DWDM Principle Training Manual

DWDM Principle Training ManualZTE CORPORATION COPYRIGHT Copyright © ZTE Corporation All rights reserved. All information contained herein are confidential information of ZTE and must be handled with highest care. Nobody can, for any purpose, copy, save, link to searching tools, or distribute by any means (including but not limited to electronic, mechanical, photocopying, recording means) of the above mentioned information without prior written consent of ZTE. Author: Randy Editor: Guo Yali * * * * ZTE UNIVERSITY ZTE University, Dameisha, Yantian District, Shenzhen, P.R.China Postcode: 518083 Tel: (+86755) 26778000 Fax: (+86755) 26778999 ZTE CORPORATION ZTE Plaza, Keji Road South, Hi-Tech Industrial Park, Nanshan District, Shenzhen, P.R.China Postcode: 518057 Technical Support Websit：http://support.zte.com.cn Client Support Hot line：（+86755）26770800 Fax: (+86755) 26770801 * * * * 800-830-1118 Version: 1.0 S.N.:PXJCSW200512097 General Preface Thanks for using DWDM Principle Training Manual. In order to use the Manual properly, please read the Preface first. 1. Application This Manual should not be used for the purpose of on-site installation or trouble shooting. 2. About This Manual This manual is composed of one volume，and the table of contents of the volume is shown below: Volume I Course Code WM_001_E1 Course Name DWDM Principle This manual is based on DWDM fundamentals. We will update this manual while the product is upgraded. We apologize if there is any discrepancy between the manual and the products used in your company. 3. Conventions Key points Indicates the key points in one section. O Note A Note statement is used to alert the reader of installation, operation, or maintenance information that is important. 5 Caution Indicates a potentially hazardous situation which, if not avoided, could result in damages to the equipment or personal injury. It may also be used to alert against unsafe practices. & Tips Indicates a suggestion or hint to make things easier or more productive for the reader. 4. Manual Update history Version 1.0 Date Dec. 2005 Comments New 5. From the Author Thank you for using this manual and your continuous support. We would appreciate your comments and suggestions on this Manual. We can be reached at Telephone： （+86755）26778806 Fax： （+86755）26778999 ZTE UNIVERSITY 2. Maintenance Experience and training materials. so as to help you operate and maintain ZTE equipment more efficiently. 01 Manual name Guide documentation to Description Introduces the structure of the whole set of manuals. the purpose and contents of each manual. 3. grounding wires. including power cables. such as outsourced parts. networking scenario and configuration of the product. The composition. 5. PCB layout. including product manuals. jumpers and specifications of each board as well as whether the board is hot swappable. 2. shelf and circuit board. ZTE founded its Documentation R&D Department in 2005. The Documentation R&D Department is responsible for writing. 03 Hardware manual 2. Preparation for installation project Hardware installation flow Installation of cabinets and shelves Connection of cables. function. internal cables .Introduction to ZTE Customer Documentation In order to provide more professional and useful documentation and offer better services to you. configuration. principle. Introduces the hardware installation method and points for attention. including: 1. panel description. Introduces the principle. 02 Technical Manual 3. principle. communication relationship between shelves and the technical specifications of the cabinet. assembly diagram and wiring diagram of each cabinet as well as the functions of the configured shelves. and how to use each manual. 4. including: 04 Installation manual--Hardware 1. 4. A suite of ZTE product documentation consists of the following manuals (will vary with products): No. publishing and issuing various kinds of customer documentation for ZTE Corporation. 6. including: 1. The structure. specifications. DIP switches. The Documentation R&D Department is committed to continually improve the quality of customer documentation. 7. alarm box and network management console. Structures of other hardware. Related fundamental knowledge Principle of hardware and software Structure of hardware and software Specifications of the overall system and each part/module Internal and external interfaces provided and signaling protocols used Service functions available Connection and networking scenarios and specific configurations Introduces the hardware system of the product from the viewpoint of cabinet. backplane and interfaces of each shelf as well as the DIP switches and jumpers on the backplane The function. 3. 4. and power-on/off procedures Introduces how to install the software of the equipment and the points for attention. including the name. installation flow and the environment requirements for software installation Installation of the operating system Installation of software for various servers Installation of client software Installation of remote maintenance system Introduces in detail all the man-machine interfaces of the operation and maintenance system and the parameters on these interfaces. 4. equipment room and grounding). Connection mode between the foreground and background. 3. please refer to the on-line help or the operation 06 Man-machine interface manual manual for this information. board) Alarm system configuration Service interface configuration. shutdown and man-machine interfaces of each server Startup. cabling. 3. including: 1. function. Manual name and external cables 5. 07 Operation manual 2. How to insert and extract circuit boards. 5. shutdown and man-machine interfaces of clients This task-oriented manual describes in detail the purpose. How to commission an office/site Configuration of background operation and maintenance system Network (topology) configuration Hardware configuration (rack.g. pre-operation setup (including hardware and software environment.No. Structure and networking mode of the operation and maintenance system Startup. 2. e. parameter description and examples for each command. format. the software to be installed. 7. 4. 05 Installation manual—software 2.. 09 Maintenance Serves as a reference manual for guiding equipment maintenance and . Description Installation of alarm system and background system Installation of antenna and feeder system for radio equipment Check list and standards for judging whether the hardware installation is up to standard. operation steps and result verification for the following tasks: 1. A typical man-machine interface manual includes the following contents: 1. 6. 6. 5. shelf. user interface configuration Introduces in detail all the man-machine commands of the operation and 08 Command manual maintenance system. 8. If no man-machine interface manual is available. 3. pre-power-on check. with your username being your phone number and your password being the last 6 digits of your phone number. 3. CD and email. 2. Description Routine maintenance.No. You will receive your username and password in your registered mailbox within two days after your registration. We welcome you to participate in this action. all trainees of ZTE university will be registered automatically.zte.com. ◆ You can query and download the latest product manuals and Maintenance Experience from http://ensupport. Structure and principle of the product Installation and debugging of hardware and software Power-on and power-off procedures. 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WM_000_E1 DWDM Basic Principles Course Objectives: z z Grasping the significance and application environment of the DWDM networks Grasping the DWDM principles and the key technology for implementing the DWDM References: z z z Optical Wavelength Division Multiplexing System Modern Communication Base and Technology Principle and Test of DWDM Transmission System . . ....10 1.......................................................2...1..................................................................1.........15 2........Contents 1 DWDM Overview .............................2 DWDM Technology Overview...............................................1 Different between DWDM Technology and Other Multiplexing Technologies...............................................................................................................2....5 1....13 2 Overview of Optical Fiber Communication .......................................20 2................1 Basic Knowledge of Optical Fibers................................3...........................1 1...........................1 Introduction to Working Wavelength Area ..........26 2..................................19 2.........................................................................................................................3................................................................................................................................2........2 Dispersion..15 2....3 Operation Wavelength Range ..................1 Fiber Loss ..............2 Relationship between DWDM and SDH......29 2............................................................................20 2.....................................................................................................................2 PDH.................1...................3 DWDM Features and Advantages ...1 1............................... SDH and DWDM .......18 2................................................................2 Wavelength Allocation......................1 Development of Multiplexing Technology in Optical Network ....................................25 2.............................2 Working Wavelength of DWDM System ..........................2 1......3..........................4 New Optical Fiber Types..........................................................................................................................................15 2................1........................................1..................................33 i ......12 1................................................................................................................................................................3 Types and Features of Common SMFs................2.................................7 1.........................................2...........................................................5 1.......................................3 Fiber Transmission Features.............25 2.....1 Brief Introduction to Optical Fibers ..21 2.................1 Background of DWDM Technology ............................4 DWDM Development Trend ..................................................1 1......................................................2 Usage Overview of Applicable Frequencies of Optical Fiber ................................................3 Non-Linear Effect of Fiber ............................................................. .................................................................2 Light Source Technology .................................................................... 45 3...................5...........................................1 Brief Introduction to DWDM System Hierarchy ........................................4... 36 3......................................................................................................................................6................................. 58 4 Protection Principle of DWDM System ........................3 Key Technologies of DWDM System .............................................3 Implementation of OSC .........2 Introduction to OM ..........6............................................5.......................................2..............3 Classification and Applications of OTU ........................................................................................................................ 44 3............................... 42 3.............................................................3.............................2 Requirements for OSC ............................................... 35 3...........................................................................1 Overview............................................... 39 3...............3 Optical Wavelength Division Multiplexing and De-Multiplexing Technologies...4.2 Working Principle and Performance Indices............... 61 4.2....................4.....6................................................................................................................................ 65 4.....3..........................................1 Functions of Optical Supervision Channel (OSC) ............................................. 62 4................................................................ 49 3...2 Ring 1+1 Protection ........................1 EDFA Technology ..........................................................................................5 Optical Amplifying Technology................... 65 4..... 64 4...... 44 3...................................2 1+1 Protection....................2..........................................2 Raman Amplifying Technology ......................... 49 3.................. 62 4.......................................... 48 3...........................................3 Features of 1+1 Protection ..............................1 Working Principle ...............................3 1: N Protection ......................................... 56 3................................................................................................ 39 3............................................................................... 35 3..................................................................................................................................... 