NT1 checks its list of addresses currently in use and then sends a new address to the new device. NT1 also helps towards contention resolution. If several devices try to access the bus at the same time, NT1 will determine which one will get the bus. 126 TE1 ISDN Terminal TE1 ISDN Telephone R U TA LAN S U NT2 ISDN PBX Digital Bit pipe T U NT1 U ISDN Exchange Non-ISDN Terminal ISDN system with a PBX for large commercial complex When we have required more devices to working simultaneously, then NT2 (Network Termination 2), which is nothing but ISDN PBX (Private Branch eXchange), need connected to NT1. This provides the actual interface for telephones, terminals and other equipment. An ISDN PBX is more or less similar to an ISDN exchange; although an ISDN PBX is smaller and can not handle as many conversions at the same time as an ISDN exchange can. An ISDN PBX can be directly connected to any ISDN terminal and telephone. Non-ISDN terminals can be connected to ISDN PBX by using a Terminal Adapter (TA). Devices that connect to the S/T interface, such as ISDN telephones, ISDN fax machines, ISDN terminal, ISDN alarm, are referred as terminal equipment 1 (TE1). Devices that are not ISDN capable, such as standard analog telephones, fax machines, as well as computers are called terminal equipment 2 (TE2). To connect a TE2 device to the S/T interface, there must have an intervening Terminal Adapter. CCITT defined four reference points; these are R, S, T and U, connecting the various devices. The R reference point is the connection between the terminal adapter and nonISDN terminals. The S reference point is connecting the ISDN PBX and the ISDN terminals. The T reference point is the connection between NT1 and ISDN PBX. The U reference point is connecting the ISDN exchange and NT1. Nowadays it is a two-wire copper twisted pair, but in future it may be replaced by optical fibers. ISDN Communications 127 The connection procedure is as follows: 1. The caller transmits a SETUP message to the switch. 2. If the SETUP message is acceptable, the switch returns a CALL PROC (call proceeding) message to the caller and forwards the SETUP message to the receiver. 3. If the receiver accepts the SETUP message, it rings the phone and sends an ALERTING message back to the switch, which forward to the caller. 4. When the receiver answers the call, it sends a CONNECT message to the switch, which forwards it to the caller. 5. The caller then sends a CONNNECT ACK (connection acknowledgment) message to the switch, which forwards it to the receiver. The connection is now established. ISDN supports multiple channels by time division sharing. Several channels have been standardized: A B (bearer channel) C D (delta channel) E H 4 kHz analog telephone channel 64 kbps digital PCM channel for voice or data 8 or 16 kbps digital channel 16 kbps digital channel for out-of-band signaling 64 kbps digital channel for internal ISDN signaling 384. 1536 or 1920 kbps digital channel There are three main types of ISDN interfaces. The services types are as follows: BRI (Basic Rate Interface): Also called 2B+D, because it consists of two 64-Kbps B channel and one 16 Kbps D channel. Adding those rates, one avails 144 Kbps. However, the bit rate of 192 Kbps is achieved including framing, synchronization and other overhead bits. The basic rate is use for home user or small business or the Internet. It simultaneously allows voice and data applications like packet-switched access, facsimile and teletex services. A single multifunctional terminal or several separate terminals can be access those services. BRI provides 128 kbps bandwidth. PRI (Primary Rate Interface): Primary rate consists of 23 64-Kbps B channels and one 16 Kbps D channel. It provides 1.5442 Mbps bit rate that is used by the United States, Canada and Japan. Primary rate may also consist of 30 64-Kbps B channels and one 16 Kbps D channel. It provides 2.048 Mbps bit rate that is second-hand in Europe. Hybrid Interface: Hybrid interface consists of one 4 kHz analog telephone A channel and one 8 or 16 kbps digital channel. Broadband ISDN (B-ISDN) Broadband ISDN is based on ATM technology. 128 Properties of B-ISDN 1. B-ISDN is based on ATM technology. 2. B-ISDN can run at 155.52 Mbps and 622 Mbps or more. 3. In B-ISDN optical fiber will be used. 4. B-ISDN use packet (cell) switching technology. 5. It provides different services like video on demand, live television, full motion multimedia e-mail, CD-quality music, LAN interconnection, high speed data transfer and many other services. Properties of N-ISDN 1. Bandwidth of B-ISDN is 2500 times than N-ISDN. 2. N-ISDN can run at 144 Kbps. 3. N-ISDN can be support with twisted pair 4. N-ISDN use circuit switching technology. Asynchronous Transfer Mode (ATM) The basic idea behind ATM is to transmit all data in small, fixed-sized packets called cells. The cells are 53 octets long, among the cell 5 octets are header and 48 octets are used for user data. 5 octet Header 48 octet User data The header is composed of six fields: • Reserved (1 bit) • Header Error Control (HEC: 8 bits) • Generic Flow Control (0 or 4 bits) • Maintenance Payload Type (2 bits) • Priority Type Identifier (PTI: 1 bit) • Virtual Path/Circuit Identifier (VPI/VCI: 8 or 12 bits) Reserved (1 bit) Header Error Generic Flow Maintenance Control (HEC: Control (0 or Payload 8 bits) 4 bits) Type (2 bits) Priority Type Identifier (PTI: 1 bit) Virtual Path/ Circuit Identifier (VPI/VCI : 8 or 12 bits) There are two important advantages of using small, fixed-size cells. Firstly, the use of small cells may reduce queuing delay for a high-priority cell, since it waits less if it arrives slightly behind a lower-priority cell that has gained access to a resource (e.g. transmitter). Secondly, the fixed-sized cells can be switched more efficiently for achieving a very high data rate. 129 In ISDN, evolution begins from old tradition circuit switching into a cell switching in telephone system. Advantages of cell-switching over circuit-switching Item Constant/variable transmission Cell switching It can handle with both constant rate transmission (audio, video) and variable rate transmission (data). Broadcasting for television It cannot be use in It provides broadcasting for distribution broadcasting for television television