65 ii ...........3.....................................................................1 Basic Structure of DWDM System ......... 57 3...........................................................................................1 Link 1+1 Protection ............... 57 3................................................................... 55 3.........................3 Key Performance Indices ................4 OTU Technology.................................................. 61 4..............................................3..........1 Overview...........................6 Supervision Technology...................................................................................................... 39 3.. .....4...66 4................2 Protection Implementation ............5 Bidirectional OMS Protection .........3................................................................3 Features of 1:N Protection..........69 iii ........66 4.............................................................4 Bidirectional Optical Channel Protection........................................67 4..................................................................................3......................................................................................... . 1 Background of DWDM Technology Prior to the introduction to DWDM technology. small attenuation. the multiplexing technology is introduced in the optical transmission system. the optical fiber transmission features large transmission capacity. coaxial cable. the multiplexing mode of signals greatly affects system performance and cost. but also expected to be interactive. Among them. optical fiber and wireless transmission. Therefore. The multiplexing technology of optical transmission network goes through three development phases: Space Division Multiplexing (SDM). good quality. The TDM 1 . Since the broadband high-speed service ceaselessly develops in the information age.1 Development of Multiplexing Technology in Optical Network The communication network covers diversified transmission media.1 DWDM Overview Highlights: z z DWDM definitions and generation background. This section will introduce the background of the DWDM technology in terms of multiplexing technology and transmission technology. In the multi-channel signal transmission system. fast and convenient. the SDM technology requires that the quantity of fiber transmission cores must be configured in accordance with quantity signal multiplexing channels. With simple design and practical feature. The multiplexing technology means to use the broadband and large-capacity features of the optical fiber to simultaneously transmit multiple channels of signals on one optical fiber or cable.1. such as twisted pair. 1. which means poor investment profit. 1. DWDM development trend. Time Division Multiplexing (TDM) and Wavelength Division Multiplexing (WDM). strict security and large trunk distance. we should know about the development of the optical network. the optical transmission system is not only expected to have larger capacity and longer distance. while leaving the massive bandwidth resources of fibers far from being fully exploited. So it is the major measure for expanding the current fiber communication network and is mostly used in trunk network. as shown in Table 1. North America and Japan. use "one-wavelength-in-one-fiber" mode. as well as interface specifications of each technology. SDH and DWDM The traditional fiber transmission technologies.WM_000_ E1 DWDM Basic Principles technology is widely applied.1-1. The PDH system covers three regional rate level standards in Europe. which introduces in Pulse Coding Modulation (PCM) digital transmission technology based on the former analog telephone network. such as Plesiochronous Digital Hierarchy (PDH) and Synchronous Digital Hierarchy (SDH). ATM and IP. It can fully meet the current need of the network broadband service development and lays a solid foundation for the development of the future fully-optical transmission network. The primary signals of the PDH system are in synchronous TDM mode. The new Dense Wavelength Division Multiplexing (DWDM) becomes the most effective and practical means for the fiber expansion. 2 . But its disadvantage is low line utilization ratio. It multiplexes signals of low rate level into high-speed signals by means of bit filling and digit interleaving. The WDM technology supports multiple wavelengths (channels) to be carried on one fiber. and the multiplexing of other high order groups are in plesiochronous (or called asynchronous) TDM mode. PDH The early optical transmission system uses PDH. SDH and DWDM are briefed below. The development processes of PDH. 1. They are so restricted by the characteristics of their own equipment that neither the transmission capacity nor expansion mode can meet the requirements of the communication network developing at a high speed. It is the base of PDH.1.2 PDH. With its unique technical advantages. SDH. the DWDM technology becomes a simple and economical means to expand the fiber transmission capacity in a rapid and effective manner. 1. 3) PDH is a kind of multiplexing structure based on point-to-point transmission.448Mbit/s 120 channels (30 × 4) 6. 1) The compatibility between three rate standards is not available.312Mbit/s 96 channels (24 × 4) North America Japan From early 1970's to 1980's. 5) Along with rate increase. the PDH disadvantages are more and more remarkable. 4) The operation. However.544Mbit/s 24 channels PDH Code Rate Tertiary Group 34. it is more and more difficult to implement multiplexing of high-order groups through PDH technology. which limits networking flexibility and increases network complexity and operation costs.064Mbit/s 480 channels (96 × 5) Quartus Group 139. It only supports point-to-point transmission.264Mbit/s 1920 (480 × 4) 274.Chapter 1 DWDM Overview Table 1. management and maintenance must depend upon manual digital signal cross-connection and service-suspension test.368Mbit/s 480 channels (120 × 4) 44.1-1 Country/Region Europe China and Primary Group (Primary) 2. 2) There is no worldwide standard optical interface specification.544Mbit/s 24 channels 1. which cannot meet monitoring and NM requirements of modern communication network. the PDH system and devices are popularly used in the digital network.312Mbit/s 96 channels (24 × 4) 6.048Mbit/s 30 channels 1. but cannot accommodate complicated networking. Private optical interfaces developed by different manufacturers are not compatible with each other. which obstructs development of international interconnection. 3 . and requirements of fiber digital communication for large-capacity and super-high speed transmission cannot be met.736Mbit/s 672 channels (96 × 7) 32.176Mbit/s 4032 (672 × 6) 97. along with the developing fiber communication technology and user's increasing demands for communication services.728Mbit/s 1440 (480 × 3) channels channels channels Secondary Group 8. 520 622. the WDM technology is introduced in the backbone network.WM_000_ E1 DWDM Basic Principles 2. The SDH signals use synchronous multiplexing mode and flexible multiplexing and mapping structure. multiplexing mode.280 SDH Level (ITU-T) STM-1 STM-4 STM-16 STM-64 OC-3 OC-12 OC-48 OC Level (SONET) OC-192 The SDH standardizes the features of the digital signals.1-2 SDH Signal Levels Line Rate (Mbit/s) 155. This kind of transmission network is easy to expand and is applicable to the development of new telecom services. Table 1. and rename it as SDH. called “one-step” de-multiplexing”. it makes possible the interworking between the devices of different manufacturers. so software can be used to directly de-multiplex a high-speed signal into low-speed tributary signals at a time. The single-wavelength transmission cannot fully utilize the broad bandwidth of fiber. greatly enlarging the transmission capacity of fiber. the Bell Communication Research Institute in America put forward the concept of Synchronous Optical Network (SONET). The rate specifications of the SDH system are shown in Table 1.080 2488. 4 . the SDH system is the TDM system based on the single wavelength. SDH In mid-1980's. the CCITT (former ITU-T) accepted the SONET concept. featuring flexibility. formed the worldwide unified technology standard for transmission network. on which a world-class telecom transmission network has been developed. Code streams of different levels are arranged regularly in the payload of the frame structure. It provides a frame that is supported globally. In addition. Furthermore. the system dispersion and other negative influences will increase difficulty of long-distance transmission. such as frame structure. But. after the transmission rate is higher than 10 Gbit/s. Therefore. transmission rate level and interface code pattern. reliability and convenient management. The payload is synchronous with the network.1-2.320 9953. In 1988. 2 DWDM Technology Overview The DWDM technology is this kind of fiber communication technology: Transmitting multiple optical carriers with information (analog or digital) on one fiber and expanding system only through wavelength (channel) increase. N × 2. it can provide multiple virtual fiber channels on one physical fiber.Chapter 1 DWDM Overview 3.2. To meet the horizontal compatibility between systems. After the transmission. it is called DWDM. TDM TDM means that different channels of signals use different time intervals (timeslot) for signal transmission on the same fiber. the practical DWDM system works in 1550 nm window for the purpose of using the gain spectrum feature of the EDFA to directly amplify the composite optical wavelength signals. It can combine (multiplex) optical signals with different wavelengths and then transmit them.1 Difference Technologies between DWDM Technology and Other Multiplexing This section compares the multiplexing technologies often used in the fiber communication system. such as SDH signal. 1. 1. 5 . each optical channel can bear different customer signals. At present.692 standard. In other words.5Gbit/s DWDM system and N × 10Gbit/s DWDM system become majority and backbone of the core network. The TDM has fixed timeslot allocation. In the DWDM system. Due to small interval (1 nm ~ 10 nm order) between adjacent wavelengths. it can separate (de-multiplex) the combined optical signals and then send them to different communication terminals. Due to unique advantages of fiber communication and its networking technologies for accommodating multi-service and broadband requirements. the central wavelength of the optical channel must accord with G. DWDM DWDM is one of WDM technologies. PDH optical signal and ATM signal. high-speed SDH system. which facilitate adjustment and control and is applicable to the digital information transmission. 1. SCM Microwave Sub-Carrier Multiplexing (SCM) technology means to modulate multiple baseband signals into the microwave carriers with different frequency for the sake of electrical Frequency Division Multiplexing (FDM). 4. it is difficult to implement the system with a capacity over 40 Gbit/s. and then use this bit stream to modulate a single optical carrier into the fiber. WDM WDM means to bear multiple wavelength (channel) systems on one fiber and convert one fiber into multiple "virtual" fibers. 2. 3.WM_000_ E1 DWDM Basic Principles Its disadvantage is low line utilization. SDH. The SDH performs optical intensity modulation to each channel of baseband signals respectively. each of which independently works on different wavelengths. restricted by modulation capacity of high-speed electronic components and laser. Each channel of signals are transmitted by one fiber. At the receiving end. because when a signal source has no data for transmission. The SDM technology has simple design and practical feature. ATM and IP. This technology is mostly used in the CATV multi-band transmission system of access network. leading to best transmission performance. For example. leading to poor investment profit. Boasting economical efficiency and practicability. In addition. such as PDH. but it requires that the fiber transmission core quantity must be configured in accordance with the signal multiplexing channel quantity. the WDM system technology is the major wavelength 6 . different channels will not affect each other. The TDM technology is widely applied. the corresponding channel will be idle but the other busy channels cannot occupy this idle channel. pick the electrical FDM line signals through the photoelectrical detector and then use microwave technology to de-multiplex the different microwave carriers. SDM SDM means the technology that divide space into different channels to implement wavelength multiplexing. you can add the core quantity in the cable or use more fibers to form different channels. to restore the former baseband signals. 2. the network communication inside the building or between buildings. the CWDM technology applies to the communication cases with short distance. this multiplexing technology only uses two wavelengths: one in 1310 nm window and the other in 1550 nm window. please refer to other chapters in this manual. Usually. It is the initial wavelength division multiplexing case. 1) 1310 nm/1550 nm-wavelength multiplexing In early 1970's.2 Relationship between DWDM and SDH 1. It can bear 8 ~ 160 wavelengths on one fiber. It implements single-fiber dual-window transmission through the WDM technology. Its disadvantages are small capacity and short transmission distance. leading to a lower cost than that of DWDM. the DWDM technology refers to the WDM technology with small interval between adjacent wavelengths. Therefore. 1. with working wavelength in the 1550 nm window. 3) CWDM The CWDM technology refers to the WDM technology with large interval (usually greater than 20 nm) between adjacent wavelengths. for example.Chapter 1 DWDM Overview multiplexing technology commonly used in the current fiber communication network. For the details. its wavelength quantity is 4 or 8 (16 at most). It uses 1200 nm ~ 1700 nm windows. 2) DWDM In simple words. It adopts non-cooling laser and passive amplifier component. mostly used in long-distance transmission system. Coarse Wavelength Division Multiplexing (CWDM) and DWDM. Relationship between DWDM and SDH on the transmission layer of optical network 7 . The WDM is divided into three multiplexing modes: 1310 nm/1550 nm-wavelength multiplexing. broad bandwidth and dense access points. 2-1 Relationship between DWDM and SDH in Transport Network The SDH system implements multiplexing. the system dispersion and other negative influences will increase the difficulty of long-distance transmission. 1. fully utilizing the bandwidth resources of fiber and increasing system transmission capacity. 1. The DWDM technology simultaneously transmits multiple optical carrier signals of different wavelengths in the same fiber. DWDM capable of transmitting different types of signals At present. cross-connection and networking on the electrical channel layer. Their relationship in the transport network is shown in Fig. But the wavelengths used in the DWDM system are mutually separated and 8 . most customer-layer signals in the DWDM system are SDH signals. 3. 2.2-1. They are the transmission means established on the fiber transport media. When the transmission rate exceeds 10 Gbit/s. The WDM system implements multiplexing. cross-connection and networking on the optical domain.WM_000_ E1 DWDM Basic Principles Both DWDM system and SDH system belong to the transmission network layer. Circuit layer (for example. Multiplexing modes of DWDM and SDH for carrier signals The SDH is the TDM system based on a single wavelength (one fiber transmitting one wavelength channel). ATM and IP) SDH channel layer DXC SDH system ADM DWDM optical channel layer OADM DWDM system OTM Fig. 692 recommendation. 9 . the signals accessed through "Open interface" shown in Fig. z Open system: The transmitting side of the WDM system provides the optical wavelength converter (OTU) to converts the customer signals with non-standard wavelength into the standard wavelength compliant with G.692 standard. Therefore.957 recommendation.2-2 Relationship between DWDM and Other Services 4. IP ATM SDH ATM Ethernet Other SDH Open optical interface DWDM Fiber physical layer Fig. The relationship between DWDM system and some common services is shown in Fig. 1. nominal central frequency (central wavelength). for example. the DWDM system can be either open DWDM system or integrated DWDM system. so each wavelength can transmit the optical signals with totally different features. central frequency offset and other parameters. "Open" means that the DWDM system has no special requirement for the working wavelength of the input signals.2-2. Optical interface standards of DWDM and SDH signals The optical interfaces of the SDH device should accord with the ITU-T G. 1. 1. channel interval. which specifies the reference frequency of each optical channel. which does not stipulate the working central wavelength.2-2. for the sake of hybrid transmission of multiple kinds of signals.Chapter 1 DWDM Overview irrelated with the service signal formats. The optical interfaces in the DWDM system must accord with the ITU-T G. WM_000_ E1 DWDM Basic Principles z Integrated system: All the customer signals accessing the DWDM system must accord with G.692 standard, for example, the signals accessed through "Non-open interface" shown in Fig. 1.2-2. 5. Integrated application of DWDM and SDH The transmission capacity of the fiber network can be effectively improved through integrated application of DWDM and SDH. 1.2.3 Operation Wavelength Range The quartz fiber has three low-loss windows: 860 nm, 1310 nm and 1550 nm, as shown in Fig. 1.2-3. 3.0 ~140THz 2.5 OH- absorption peak 2.0 Loss (dB/km) 1.5 1.0 0.5 O 0 800 1000 1200 E 1400 S C L 1600 Wavelength (nm) L: Long Band ~50THz OH- absorption peak OH- absorption peak O: Original Band E: Extend Band S: Short Band C: Conventional Band Fig. 1.2-3 Low-Loss Windows in Fiber Communication 1. 860 nm window Its wavelength range is 600 nm ~ 900 nm. It is used in multi-mode fiber, with large transmission loss (2 dB/km averagely). It is applicable to the short-distance access network, such as Fiber Channel (FC) service. 2. 1310 nm window The lower limit of the available wavelength here depends on the fiber cut-off wavelength and fiber attenuation coefficient, and the upper limit depends on OH 10 Chapter 1 DWDM Overview root absorption peak at 1385 nm. The working wavelength range is 1260 nm ~ 1360 nm. The average loss is 0.3 dB/km ~ 0.4 dB/km. This window is applied to intra-office, short-distance and long-distance communication of STM-N signal (N = 1, 4 or 16). The light source can be multi-longitudinal mode laser (MLM) and LED. Since the broadband optical amplifier working in 1310 nm window is not available at present, this window is not suitable for the DWDM system. 3. 1550 nm window The lower limit of the available wavelength here depends on OH root absorption peak at 1385 nm, and the upper limit depends on infrared absorption loss and bending loss. . The working wavelength range is 1460 nm ~ 1625 nm. The average loss is 0.19 dB/km ~ 0.25 dB/km. The loss in the 1550 nm window is the lowest, so it can be applied to short-distance and long-distance communication of SDH signals. In addition, the EDFA often used currently has sound gain flatness in this window, so this window is applicable to the DWDM system as well. The working wavelength in the 1550 nm window is divided into three parts (S band, C band and L band), with the wavelength range shown in Fig. 1.2-4. Fig. 1.2-4 Division of Working Wavelength in 1550 Window 1) S band (1460 nm ~ 1530 nm): Since the working wavelength range of EDFA is in C band or L band, S band is not used in the DWDM system at present. 2) C band (1530 nm ~ 1565 nm): It is often used as the working wavelength area of the DWDM system under 40 waves (with band interval as 100 GHz), DWDM system under 80 waves (with band interval as 50 GHz) and SDH system. 3) L band (1565 nm ~ 1625 nm): Working wavelength area of the DWDM system above 80 waves. In this case, the band interval is 50 GHz. 11 WM_000_ E1 DWDM Basic Principles 1.3 DWDM Features and Advantages 1. Fully utilizing fiber bandwidth resources and featuring large transmission capacity The DWDM technology makes full use of the huge bandwidth (about 25 THz) resource of fibers to expand the transmission capacity of the system. 2. Super-long transmission distance Through EDFA and other super-long distance transmission technologies, the channel signals in the DWDM system are amplified at the same time, for the sake of long-distance transmission of the system. 3. Abundant service access types The wavelengths in the DWDM system are separated to each other, capable of transmitting different services in transparent way, such as SDH, GbE and ATM signals, for the sake of hybrid transmission of multiple kinds of signals. 4. Saving fiber resource The DWDM system multiplexes multiple single-channel wavelengths for transmission in one fiber, greatly saving fiber resource and reducing line construction cost. 5. Smooth upgrading and expansion Since the DWDM system transmits the data in each wavelength channel in transparent way and does no process the channel data, only more multiplexing wavelength channels should be added for expansion, which is convenient and practical. 6. Fully utilizing well-developed TDM technology At present, the optical transmission technologies in TDM mode, such as SDH, have been well developed. Through the WDM technology, the transmission capacity can be enlarged by several times or even dozens of times, with expansion cost lower than that in the TDM mode. 7. Forming full optical network Full optical network is the development trend of the optical transport network. In such network, the WDM system is connected with Optical Add/Drop Multiplexer (OADM) and Optical Cross-connection (OXC) device, to directly 12 Chapter 1 DWDM Overview performing optical channel adding/dropping and cross-connection to the services on the optical wavelength signals, and thus forming full optical network with high flexibility, reliability, survivability and economical efficiency to meet the requirements of future information society for the broadband transport network. 1.4 DWDM Development Trend 1. Higher channel rate The channel rate of the DWDM system has developed to 10 Gbit/s from 2.5 Gbit/s, and the system at 40 Gbit/s rate is in experimentation and its technology becomes more and more mature. 2. Greater wavelength multiplexing quantity The DWDM system at early phase usually works at 8/16/32 wavelengths, with channel interval as 100 GHz and working wavelength in C band. Along with ever development of the technology, the working wavelength of DWDM system can cover C and L bands, with interval as 50 GHz. For example, ZTE's ZXWM M900 device can provide the multiplexing of up to 160 waves. 3. Super-long fully-optical transmission distance The initial cost and operation cost for network construction can be reduced through extending full optical transmission and reducing electrical regeneration points. The traditional DWDM system uses EDFA to extend the current-free delay transmission distance. At present, this distance can be extended from 600 km to above 2000 km, through distributed Raman amplifier and super-powerful Forward Error Correction (FEC) technology, dispersion management technology and optical balancing technology and effective modulation formats. 4. Evolving from point-to-point WDM to full optical network The ordinary point-to-point DWDM system consists of Optical Terminal Multiplexer (OTM). Although it has a huge transmission capacity, it only provides primitive transmission bandwidth and features inflexible networking capability. Along with ceaseless development of electrical cross-connection system, the node capacity ever increases, and the point-to-point networking obviously cannot keep up with the growth of network transmission link capacity. 13 In the full optical network. OXC is the route switch of next generation optical communication. Development of IP over DWDM technology The bandwidth of the Internet backbone network increases quickly. Internet data streams alone will occupy the capacity of the whole single-wavelength fiber system (at present. 5. the IP over DWDM will be the major technology of the network communication in the future. The OADM controls the optical signals of different wavelength channels to be sent to the proper locations. it provides these functions: Providing connection function based on wavelengths. converging input of different wavelengths from different directions and then output signals with proper wavelengths. we can construct chain and ring optical networks. providing wavelength add/drop function of optical channels. Through OADM and OXC. Through OADM. The OXC is set at the important tandem point of the network. and implementing protection and restoration on wavelength.WM_000_ E1 DWDM Basic Principles The further expansion opportunity depends on the optical nodes. OADM and OXC. we can construct more complicated ring network. 14 . wavelength and fiber levels. for the sake of protection and restoration of optical services. In the next generation IP Over DWDM telecom/network architecture. leading the wavelength channels for the sake of best utilization of fiber infrastructure. Therefore. the OXC may replace the existing electrical switching/route by optical signals. that is. so if the DWDM technology is not used. the maximum transmission rate of the single-wavelength fiber system for commercial use is 10 Gbit/s). cladding and coating layer. 2. Transmission features of optical fibers. Types and applications of common optical fibers. such as GeO2.1 Basic Knowledge of Optical Fibers 2. 2. 2. 15 . Structure of optical fibers Optical fiber is a kind of cylinder glass fiber with sound light conducting performance and small diameter.2 Overview of Optical Fiber Communication Highlights: z z z Basic knowledge on optical fiber communication. The diameter of the fiber core usually ranges 5 µm ~ 50 µm. as shown in Fig. Coating layer Cladding Fiber core n2 n1: Refractive index of fiber core n2: Refractive index of cladding n1 Fig.1-1 Structure of the Optical Fiber 1) Fiber core It is made of SiO2 (quartz). consisting of fiber core. to improve refractive index (n1) of the fiber core.1.1-1. It also comprises few doped chemical.1 Brief Introduction to Optical Fibers 1. when the shooting-in angle of the light meets the full reflection condition. with outer diameter as about 250 µm.1-2 Comparison between Step-Type Fiber and Graded Fiber 16 . 2. Depending on the refractive index radio distribution on the fiber cross section. Since the refractive index (n1) of the fiber core is greater than that (n2) of the cladding. the light can be repeatedly reflected on the juncture and move forwards in "zigzag" way. The relationship between refractive index and fiber structure as well as the transmission of light in the fiber are shown in Fig. 1) Fiber categories By distribution shape of refractive index When the light is transmitted in the fiber. such as epoxide resin and silicone rubber. the fiber is divided into step-type fiber and graded fiber. it is made of SiO2. n2 Cladding Fiber core Light n2 Cladding Fiber core Light n1 a: Step-type fiber n1 b: Graded fiber Fig. Through adding coating.WM_000_ E1 DWDM Basic Principles 2) Cladding Usually. 3) Coating layer It is made of high molecular materials. we can improve flexibility. 