distribution. distribution. Data rate It runs with very slow rate It runs at very high rate (kilobits per second) (gigabits per second). The intended speeds for ATM networks are 155.52 Mbps and 622.08 Mbps with the possibility of gigabit speeds in future. ATM network was based on AT&T SONET (Synchronous Optical Network) transmission system as the physical layer carrier. SONET is preferred for two reasons. First, the high data rate of SONET. Second, in using SONET, the boundaries of cells can be clearly defined. The transmission rates of SONET are in range from 51.84 Mbps to 2.48 Gbps. Circuit switching rate It can handle only with variable rate transmission (data). The B-ISDN ATM Reference Model The B-ISDN ATM reference model consists of three layers: Physical Layer, ATM Layer and ATM adaptation Layer. On the top of ATM Adaptation Layer users can put whatever they want. Unlike ISO OSI model or TCP/IP model, the ATM model is defined as being three-dimensional. ATM provides for two types of connections: permanent virtual connection (PVC) and switched virtual connection (SVC) PVC: A permanent virtual circuit connection is established between two endpoints by network provider. The VPI and VCI (Virtual Path/Circuit Identifier: 8 or 12 bits) are defined for the permanent connections. SVC: In a switched virtual connection, each time an endpoint wants to make a connection with another endpoint, a new virtual circuit must be established. 130 Plane management Layer management Control plane User plane Upper layers Upper layers ATM adaptation layer ATM layer Physical layer CS SAR TC PMD The B-ISDN ATM reference model Physical Layer The physical layer handle with the physical medium, voltages, bit timing and various other services. The physical layer is divided into two sublayers: PDM (Physical Medium Dependent) and TC (Transmission Convergence). The PDM sublayer interfaces to the actual cable. PDM is medium dependent. It deals with bit timing and physical network access. For data reception, PDM accepts inbound cell, verifies the checksum and then forwards to the ATM layer. The other sublayer of the physical layer is the TC sublayer. It responsible for numerous functions, including the following: • Cell rate decoupling. • Maintaining cell boundaries. • Generation of Hardware Error Control (HEC) sequence; that is nothing but header checksum generation and verification • Cell generation • Packing or unpacking cells from the enclosing envelope • Transmission frame adaptation, generation and recovery functions. ATM Layer The ATM layer handles with cells and cell transport. It defines the layout of a cell and cell header. It is responsible for establishment and release virtual connections. The cells are received from the AAL through those connections. Congestion control is also done in this layer. The ATM layer is responsible for numerous functions, including the following: • Flow control • Cell header generation/extraction • Virtual circuit/path management 131 • Cell multiplexing and demultiplexing. ATM Adaptation Layer The ATM Adaptation Layer (AAL) is divided into two sublayers: SAR (Segmentation And Reassembly) and CS (Convergence Sublayer). The lower sublayer SAR splits packets into cells on the sender side and puts them back together again at the destination side. The upper sublayer CS makes it possible to have ATM systems offers different types of services to different applications like file transfer and video on demand have different requirements concerning error handling, timing, etc. The ATM defines four different AAL: AAL1, AAL2, AAL3/4 and AAL5. The AAL offers four categories of services: Class A, Class B, Class C and Class D. an example of a Class A service is circuit emulation. An Example of Class B service is variable bit-rate video. Class C and Class D correspond to data transfer applications. CCITT has defined four AAL protocols, one to support each of four classes of service. The type 1 protocol supports Class A, type 2 supports Class B and so on. The protocol reference model makes reference to three separate planes: User Plane The User Plane provides for user information transfer along with associated controls. It handles with data transfer, flow control, error correction and other user functions. Control Plane Control Plane responsible for cell-control and connection-connection control functions. Management Plane Management Plane performs management functions related to a system as a whole and provides coordination between all the planes and layer management, which provides management functions relating to resources and parameters residing in its protocol entities. 132 Bibliography 1. Computer Networks- Andrew S. Tanenbaum. 2. Data Communications and Networking- Behrouz A. Forouzan. 3. Data Communications and Networking- Dr. Madhulika Jain and Satish Jain. 4. Peter Norton’s Complete Guide to Networking-Peter Norton, Dave Kearns. 5. Networking: The Complete Reference-Craig Zacker 133 VSAT VSATs (Very Small Aperture Terminals) are low-cost microstations used in satellite communication. These tiny terminals have 1 meter antennas and can put out about 1 watt power. In many VSAT systems, the microstations do not have enough power to communicate directly with another via satellite. Hub is a special ground station, with a large high-gain antenna which needed to relay traffic between VSATs. In this mode of operation, either the sender or the receiver has a large antenna and a powerful amplifier. Depending on the distance between the user and the ground station and the elevation of the satellite, the end-to-end transit time is 250 to 300 msec; typically 270 msec. There is a time delay of 540 m second between a transmitted and received signal for a VSAT system with a hub. VSATs can be describes technically as an intelligent earth station connecting to the geosynchronous satellite suitable for supporting a variety of two-way telecommunication and information services such as voice, data and video. The major benefits of VSAT network are: i. Very simple and easy to install and greater reliability. ii. High throughput and low bit error rate (BER) for data applications. iii. Integration of data and voice in one communication medium. INDEX A B C 134 D DQDB E F G Gateway H Hub I ISDN J K L M N O P Protocol Q R Router S T U V VSAT W X Y Z 135