2. mechanical strength and aging-resistance features of the optical fiber. with outer diameter as 125 µm. and thus restricting the light inside the fiber core and forming transmission wave. The refractive index (n2) of cladding is less than that (n1) of fiber core. 2. each light shoots into the juncture between fiber core and cladding in proper angle.1-2. and full-plastic fiber with plastic core and plastic cladding. Such specified electromagnetic wave is called optical fiber mode. and each light in its core is in a transmission mode.1-1. For the specified fiber structure. this fiber is called Single-Mode Fiber (SMF) and its core transmits only one light. the light transmitted in the fiber should not only meet full-reflection condition between fiber core and cladding. Therefore. only a series specified electromagnetic wave can be effectively transmitted in the fiber. If the fiber supports multiple conduction modes. this fiber is called Multi-Mode Fiber (MMF). 17 . In the fiber. the conductible mode quantity depends on structure and refractive index radial distribution of the fiber. but also meet the related conditions for electromagnetic wave in the transmission process. the fibers can be divided into quartz fiber.1-2 shows two typical multi-mode fibers. 2. The differences between SMF and MMF are listed in Table 2. quartz-plastic fiber with quartz core and plastic cladding. If the fiber supports only one conduction mode (base mode). by material.Chapter 2 Overview of Optical Fiber Communication 2) By fiber material Besides quartz fiber. glass fiber with many compositions. Such fibers have greater loss than the quartz fiber. 3) By transmission mode Light is a kind of electromagnetic wave. so they are usually used by the short-distance systems inside buildings or rooms. Fig. increasing along with increased optical signal spectrum width Ordinary SMF.absorption peak 2. 2.WM_000_ E1 DWDM Basic Principles Table 2. At present. Dispersion Shifted Type Working window Applications Fiber (DSF) and Dispersion Compensation Fiber (DCF) 1310 nm and 1550 nm Long-distance fiber communication 850 nm and 1310 nm Short-distance fiber communication system at low rate system with large capacity 2. 3.5 OH. there are five low-loss windows.5 O 0 800 1000 1200 E 1400 S C L 1600 Wavelength (nm) L: Long Band ~50THz OH.absorption peak II V III IV O: Original Band E: Extend Band S: Short Band C: Conventional Band Fig.5 1.0 0. the fiber transmission loss is lower and lower. directly affecting transmission bandwidth and transmission distance Ordinary MMF SMF Transmission mode Fiber core Dispersion influence Only supporting transmission in base mode Smaller (about 5 µm ~ 10 µm) Caused by transmission rates different frequency components in the optical signal.absorption peak I OH.1-1 Differences Between SMF and MMF MMF Supporting multiple conduction modes Greater (about 50 µm) of Large mode dispersion due to different transmission rates of different modes.2 Usage Overview of Applicable Frequencies of Optical Fiber Due to improving fiber manufacture technique.1.0 Loss (dB/km) 1.1-3 Division of Low-Loss Windows 18 . as shown in Fig.1-3.0 ~140THz 2. 2. G.652/G.653/G . G. Therefore. it has dispersion close to zero.652. 19 .652/G.655.653 and G. G.5 Gbit/s. It is applicable to the high-rate and long-distance single-wavelength communication system. resulting in optical signal attenuation in multiplexing channels and channel crosstalk. its loss is the smallest.653 (dispersion shifted SMF) It has the smallest loss and the smallest dispersion in the 1550 nm. The fiber types applied in the DWDM system are also involved. serious non-linear Four Wave Mixing (FWM) problem will occur in zero-dispersion wavelength area. it is applicable to both SDH system and DWDM system. wavelength range.655 V 1360 ~ 1530 (E + S bands) 1360~1530 Full-wave fiber 2.653/G . applied fiber types and application of each window are described in Table 2.655 IV 1600 (L band) 1565~1625 G. it is only applicable to the SDH system. When it is used in the 1310 nm window. with dispersion of 17 ps/km nm. But in the 1550 nm window. it usually works in the 1550 nm window. In the 1310 nm window. when it is used in the 1550 nm window. When the DWDM technology is used.652/ G. 1.652 (ordinary SMF) It is also called dispersion non-shifted SMF.1-2. Table 2.1-2 Window Mark (nm) Wavelength range (nm) Fiber type 850 600~900 MMF Short Applications distance and low rate I 1310 band) 1260~1360 MMF/G. 2.3 Types and Features of Common SMFs This section briefly introduces features and functions of three SMFs.653 Short distance and low rate Long distance and high rate Feature Comparison between Low-Loss Windows II (O III 1550 (C band) 1530~1565 G. G. used in 1310 nm and 1550 nm windows.Chapter 2 Overview of Optical Fiber Communication The optical signal mark.1. requiring dispersion compensation when the single channel rate is over 2. 1 THz ~ 196. G. the working wavelengths of the systems below 40 wavelengths.0 THz) Channel interval: 50 GHz Central frequency offset: ±5 GHz 3.5 GHz (at rate 10 Gbit/s) 2.WM_000_ E1 DWDM Basic Principles 3.1 Introduction to Working Wavelength Area Based on multiplexing channel quantity and frequency interval of the DWDM system. the absolute value of its dispersion is not zero and within a certain range (ensuring smallest loss and small dispersion in this window). Therefore. this kind of fiber is usually used in the DWDM system. In addition.1 THz ~ 196.0 THz Channel interval: 100 GHz Central frequency offset: ±20 GHz (at rate lower than 2.5 Gbit/s). non-zero dispersion suppresses the influence of non-linear FWM over DWDM system. 1.655 (non-zero dispersion shifted SMF) In the 1550 nm window. 80-wavelength system Working wavelength range: C band (1530 nm ~ 1565 nm) Frequency range: C band (192. ±12. 160-wavelength system Working wavelength range: C band (1530 nm ~ 1565 nm) + L band (1565 nm ~ 1625 nm) 20 . 80-wavelength system and 160-wavelength system are introduce below.2 Working Wavelength of DWDM System 2. 2. 8/16/32/40-wavelength system Working wavelength range: C band (1530 nm ~ 1565 nm) Frequency range: 192.2. It is applicable to the high-rate and long-distance optical communication system. 79 1558.75 1554.36 1556.8 192.0 194.2-1 No.12 1545.32 1544.1 194.51 1547.9 193.98 1558.13 1553.2 193.94 Wavelength (nm) .7 192.5 192.3 21 1560.2.4 192.95 THz) Channel interval: 50 GHz Central frequency offset: ±5 GHz 2.52 1551.53 1543.0 THz) + L band (190.17 1557.72 1550.5 193.1 193.7 193. Table 2.32 1548.2-1.6 192.72 1546.73 1542.2 194.6 193.2 192. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 Wavelength Allocation of 40CH/100 GHz Interval on C Band Central Frequency (THz) 192.90 THz ~ 186.12 1549.3 193.1 192. as stipulated by the ITU-T Recommendation G.4 193.9 194.692.55 1555.Chapter 2 Overview of Optical Fiber Communication Frequency range: C band (192.61 1559. The wavelength allocation for C band 40-wavelength system with wavelength interval of 100 GHz is shown in Table 2.92 1550.0 193.1 THz ~ 196.8 193.94 1554. 1.33 1552.92 1546.3 192.2 Wavelength Allocation The working wavelength of the DWDM system complies with the specific central wavelength and central frequency values in the multi-channel system. 65 193.0 1542. Table 2.61 1535.12 1546.14 1541.55 193.68 1531.55 Wavelength (nm) 2.85 195.35 1540. The wavelength allocation for C/C+ band 80-wavelength system with wavelength interval of 50 GHz is shown in Table 2.11 1548.92 1547.86 22 41 42 43 44 45 46 47 48 49 50 51 52 53 Wavelength No.47 1533.19 1537.47 1532.85 193.32 1549.52 1546.05 196.12 1530.75 193.80 195.2-2.4 195.94 1530.90 1531.82 1535.6 195.70 195.9 196.68 1533.45 Nominal Central Wavelength nm 1544.5 194.05 194.00 195.55 1529.8 194.70 193.2-2 Wavelength Allocation of 80CH/50 GHz Interval on C/C+ Band Nominal Central Wavelength nm 1529.60 193.00 193.2 195.40 1536.8 195.65 195.72 1531.07 1533.50 193.29 1532.95 195.77 1538. Nominal Central Frequency THz 194.80 193.60 195.7 195.91 1549.5 195.72 Nominal Wavelength No.55 195.95 193.72 1548.75 195.6 194.04 1534.3 195.45 .12 1531.51 1531.33 1529.90 1532.WM_000_ E1 DWDM Basic Principles No.4 194.25 1533.0 195.72 1546.51 1548.33 1530.92 1545.90 193.32 1545.98 1538.32 1547.1 195. 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 Central Frequency (THz) 194.50 195.90 195. 1 2 3 4 5 6 7 8 9 10 11 12 13 Central Frequency THz 196.7 194.9 195.16 1529.56 1539. 50 194.53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 Wavelength No.17 1558.20 193.55 192.30 193.94 1555.55 1556.20 192.14 1542.35 193.35 195.37 1539.22 1536.61 23 .20 195.72 1552.45 192.64 1535.04 1535.12 1552.45 194.90 192. 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 Central Frequency THz 195.39 1559.33 1543.25 193.94 1543.80 192.00 192.82 1536.54 1554.32 1551.33 1553.10 Nominal Central Wavelength nm 1550.12 1550.90 194.92 1551.50 192.75 192.15 195.40 195.75 1556.75 1542.35 1541.40 1537.58 1558. Nominal Central Frequency THz 193.25 192.95 192.61 1537.98 1539.79 1560.56 1540.25 1534.96 1557.30 195.95 1541.43 1535.35 192.16 1540.77 1558.73 1544.65 192.36 1557.25 194.70 192.15 193.34 1555.70 194.40 193.30 192.55 194.10 195.05 193.75 194.20 194.98 1559.52 1550.65 194.35 194.73 1554.25 195.60 192.85 194.00 194.40 194.05 195.52 1552.54 1542.Chapter 2 Overview of Optical Fiber Communication Nominal Wavelength No.30 194.10 193.60 194.93 1553.58 1538.95 194.15 1556.85 192.10 Nominal Central Wavelength nm 1534.15 192.00 1537.15 194.77 1540.40 192.13 1554.79 1538.20 1560.19 1538.80 194.13 1544. 46 1587. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 Central Frequency THz 190.73 1589.68 1592.45 187.05 188.10 1579.04 1598.85 24 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 Wavelength No.90 188.93 1580.60 1582.83 1571. The wavelength allocation for L/L+ band 80-wavelength system with wavelength interval of 50 GHz is shown in Table 2.70 190.65 190.65 1572.13 1574.27 1578.86 1578.50 187.25 188.60 187.45 190.89 1599.22 1594.79 1594.06 1595.04 1587.30 188.40 188.48 1572.60 189.75 188.85 189.37 1575.83 1591.80 189.70 187.75 190.2-3 Wavelength Distribution of 80CH/50 GHz Interval on L/L+ Band Nominal Central Wavelength nm 1570.06 1572. Nominal Central Frequency THz 188.10 190.40 .78 1576.95 1593.20 190.26 1591.80 188.20 1576.91 1596.05 190.52 1592.00 189.18 1581.60 188.35 188.50 188.34 1596.03 1577.WM_000_ E1 DWDM Basic Principles 3.15 188.47 1598.41 1590.88 1588.35 1580.85 187.55 189.15 1589.45 188.50 189.90 189.42 1570.30 1588.00 187.71 1574.65 187.57 1589.75 189.55 188.24 1571.52 1579.54 1574.65 188.99 1590.60 190.30 1573.65 189.95 189.77 1581.70 188.35 190.40 190.75 187.30 190.89 1573.10 188.45 189.85 188.2-3.20 188.40 Nominal Central Wavelength nm 1587.50 190.02 1582.62 1598.19 1597.76 1597.85 190.49 1595.44 1582.70 189.15 190.44 1577.37 1593.90 190.75 Nominal Wavelength No.80 190. Table 2.32 1599.95 1575.61 1577.55 190.80 187.90 187.25 190.55 187.10 1592.69 1579.95 187.64 1595. 10 189. absorption loss of material impurity (especially the loss caused by the remained OH component in the fiber).35 189. and dispersion loss due to incomplete fiber structure. The fiber attenuation spectrum is shown in Fig. Nominal Central Frequency THz 187. 2) The fiber additional loss caused by optical cable layout.36 1585.17 1600.95 1585.Chapter 2 Overview of Optical Fiber Communication Nominal Wavelength No. and the one in Window III is 0. the optical power transmitted in the fiber will attenuate by index with the increase of transmission distance.1-3.46 1601. The 1380 nm point in Window V exist OH absorption peak.03 1601.25 189.00 188.20 1586.3 dB/km ~ 0.3 Fiber Transmission Features 2. 1.00 186.19 dB/km ~ 0.15 187.62 72 73 74 75 76 77 78 79 80 Wavelength No. the one in Window II is 0.10 187.31 1602.11 1584. Due to existence of fiber loss.15 189.25 187.05 189.95 Nominal Central Wavelength nm 1600. This aspect involves bending loss and minor bending loss of optical fiber/cable. and coupling loss between optical components. 25 .05 187.30 189. Raileigh dispersion loss.17 1603.30 187.95 Nominal Central Wavelength nm 1583.25 dB/km. 2.4 dB/km.74 1602. fiber connection and system coupling/connection in all kinds of environment.20 189.3.20 187.57 2. The average loss in Window I is 2 dB/km. because the fibers are bundled into cable. 32 33 34 35 36 37 38 39 40 Central Frequency THz 189. connection loss in the fiber line.1 Fiber Loss Power transmission loss is a basic and important parameter of the fiber.35 187.60 1601.27 1583.53 1584. Generation of fiber loss and low-loss window The fiber loss covers two aspects: 1) Loss coming from fiber.69 1584.88 1602.78 1586. including inherent absorption loss of fiber materials. sectional loss (dB) NF: Noise index of the EDFA. N: Number of the EDFAs between optical multiplexer and optical de-multiplexer of the WDM system.NF . greater line loss leads to lower OSNR. the OSNR at the receiving end. the pulse wave shape spreads by time at the fiber output end. The formula shows this: When the other parameters keep unchanged.25 dB/km 1550 nm G.10logN Here. Q value and BER should also be considered. 2. the calculation formula is: OSNR = Pout – 10logM . Relationship between fiber loss and OSNR OSNR means the ratio between optical signal power and noise power. The design is qualified only when these three factors are qualified.652 0.19 dB/km ~ 0. except loss limit and dispersion limit.15 dB/km dB/km ~ 0. Pout: In-fiber optical power (dBm).655 Typical loss value (1310 nm) Typical loss value (1550 nm) Working window 1310 nm and 1550 nm 2.25dB/km 1550 nm SMF Loss G.3 dB/km ~ 0.2 Dispersion After the optical pulse signals entering the fiber through input end are transmitted for a long distance.19 dB/km ~ 0.4 dB/km 0. engineering design and maintenance. In the initial design of the DWDM. which means decreased transmission quality of the optical line. The line loss values of the common fibers are shown in Table 2. Table 2.653 0. M: Number of multiplexing channels of the WDM system L: Loss between any two optical amplifiers. Take the OSNR at the receiving end of the DWDM system as an example. this 26 .3-1.3-1 Fiber Type G. that is.WM_000_ E1 DWDM Basic Principles 2.3.25 0. It is very important for estimating and measuring bit error performance.L + 58 . 1 Chroma Dispersion 1. We take the dispersion in the SMF as an example.Chapter 2 Overview of Optical Fiber Communication phenomenon is called dispersion. as shown in Fig. This dispersion is related to the waveguide effect of fiber structure. 2. fiber material. 1) Material dispersion: The quartz glass.3-1. According to the dispersion calculation formula. as well as their influences to the DWDM system.3-1 Dispersion in Fiber Dispersion will cause inter-symbol interference. so it is also called structure dispersion. In the area with negligible chroma dispersion. While the light source has certain spectrum width. 2) Waveguide dispersion: For a transmission mode of the fiber. Brief introduction to chroma dispersion Chroma dispersion is divided into material dispersion and waveguide dispersion. the material dispersion at a specific wavelength 27 .3. has different refractive index for different optical wavelengths. Material dispersion is greater than waveguide dispersion. the polarization mode dispersion is the major part of SMF dispersion. The phenomena and causes of chroma dispersion and polarization mode dispersion will be introduced below. 2. and is called chroma dispersion. and different wavelengths results in different group rates. the pulse spreading occurs due to different group rates in different optical wavelengths. affects correct judgment of optical pulse signal at the receiving end. 2. The dispersion in the SMF is caused by different transmission rates of different frequency components in the optical signal. deteriorates BER performance and even affects information transmission.2. so the optical pulse spreading will occur. Optical power SMF Optical power Incoming optical pulse waveform Time Time Outgoing optical pulse waveform Fig. Influence of chroma dispersion Chroma dispersion will result in pulse spreading and chirp effect. In the normal dispersion fiber. it is required to use the DCF with negative wavelength dispersion to compensate the dispersion and reduce the total dispersion value of the whole transmission value. 1) Pulse spreading Pulse spreading is the major influence of chroma dispersion to system performance. Removing influence of chroma dispersion to DWDM system Since the DWDM system is mostly used in the 1550 nm window.WM_000_ E1 DWDM Basic Principles may be zero. In the transmission line. Normal transmission can be ensured when the generated dispersion is within the tolerance. 2. At this time. and this wavelength is called the zero dispersion wavelength of the material. Due to chirp effect. When the transmission distance is longer than the fiber dispersion length. 3. there is the tolerable maximum dispersion value (that is. the low-frequency component of the pulse is located at the rear edge of the pulse and the high-frequency component is located at the front edge of the pulse. Luckily. we can properly use these two fibers to offset the chirp effect and remove the pulse dispersion spreading. Such phase modulation makes different parts of the pulse make different offset from the central frequency with different frequencies. In the abnormal dispersion fiber. the high-frequency component of the pulse is located at the rear edge of the pulse and the low-frequency component is located at the front edge of the pulse. the system will have serious inter-symbol interference and bit errors. For example. G. dispersion tolerance). this wavelength is in the low-loss window near 1310 nm. if G. the fiber is divided into normal dispersion fiber and abnormal dispersion fiber. the pulse spreading is too large. Although the optical components are heavily affected by the dispersion.652 fiber is used. 28 .652 fiber is the zero dispersion fiber. 2) Chirp effect Dispersion not only results in pulse spreading but also makes pulse generate phase modulation. which is called chirp effect of pulse. Chapter 2 Overview of Optical Fiber Communication 2. the transmitting optical power is low and the fiber has linear transmission feature. the PMD will result in pulse separation and pulse spreading.2. 2. the fiber has non-linear effect after the EDFA is used. the PMD can almost be omitted. Compared with other dispersions. Usually. Due to geometrical and pressure asymmetry. 29 . The narrower the pulse in the super-speed system is.3-2. Optical fiber Incoming light Outgoing light Delay Fig. which will lead to additional attenuation of the fiber communication system as well as restriction of optical power. EDFA amplifying performance and current-free regenerative relay distance. But. In the ideal case.2 Polarization Mode Dispersion (PMD) PMD is a kind of physical phenomenon existing in optical fiber and optical component fields. The baseband in the SMF has two polarization modes that are orthogonal. two polarization modes should have the same feature curve and transmission characteristics.3 Non-Linear Effect of Fiber In the common fiber communications system. 2. degrade transmission signal and limit transmission rate of carriers. But it cannot be totally extinguished. the greater the PMD influence is. 2. for the DWDM system.3. as shown in Fig. but can be minimized through optical components. the unit of PMD is ps/km1/2. The non-linear effect of the fiber will result in serious crosstalk between multi-wavelength channels of the DWDM system.3-2 PMD in SMF In the digital transmission system. two polarization modes have different transmission rates.3. resulting in delay and PMD. 652/G. the SPM will result in greater dispersion limit distance.653 fiber or working area with negative dispersion of G. 2. 1. Four Wave Mixing (FWM). Stimulated Raman Scattering (SRS) and Stimulated Brillouin Scattering (SBC). which is called SPM. 2. the SPM will result in smaller dispersion limit distance. the long wavelength area of G. Cross-Phase Modulation (XPM). SPM Due to dependency relationship between refractive index and light intensity. as shown in Fig.655 fiber) with negative dispersion index. refractive index changes during optical pulse continuance.653 fiber or working area with positive dispersion of G. with the pulse peak phase delayed for both front and rear edges. With more transmission distance. As a result. the spectrum spreading results in pulse spreading. the phase shift is accumulated continuously and represents large phase modulation upon certain distance.3-3.WM_000_ E1 DWDM Basic Principles The non-linear effect involves Self-Phase Modulation (SPM).3-3 SPM When the system works in the fiber working area (for example. the short wavelength area of G. When the system works in the fiber working area (for example.655 fiber) with positive dispersion index. 30 . Intensity Pulse width before transmission Optical spectrum before transmission Intensity Pulse width after transmission Optical spectrum after transmission Fig. FWM FWM refers to a physical process of energy exchange between multiple optical carriers caused by non-linear effect of the fiber. due to FWM. when multiple frequencies of optical carriers with high power are simultaneously transmitted in the fiber. the XPM will also affect wave shape and spectrum of pulse. As shown in Fig. which is called XPM. XPM When two or more optical waves with different frequencies are simultaneously transmitted in the non-linear media. 2. resulting in non-linear phase modulation of the optical wave with other frequencies. FWM results in optical signal energy attenuation in multiplexing channels and channel crosstalk. 3. 2. In addition the low-dispersion fiber can also reduce the influence of SPM to the system performance. a new optical wave appears on another wavelength. Meanwhile. the mixing 31 . Adding dispersion properly can reduce the XPM influence. leading to unstable polarization of the fiber transmission. Incoming light Outgoing light New optical wave Fig. XPM will result in a series of non-linear effects. such as signal interference between DWDM system channels and non-linear dual-refraction of the fiber. 2.Chapter 2 Overview of Optical Fiber Communication The SPM effect occurs in certain distance from the transmitter end. Along with dispersion increase.3-4 FWM The generation of FWM is related to the fiber dispersion. the amplitude modulation of each frequency wave will result in the corresponding change of the fiber refractive index. For zero dispersion.3-4. XPM often occurs along with SPM. the mixing efficiency is the highest. 32 . It comes of mutual action and energy exchange between photon and acoustic phonon (crystal vibration status).3-5. along with increase of system transmission rate. On the other hand. making distributed Raman amplifier based on Raman gain. So. The DWDM system uses G. SRS effect can be used to make fiber Brillouin laser and amplifier.. SBS will result in unstable signal light source and crosstalk between reverse transmission channels. SBS will not greatly affect the high-speed fiber transmission system.. The DRA board of ZTE DWDM device implements the optical amplifying function through the SRS effect. the SBS peak gain obviously reduces. SBS SBS belongs to the stimulated non-elastic scattering process caused by non-linear effect. However.3-5 SRS λ1 λ2 λ3 λ SRS effect is widely applied in the fiber communication. SRS introduces in negative influence to the communication system. for example. Outgoing light Power . It comes of mutual action and energy exchange between photon and optical phonon (molecular vibration status). 2. 4. light in the short-wavelength channel will serve as pump light to transfer energy to the long-wavelength channel. 5.WM_000_ E1 DWDM Basic Principles efficiency reduces fast. resulting in Raman crosstalk between channels. 2. λ1 λ2 λ3 λ Fig. SRS effect results in attenuation of signals with short wavelength and reinforcement of signals with long wavelength. as shown in Fig.. Incoming light Power .. In the DWDM system. to provide distributed broadband amplifying for optical signals.655 fiber to slider over the FWM effect in 1550 nm zero-dispersion wavelength area. SRS SRS belongs to the stimulated non-elastic scattering process caused by non-linear effect. On the other hand. 0 ps/nm·km ~ 3. The attenuation of the full-wave fiber becomes even at the band of 1310 nm~ 1600 nm.ions are already eliminated.0 ps/nm·km. water peak free fiber. Real-wave fiber Real-wave fiber is a kind of non-zero dispersion shifted single-mode fiber (G.15 dB/km ~ 0. with average loss as 0. The available wavelength range is about 1.4 New Optical Fiber Types This section will briefly introduce features and applications of some new-type fibers. It is applicable to the long/medium-distance optical transmission system. the dispersion index is 2. In this way. which is less than those of the other types of fibers. Its fiber characteristics are similar to those of G. It is applicable to large-capacity optical transmission system to reduce network construction cost.5 times of the wavelength range of ordinary fibers.ions near the 1385 nm wavelength and thus also eliminates the appended water peak attenuation caused by the OH.Chapter 2 Overview of Optical Fiber Communication 2. 1. Its zero dispersion point is in short-wavelength area below 1530 nm. The zero dispersion point is still in the 1310 nm window. capable of tolerating higher non-linear effect. no water peak attenuation will occur even when the fiber is exposed to hydrogen gas and the long-term attenuation is reliable. Full-wave fiber is numbered as G.655 fiber.652 fiber.19 dB/km.654 optical fiber The G. 2. eliminates OH. Its full name is wavelength-expanded dispersion non-shifted single-mode fiber. 3.655 fiber) widely used at present. the fiber attenuation is only determined by the internal scattering loss of the silicon glass. Full-wave fiber The full-wave fiber.652 C&D in ITU-T Recommendations. As internal OH.654 fiber works in the 1550 nm window. Full-wave optical fiber can provide a complete transmission band from 1280 nm to 1625 nm.ions. It has small dispersion slope and dispersion coefficient. G. 33 . In 1549 nm ~ 1561 nm band. It is one kind of G. 655 fiber). it improves non-linear resistance capability of the system. Fiber with large effective fiber core area It also belongs to non-zero dispersion shifted single-mode fiber (G. Larger effective area means high affordable optical power. The super-speed system performance is mostly limited by dispersion and non-linear effect. But the non-linear effect cannot be distinguished only through linear compensation. better resistance to non-linear effect.WM_000_ E1 DWDM Basic Principles 4. The effective area of the fiber determines the fiber non-linear effect. 34 . that is. Usually. dispersion can be distinguished through dispersion compensation. Essentially. Supervision technology. The basic structure of a unidirectional DWDM system is shown in Fig... with each optical channel bearing one service signal. 3. Optical transmitter end 35 .692 Optical transponder Optical transponder Optical transponder Optical receiver Optical relay amplifier λ1 λ2 λ1 λ2 OM OBA OLA OPA OD RX1 Optical transponder Optical transponder Optical transponder RX2 RX3 λ3 λn λ3 λn TXn . Optical amplifying technology.. 3. Light source technology. 3.1-1. OTU technology. Optical transponder RXn Receiver/transmitter of optical supervision channel Transmitter of optical supervision channel Receiver of optical supervision channel Fig. Optical wavelength division multiplexing and de-multiplexing technologies. Optical transmitter TX1 TX2 TX3 G.. Optical transponder .1-1 Composition of DWDM System 1.1 Basic Structure of DWDM System The DWDM system multiplexes several or dozens of optical channel signals with different nominal wavelengths to one fiber for transmission.3 Key Technologies of DWDM System Highlights: z z z z z z Basic structure of the DWDM system. 3. The information in the NMS is borne by the monitoring signals in the optical monitoring channel. 2. the semi-conductor optical sources widely used now are Laser Device (LD) and Light-Emitting Diode (LED). The DWDM NMS should be capable of managing optical amplifying units (such as OBA. After that. for the sake of effectively management of network management system over the DWDM system. wavelength division multiplexer. 4. Type of optical sources At present. 3. 3. which will be output by the OBA to the fiber for transmission. the optical channel signals are de-multiplexed by the optical de-multiplexer and then respectively input to the corresponding multiplexing channel optical receivers. such as standard SDH signal. Optical regenerating amplifier end Located in the middle of the optical transmission section. the optical multiplexer combines these signals into one beam of optical wave. RX1…RXn. The optical monitoring signals are used to bear NE management and monitoring information of the DWDM system. fault. 36 . Each optical channel bears different service signals. the optical transmitters of all the multiplexing channels. ATM signal and Ethernet signal.2 Light Source Technology 1. It can manage the device in terms of performance. Network management system This module is omitted in Fig. 3. configuration and security. Optical monitoring channel In the DWDM system shown in Fig.1-1. respectively transmit the optical signals (λ1. Optical Transponder Unit (OTU) and channel performance supervision on one platform. λ2 …λn. Optical receiver end After the line fiber goes through amplifying of OPA. OLA and OPA). it uses OLA to amplify the optical signals. f2…fn) with different nominal wavelengths. with the corresponding frequencies as f1.WM_000_ E1 DWDM Basic Principles TX1…TXn.1-1. 5. an independent wavelength (1510 nm) is used as the optical monitoring channel for transmitting optical monitoring signals. 2. Modulation modes of DWDM system laser At present. It is applicable to the long-distance high-speed system. the dispersion influence is larger. minimizing spectral width of light source devices is an effective measure for solving dispersion limit. When the electrical pulse signal is "0". with small in-fiber power. external modulation). For example. with large in-fiber power. It is applicable to short-distance low-speed system. After the optical cables are laid. when the electrical pulse signal is "1". 1) Features of DWDM system light source Providing standard and stable wavelength The DWDM system has very strict requirements for the working wavelength of each multiplexing channel. large spectral line width and low modulation rate. The LED is non-coherence light source. dispersion limit can be solved through using optical fibers/cables with small dispersion coefficient or semi-conductor laser with small spectral width. the working current of the laser is smaller than its current threshold. and thus making it generate the optical pulse stream corresponding to the electrical signal pulse. Along with increased transmission rate. The common wavelength stabilization measures are temperature feedback control method and wavelength feedback control method. small spectral line width and high modulation rate. there are two methods of light source intensity modulation: Direct modulation and indirect modulation (that is. 2) Providing rather large dispersion tolerance Fiber transmission may be limited by system loss and dispersion.Chapter 3 Key Technologies of DWDM System LD is coherence light source. Wavelength drift will cause unstable and unreliable operation of the system. therefore it does not generate optical pulse. the working current of the laser is greater than its current threshold. 37 . The light source of the DWDM system adopts the semi-conductor laser. therefore it generates an optical pulse. 1) Direct modulation Direct modulation means directly controlling the working current of semi-conductor laser through electrical pulse code stream. Here. 3. z Waveguide M-Z modulator At the input end. and thus making optical pulse stream under indirect control of electrical pulse signal stream. in external modulation case. low driving power and low power consumption. At present. the external modulators often used are electrical absorption modulator (EA) and waveguide Mach-Zehnder (MZ) modulator. high integration. Therefore. But.5 Gbit/s. when the 38 . it performs phase modulation to the optical signals. with low loss and low cost.It is applicable to the long-distance transmission system at rate over 2. The direct modulation mode is often used in the transmission system composed of G. The optical wave emitted by it is divided by the optical de-multiplexer into two equal signal channels. the CW is in continuous wave working status. z EA modulator It uses absorber controlled by electrical pulse signal to absorb or not absorb the optical wave transmitted by the continuous wave semi-conductor laser (CW). the super-speed change of working current of the laser will make modulation chirp easily. the optical multiplexer has no optical signal output. The EA light source features small size. And chirp will limit transmission rate and distance of the system. The maximum dispersion can reach 12 000 ps/nm.5 Gbit/s. Under control of electrical pulse stream. the laser nit generates stable high-power laser. and thus obtaining optical pulse stream. At the output end. 2) Indirect modulation (external modulation) The external modulation mode refers to indirectly control (modulate) the continuous light generated by the laser which is in the continuous light emitting status.WM_000_ E1 DWDM Basic Principles The direct modulation mode is simple.652 fiber. two optical tributaries are combined by the optical multiplexer. to obtain the maximum dispersion value much greater than that in the case of direct modulation. with transmission shorter than 100 km and rate lower than 2. and the external modulator will modulate it in low chirp. which will respectively enter two optical tributaries of the modulator. When the signal phases in two optical tributaries are reverse to each other. 1) Brief introduction to common OMs Grating type of OM 39 . that is. and large extinction ratio. which is actually a kind of optical filter. large maximum dispersion value. Since the performance of OM and OD determine the system transmission quality.3 Optical Wavelength Division Multiplexing and De-Multiplexing Technologies 3. offset and channel crosstalk of OM and OD must be small values. the Optical De-multiplexer (OD) is used to divide optical wave in the fiber into optical signals of each multiplexing channel with formal nominal wavelength. The M-Z light source features high modulation rate. as well as OM types often used in the DWDM systems with different wavelength numbers. the attenuation. 1. Its disadvantage is that polarization maintaining fiber must be used to connect the laser and the modulator.3. Its chirp coefficient can be zero in theory. and then transmit it into the fiber for transmission. At the receiving end.1 Overview The optical wavelength division multiplexer and de-multiplexer. also called optical multiplexer and de-multiplexer. belong to optical wavelength division multiplexer. de-multiplexing optical wave.Chapter 3 Key Technologies of DWDM System signal phases in two optical tributaries are the same.3. At the transmitting end. 3. 3. and then input them into the corresponding optical channel receivers. multiplexing optical wave. In this way. that is. the optical multiplexer has optical signal output. the optical pulse stream is controlled by the electrical pulse stream. the Optical Multiplexer (OM) is used to combine the optical signals in each multiplexing channel with nominal wavelength into a beam of optical wave. because modulation status is related to light polarization status.2 Introduction to OM Four types of common OMs are briefly introduced below. 3.WM_000_ E1 DWDM Basic Principles The grating type of OM is an angular dispersion type of device.n λ1 λ2 λ3 λ4 λn Fig.5 nm. 40 . different refractive indexes and different thickness values. Its working principle is shown in Fig. 2) Dielectric thin film type of OM It is composed of Thin Film Filter (TFF). therefore TFF emerges a passband within certain wavelength range while a stopband within other wavelength ranges. The working principle is shown in Fig. λ 1 . Since the optical signals with different wavelengths have different refractive angles on the grating.3-1 Principle of Grating Type of OM It has sound wavelength selection performance.3. However. 3. and thus forming the desired filtering performance. One layer features high refractive index and the other layer features low refractive index.3-1. 3. the grating features precise manufacture requirements and is not suitable for large-batch manufacture.. TFF consists of dozens layers of dielectric films with different materials. it divides and combines the optical signals with different wavelengths.3-2. capable of narrowing wavelength interval to about 0. It is often used in research in the laboratory...2. . it is the best scheme for optical wave multiplexing/de-multiplexing in the optical transport network. Fig. 41 . It is used for OM.Chapter 3 Key Technologies of DWDM System λ 1. 3.n λ1 λ3 λ2 λ4 λ6 Fig. low insertion loss and sound channel interval. Its working principle is shown in Fig.3-2 λ5 λ7 Principle of Dielectric Thin Film Type of OM It is a kind of compact passive optical device with stable structure.3. featuring flat signal passband.2. 3.3-3.3-4..3-3 Principle of AWG OM Due to compact structure and low insertion loss. with the working principle shown in Fig.. 4) Coupling type of OM It is a kind of surface interactive device with two or more fibers which are closed to each other and are properly melted. 3. 3. 3) Array Waveguide OM (AWG) AWG OM is the flat waveguide device based on optical integration technology. 3. Table 3. with low cost and large insertion loss. 3. closely related to resolution and isolation of the device.3-1 Relationship between DWDM Systems and Corresponding Optical Wavelength Division Multiplexers OM Below 32 wavelengths √ √ √ 40 wavelengths √ √ Above 80 wavelengths √ Below 32 wavelengths √ √ OD 40 wavelengths √ √ Above 80 wavelengths √ Type of Optical Wavelength Division Multiplexer Coupling type Array waveguide type Dielectric thin film type Grating type 3.3 Key Performance Indices 1.WM_000_ E1 DWDM Basic Principles λ1 λ2 λ3 λ4 λ5 λ6 λ7 λ8 λ1，2，3…… Fig. Multiplexer/de-multiplexer of DWDM system The relationship between the system with different wavelengths and the corresponding optical wavelength division multiplexers is shown in Table 3. 2. Insertion loss The attenuation effect of wavelength division multiplexer to optical signals directly affects system transmission distance. Multiplexing channel quantity It represents the quantity of optical channels of multiplexing and de-multiplexing made by the optical wavelength multiplexer. 2. 42 .3-1.3-4 Principle of Coupling Type of OM It can only implement multiplexing function. Polarization Dependent Loss (PDL) PDL means the maximum change value of the insertion loss caused by the change of optical wave polarization status. there is the problem of wave vibration direction (polarization). the better is the frequency selection performance of the wavelength division multiplexer. It is meaningful only for the wavelength sensitive devices (TFF type and AWG type devices). The higher the channel isolation is. 3. And the same wavelength division multiplexer has different attenuation effects to the optical waves in different polarization statuses. Smaller coefficient is preferable. Isolation It represents the isolation distance between multiplexing optical channels in the optical devices. Smaller temperature coefficient of the wavelength division multiplexer is preferable. Smaller PDL value is preferable. the ratio between the reflection optical power and incidence optical power is the reflection coefficient. Light is the electromagnetic wave with extremely high frequency. It is not meaningful for the coupling devices. Reflection coefficient At the input end of the wavelength division multiplexer. the crosstalk suppression ratio becomes higher and the mutual interference between multiplexing optical channels becomes lower. their polarization statuses will not be totally consistent. Smaller coefficient means more stable central working frequency of the multiplexing channels. 6. Consequently. Temperature coefficient It means the offset of central working frequency of the multiplexing channel caused by ambient temperature change. 43 . For the optical signals of the multiplexing channels input to the wavelength division multiplexer. 5. therefore. 4. Smaller insertion loss value is preferable.Chapter 3 Key Technologies of DWDM System Different types of wavelength division multiplexers have different insertion loss values. It converts the non-nominal wavelength of the optical channel signal into the nominal optical wavelength in accordance with G. Greater bandpass value is preferable. Sound bandpass feature curve should be flat and wide. Smaller bandwidth value is preferable. It describes the stopband feature of the OD. z Channel width @ -0. OTU also provides other functions: 1.692 stipulation in ITU-T recommendations.5 dB and channel bandwidth @-20 dB. 3.5 dB It refers to the corresponding change value of the working wavelength when the OD insertion loss decreases by 0. Standard and stable light source The DWDM system needs to multiplex multiple wavelengths in a low-loss window. so the central frequency of the DWDM light source must stably work in the nominal central frequency sequence specified by ITU-T standards. z Channel width @ -20 dB It refers to the corresponding change value of the working wavelength when the OD insertion loss decreases by 20 dB. It is not meaningful for the coupling type of wavelength division multiplexer. with small wavelength interval.WM_000_ E1 DWDM Basic Principles 7.4 OTU Technology 3. and then makes it access the DWDM system.5 dB. 44 . Bandwidth It is a parameter of the wavelength sensitive devices (TFF type and AWG type devices). Bandwidth is divided into channel bandwidth @-0.1 Overview The Optical Wavelength Transponder Unit (OTU) technology is used to implement wavelength conversion. The stopband feature curve should be sharp. It describes the bandpass feature of the OD.4. Functioning as regenerator When the transponder serves as the regenerator. 3. if only shaping and timing processing (that is.Chapter 3 Key Technologies of DWDM System 2. divided into C band and L band. G. dispersion and optical transmitting power accord with G. to output DWDM optical channel signals whose wavelength. Light source with rather large dispersion tolerance Increase of current-free relay distance in the DWDM system requires greater dispersion tolerance distance of the light source. it has the data regeneration function. After O/E conversion.4.692 Optical outp O/E E/O Fig. 2. which is an optional function of the OTU. 45 .692 recommendation. timing. 1) Key performance indices System working wavelength area It is in the 1550 nm low-loss window.2 Working Principle and Performance Indices 1. (regeneration) G. and implements shaping. this OTU only implements wavelength conversion function with a small transmission distance. and then performs E/O conversion.4-1. for the sake of solving non-linear effect of the fiber. actually this OTU also has the function of the regeneration repeater (REG).957 recommendation.4-1 Working Principle of OTU OTU performs O/E conversion to the multiplexing optical channel signals which accord with the G. 3. timing processing and regeneration (that is. 3R functions) are implemented. 3. 3.957 ptical input Shaping. After O/E conversion. 2R functions) are implemented. timing extraction and data regeneration (this can be omitted) to the converted electrical signals. Working principle The working principle of the OTU is shown in Fig. if shaping. 2) Channel interval Channel interval means the nominal frequency difference between two adjacent multiplexing channels. For example.10 THz (1 THz = 1000 GHz) z L band (long-wavelength band) Wavelength range: 1565 nm ~ 1625 nm Working frequency: 190.1 THz.95 THz Hint: Usually. z When the multiplexing channels are 16/32/40 wavelengths. the working range of the DWDM system is represented through frequency.90 THz ~ 186. 3) Nominal central frequency It refers to the central wavelength (frequency) corresponding to each multiplexing channel in the DWDM system. even channel interval is used mostly. the channel interval is 200 GHz.05 THz ~ 192. when the multiplexing channels are 16/32/40 wavelengths. Smaller channel interval requires higher resolution of the OD and means more multiplexing channels. The minimum channel interval of the DWDM system is integer times of 50 GHz. z When the multiplexing channels are above 80 wavelengths. covering even channel interval and uneven channel interval. 46 . z When the multiplexing channels are 8 wavelengths. At present. the channel interval is 100 GHz and frequency increases in ascending order. the central frequency of wavelength 1 is 192. the channel interval is 100 GHz. the channel interval is 50 GHz.WM_000_ E1 DWDM Basic Principles z C band (conventional band) Wavelength range: 1530 nm ~ 1565 nm Working frequency: 196. the dispersion tolerance distance of the system light source must be prolonged.5 Gbit/s or ±12. 5) Dispersion tolerance Dispersion reflects the spreading of the optical pulse in the transmission in the fiber. the maximum central frequency offset is ±5 GHz. levels of bit "1" and bit "0" are similar. The requirements of DWDM system for the fiber chroma dispersion coefficient are basically those of a single multiplexing channel rate signal for fiber chroma dispersion coefficient. humidity and other factors taken into consideration. The pulse spreading will be more and more serious along with increased transmission distance. the maximum central frequency offset is ±20 GHz (about ±0. when input signals are located in the 1550 nm window and the BER reaches 10-12.16 nm) when the rate is below 2. it is required to take proper measures to compensate the optical pulse spreading in fiber transmission process. that is. with temperature. 6) Receiver sensitivity The receiver sensitivity refers to the minimum value of the average receiving optical power on the OTU input port. 47 .5 GHz when the rate is 10 Gbit/s. In addition. It refers to the offset between the actual working central frequency of the multiplexing optical channel and nominal central frequency. According to the national standards. leading to mistaken judgment of the receiver. The pulse spreading will result in decreased distinguish ratio of signal pulse at the receiving end. the system with frequency interval as 100 GHz.Chapter 3 Key Technologies of DWDM System 4) Central frequency offset It is also called frequency offset. To avoid bit error. For the system with frequency interval as 50 GHz. The maximum central frequency offset refers to the value which can be met when the designed life cycle of the system expires. since the current-free relay distance of the DWDM system is much greater than that of a single SDH system. Their applications in the transmission system are shown in Fig. when input signals are located in the 1550 nm window and the BER reaches 10-12. 3. 48 . 2R functions) as well as B1 byte detection function. OTUT It is located between customer devices and OM.4-2 OTU Applications 1. 2. 3. λ1 λ2 OTUT OTUT OM OA OA OD OTUG OTUG OM OA OA OD OTUR OTUR λ1 λ2 Line fiber OM: Optical multiplexer OD: Optical de-multiplexe Internal fiber OA: Optical amplifier Fig.4-2. Its input and output signals should accord with G. OTUG It is located between OM and OD. 2R functions and B1 byte detection function.WM_000_ E1 DWDM Basic Principles 7) Overloaded optical power The overloaded optical power refers to the maximum value of the average receiving optical power on the OTU input port. It implements wavelength conversion. OTUR It is located between OD and customer devices.3 Classification and Applications of OTU Depending on the locations in the DWDM system. This kind of OTU not only has E/O and O/E conversion functions. the OTU can be classified into OTUT. The optical signals output from the OD to the OTUR should accord with G.692 standards. 3. OTUR and OTUG.692 standards. The signals output from it to the OM should accord with G.692 standards. This kind of OTU has the similar functions as OTUT functions. 3.4. but also has re-shaping and re-timing functions (that is. for example. re-timing and data regeneration functions (that is. The light source for stimulation is called pump light source. In addition. certain quantity of Er3+ ions are doped to form Erbium Doped Fiber (EDF}.5. Without O/E/O conversion. 3. it is equal to a conventional REG. In the fiber manufacture process. 3R functions). Its classifications are shown in Fig. which is called stimulation.1. It also has B1 byte detection function.1 EDFA Technology Principle 1. 49 . Working principle Erbium (Er) is a lanthanon. for example. PDFA Raman fiber amplifier (SRA) Brillouin fiber amplifier (SBA) { 1550 nm fiber amplifier. Optical amplifier is the technology for solving optical power limit. 3. EDFA Fig.1 EDFA Technology 3. optical power gradually decreases along with transmission distance increase.5-1 Fiber Amplifier Classifications EDFA and Raman fiber amplifier are introduced below. the receiving power at the receiving end must always be certain value. 3.Chapter 3 Key Technologies of DWDM System This kind of OTU not only has E/O and O/E conversion functions. -28 dBm. but also has re-shaping. Therefore. 3. { Semi-conductor OA { Resonance type Progressive wave type Fiber amplifier { Lanthanon doped fiber amplifier Non-linear optical amplifier { 1310 nm fiber amplifier. Therefore. to ensure correct signal receiving.5. The light source output of the laser usually is not over 3 dBm.5 Optical Amplifying Technology For the long-distance optical transmission. otherwise. the optical power limit turns into the major factor determining transmission distance. for example. The Er3+ ions in such fiber will absorb photon energy to make own energy level change. it directly amplifies the optical signals. the laser life cycle may be unqualified.5-1. bump light source. as shown in Fig. therefore they continuously converge to metastable energy level in non-radiant transition format. the Er3+ ions are in instable status.5-2 Working Principle of EDFA The Er3+ ion free from stimulation is at the lowest energy level. 50 . the metastable particles transit to the ground status in stimulated emission format.5-2. and photons which are the same as those in the incoming signal light are generated. Its working principle is shown in Fig. the Er3+ ion absorbs energy of the pump light and transits to the higher energy level. N3~0 1550 nm signal light N2 1550 nm stimulated emission 980 nm pump light N1 1480nm Fig. coupler and isolator. Composition The EDFA consists of EDF. and thus implementing population inversion distribution. 3. At the higher energy level. 3. 3. and thus implementing amplifying of optical signals. 2.5-3.WM_000_ E1 DWDM Basic Principles and the corresponding transmitting stimulation optical wave is called pump light. When the optical signals with 1550 nm wavelength pass this segment of EDF. When the pump light is shot in. λn Isolator WDM coupler Erbium doped fiber Isolator λ1 λ2 . such as signal shot noise. NF is used. 51 .Chapter 3 Key Technologies of DWDM System λ1 λ2 . EDFA noise comes from many factors. 3. It greatly affects the system performance. The isolator is used to suppress light reflection. for example.. to ensure stable working of the optical amplifier.0 dB. & Note: ASE means the emission noise caused by such own factors as unbalance between EDFA optical transmitting area and absorption area. internal reflection noise and Amplified Spontaneous Emission (ASE) noise. Since the EDFA can amplify both optical signals and noise. 2) Noise Fig. But the major source of EDFA noise is ASE noise.5-3 EDFA Composition The coupler is used to combine signal light with pump light. Smaller NF is preferable. below 5. The pump laser generates pump light source. especially the OSNR of the whole system. 1) Key performance indices Gain (G) It is the ratio between output optical signal power and input optical signal power.. different population inversion degrees (quantity of ions in stable energy level E2 and the quantity of ions in ground energy level E1 are different). λn Pin Pout Pump laser Fig. NF value is closely related to the ASE noise of the EDFA. EDFA gain and EDFA working status. (NF) It is the ratio between SNR at EDFA input end and SNR at output end. Greater gain means more powerful amplifying capability... 3. Therefore. In the DWDM system. In the WDM system application. smaller EDFA gain flatness is preferable. to minimize the difference between output optical power signals of different multiplexing channels and facilitate optical power estimation. It is required to determine the proper power according to signal rate and transmission fiber type. one EDFA is responsible for amplifying all the multiplexing optical channel signals in the system. 52 . the EDFA output power cannot too large. On the other hand. 6) Polarization Dependent Gain (PDG) Since the EDFA generates different gains for the optical waves in different polarization statuses. especially for the WDM system with enormous multiplexing channels.WM_000_ E1 DWDM Basic Principles 3) Bandwidth The working wavelength range of the DWDM system covers C band and L band. So. To avoid fiber non-linear effect. More importantly. to ensure EDFA gain flatness and low noise performance. to avoid fiber non-linear effect. Smaller value is preferable. the optical power of a signal channel cannot be too large. In other words. aluminum doped technology is usually used in the EDF. its input/output optical power range should be large enough. we call the maximum change value of the EDFA gain caused by polarization status change of the optical wave as PDG. 4) Gain flatness Gp-p It means the allowed fluctuation of EDFA gain within the specified working band range. 5) Total input/output power range It is the optical power range at input/output end of the EDFA. For the sake of sound flatness. the DEFA should work in small signal working range. so its bandwidth should be wide enough. The optical amplifier needs to amplify all the multiplexing channel signals of the system. the input/output power range of the EDFA cannot be too large. 1) BA: It is located behind the OTM or transmitting light source of relay device. 3. It is used to directly insert the EDFA into the fiber transmission link for amplifying signals. 1. flat gain. Importance of EDFA for DWDM system To ensure the transmission quality of DWDM system. Proper gain flatness is especially important. 4.1. and in the front of the relay segment. Line Amplifier (LA) and Pre-Amplifier (PA). two EDFA classification modes are introduced below. By location By locations in the optical transport network. a few pump light leakage occurs. Smaller leakage is preferable. low NF and high output power. 8) Input/output optical reflectance It is the ratio between optical power at the EDFA input/output end and reflection optical power. Greater value is preferable. It is used to pre-amplify the small signals going through line attenuation. 2) LA: It is located in the middle of the relay segment.Chapter 3 Key Technologies of DWDM System 7) Pump light leakage Although optical isolators are set at input and output ends of the EDFA. 53 . the EDFA used in the DWDM system must have sufficient bandwidth. Pump light leakage means the ratio between pump light leakage power and input/output pump light power. to boost the power of optical signals entering the receiver and meet the sensitivity requirements of the receiver. A relay segment can be equipped with multiple LAs as required.5.2 EDFA Classifications Depending on locations of EDFA in the DWDM system and pump source types. 3) PA: It is located at the relay segment end and ahead of the optical receiving device. which is special requirement of DWDM system for EDFA. It is used to boot the transmitting power for the sake of extending transmission distance. the EDFA is divided into Booster Amplifier (BA). which is unnecessary for boosting EDFA output power.5-4. If such WDM system uses the 1480 nm pump source. In the actual LA applications.5. because these two types of pump sources have high pump efficiency.1. 3. However.WM_000_ E1 DWDM Basic Principles The locations of all kinds of amplifiers in the optical line are shown in Fig. 54 . When the optical power is increased to certain degree. Therefore. but optical power increase should be proper. the pump sources often used cover 980 nm and 1480 nm. it is required to control the value of the in-fiber optical power in a single channel. because enormous tributaries dwindle the available power range and the pump source with higher power is necessary. The 980 nm pump light source has lower NF. 3. the EDFA also introduces in some new problems. for the sake of both NF improvement and output power increase. 1. By pump source At present. the above-16-channel WDM system uses the 1480 nm pump source. the 1480 nm one has higher NF. Non-linear effect EDFA amplifies the optical power through increasing the optical power shot into the fiber. the system power attenuation will increase. Relay segment OTM BA LA LA PA OTM Fig. so a larger output power is obtainable (about 3 dB higher than that of the 980 nm pump light source). most 8-channel WDM system uses the 980 nm pump source.5-4 Locations of all Kinds of Amplifiers in Relay Segment 2. 3. because the WDM system of G. fiber non-linear effect will occur. The two-level pump can also be used.652 fiber mostly features dispersion limit other than loss limit.3 Problems of EDFA to Be Solved When solving the problems of fiber transmission system. in the usage of fiber amplifier. ±1 dB. Working principle The Raman fiber amplifier uses the gain mechanism generated by non-linear SRS in the fiber to amplify the optical signals. and thus suppressing surge.Chapter 3 Key Technologies of DWDM System 2. 3. Bandwidth Bandwidth refers to the range of the optical wavelength which can be amplified flatly. The SRS converts the energy of short-wavelength pump light into the energy of long-wavelength signal light. 4. The gain fluctuation should be limited within the allowed rage. 55 . and the one of the EDFA in L band is 1565 nm ~ 1625 nm. In other words. the current-free relay segment in the WDM system cannot be prolonged limitlessly. Optical surge When the optical line is normal. Therefore. the EDFA will automatically reduce power or shut down power upon no input light. leading to optical surge. Therefore.5. 3. for example. The way of solving optical surge is to implement Automatic Power Reduction (APR) or Automatic Power ShutDown (APSD) function in the EDFA. the bandwidth is closely related to the gain flatness. Dispersion Along with transmission distance increase. The gain flatness filter is used inside the EDFA. the total dispersion increases correspondingly. so that the EDFA has almost the same gain for each multiplexing optical channel signal within the corresponding wavelength range. The working wavelength range of the EDFA in C band is 1530 nm ~ 1561 nm. the metastable erbium ions still converge continuously.2 Raman Amplifying Technology 1. We can prolong the current-free relay distance of the multiplexing section through dispersion compensation measures. to amplify the signal light. the erbium ions stimulated by the pump light are carried off by the signal light. so energy transient will occur. and thus implementing amplifying of signal light. If the input light is interrupted. Compared with EDFA. Application If the DWDM system above 40 G only uses EDFA for amplifying. 4) 5) It has flat gain. the combination of EDFA and SRA can form the important optical amplifying technology for the transmission system above 40 G or of super great distance. Since the noise of Raman fiber amplifier reduces along with fiber distance increase. and no transient effect. the SRA has such advantages as low noise. the monitoring system is designed into the independent system separated from working channels and devices. spontaneous emission will be accumulated. 2) With the same SNR. 7) The amplifying gain is low. for the sake of compensation of line attenuation and node insertion loss. the fiber should be long enough.WM_000_ E1 DWDM Basic Principles 2. 6) The pump conversion efficiency is low. 1) Features Based on dozens of kilometers of line fibers. 3) It can generate gain for all the wavelengths. with low NF and effectively improve system SNR. The gain wavelength range depends on the pump wavelength. 3. 56 . There is no requirement for the fiber type. physically. it can reduce the optical power at the transmitting end and minimize the non-linear effect. control and management are basic requirements of all the network operations. Therefore. so the high-power pump laser source is required. it implements distributed amplifying. 3. serving as full-band amplifier (required to be divided into C band amplifier and L band amplifier). so it needs to work with the EDFA to form combined amplifier. restricting overall system performance. To ensure secure operation of the DWDM system.6 Supervision Technology Detection. introducing in no additional loss upon removal of pump light. 3.6. ZTE DWDM system uses an independent wavelength (1510 nm) and relays on no service channel. 57 . The supervision information transmitted on the OSC is the information related to all kinds of optical amplifiers.Chapter 3 Key Technologies of DWDM System For example. Therefore. such as input/output optical power of the optical amplifier and working wavelength of pump light source. it has no electrical interface connection because there is no add/drop of service signals. At this time. The OSC cannot restrict the services on the 1310 wavelength in the future. and thus eventually implementing monitoring over the NE devices of the system. there is no special byte for monitoring the EDFA in the SDH overhead. The OSC cannot restrict the optical wavelengths (980 nm and 1480 nm) of the pump light source in the optical amplifier. the OSC cannot work normally. involving fault alarm. so an electrical signal must be added to monitor the EDFA status. leading to higher difficulty of monitoring on it. 3. the DWDM system with optical amplifier can monitor and management the EDFA. control over backup line upon line interruption and EDFA supervision. 3. The OSC should still be available upon failure of the LA. Since the EDFA only amplifies optical signals without electrical signal input. when the optical amplifier is failed. In addition.2 Requirements for OSC The DWDM system has the following requirements for the OSC: 1. 4.6. the subversion is meaningless.1 Functions of Optical Supervision Channel (OSC) Different from the conventional SDH system. quality parameter supervision in the operation. The OSC is used to transmit the NE management and supervision information related to the DWDM system on a long wavelength. the network operator can effectively manage the DWDM system. fault location. to ensure that no active amplifying is required for the long distance transmission and reliability is improved. In this way. 2. Especially when it uses as the optical amplifier regenerator. The OSC cannot restrict the transmission distance between two LAs. it is required to use a 2-wavelength OM OM2 behind the OBA to add the OSC information into the main channel. at the receiving end. Working wavelength of OSC For the DWDM system working with LA. 58 . at the transmitting end. Dropping and adding of OSC information As shown in the above diagram.3 Implementation of OSC The implementation principle of OSC is shown in Fig. 3. λ1 λ2 OTUT OTUT OM OBA OM2 λosc OTUR OD2 λosc λ1 λ2 OPA OD OTUR OSC information Line fiber OM: Optical multiplexer OD: Optical de-multiplexer OSC information Internal fiber OBA and OPA: Optical amplifiers OTUT and OTUR: Optical transponders Fig. OSC transmission segment can be dropped on each optical amplifier relay station and DWDM system office station and added with new supervision signals. to ensure that the supervision information transmitted on the OSC can be dropped or added on each optical amplifier relay station and DWDM system office station without influence from optical amplifier.6. The OSC transmission is bidirectional.WM_000_ E1 DWDM Basic Principles 5. Bidirectional transmission ensures the supervision information can be received by the line terminal when one fiber is broken. it is required to use a 2-wavelength OD OD2 ahead of the OPA to drop the OSC information. 2. 3. 6. which should be able to perform adding/dropping with low enough BER in each optical relay/amplifier. an additional OSC is required. 3.6-1.6-1 Implementation Principle of OSC 1. 6-1. For example. The 10/100 Mbit/s OSC uses 4B/5B code. the receiving sensitivity should be high. Since this channel is out of the gain bandwidth of the EDFA (also called external OSC).480 nm or 1. the working rate of the OSC is set to 2 Mbit/s. Line coding The 2 Mbit/s OSC uses Code Mark Inversion (CMI) as the line code type. to ensure normal operation of the supervision channel upon optical amplifier fault.510 nm when out of service information transmission band. the supervision signals must be drop (dropping optical channel) ahead of EDFA and added (adding optical channel) behind the EDFA. a specific wavelength can be used as the OSC. ZTE DWDM can provide supervision rate of 10 Mbit/s or 100 Mbit/s. which is transmitted and exchanged in PCM32 frame format. 5. most of the information really requiring supervision involves EDFA working status. OSC rate improves as well. Frame structure of OSC information For the supervision system at working rate of 2 Mbit/s. Transmission rate of OSC In the actual DWDM system. 4. since the technology ceaselessly develops. Therefore. for example. ZTE DWDM.510 nm is preferable. so the supervision information is not huge.310 nm. For the system at supervision rate of 10 Mbit/s or 100 Mbit/s. The 1. 59 . As shown in Fig. thirty-two 64 kbit/s bytes are used to bear supervision information. 3. OSC is added behind OBA and dropped ahead of OPA. the supervision channel signals without amplifying of optical amplifier can cover the maximum transmission distance of the service major signals. In addition. the supervision channel uses 10/100 M Ethernet technology to encapsulate data in IP packet format and transmits and exchanges them in Ethernet data frame. 3. As a result. At present. Such wavelength can be 1.Chapter 3 Key Technologies of DWDM System According to ITU-T recommendation. 1. WM_000_ E1 DWDM Basic Principles 6. 60 . At this time. for the sake of OSC protection. Data Communication Network (DCN)) should be used for transmitting supervision information. OSC protection Upon the OSC bidirectional transmission interruption caused by totally break-off of the fiber. the NE management system cannot obtain the supervision information normally. the backup route (for example. 1 Brief Introduction to DWDM System Hierarchy & Note: In this chapter. and their functions are listed in Table 4. The DWDM system is divided into Optical Multiplexing Section (OMS) layer.4 Protection Principle of DWDM System Highlights: z z z z Principle of 1+1 protection. 4. 4.. First of all.1-1 Hierarchy of DWDM System 61 .1-1 OTM TX1 TX2 TX3 G.692 Optical transponder Optical transponder Optical transponder OTM OLA λ1 λ2 λ3 λn OTS OMS OTS OM OBA OLA OPA OD λ1 λ2 λ3 λn RX1 Optical transponder Optical transponder Optical transponder RX2 RX3 TXn . we will introduce the location of each layer in the system. The DWDM system protection involves protection of optical channel layer and optical MS layer. Principle of optical multiplexing section protection. Principle of optical channel protection. Optical transponder . 4.1-1. Principle of 1:N protection. we take ZTE DWDM as an example for introduction. Optical Channel (OCH) layer and Optical Access (OAC) layer.... Optical Transport (OTS) layer. The locations of the layers in the system are shown in Fig. Optical transponder RXn OAC OCH OAC Fig. Channel 1+1 protection in the chain networking can only protect device other than route. The protection channel and protected channel are transmitted in the same fiber.1 Link 1+1 Protection Depending on locations. In channel 1+1 protection. So this protection mode is called "concurrent transmitting and priority receiving". the number of OP boards configured must be consistent with that of the channels to be protected. For ZTE DWDM.1-1 Layer OMS OTS Hierarchy Meaning of DWDM System Function Multiplexing optical channel signals and de-multiplexing multiplexed optical channel signals Transmitting optical signals on all kinds of fibers Supporting OAC to convert customer signals into optical signals in accordance with G. the protection function monitors the statuses of the signals received from these two lines and selectively connects with the line with better signal quality. In other words. and between OLAs At the line side of optical OCH transponder platform At the client side of optical transponder platform OAC 4. 4. 62 .692 recommendation for transmission Accessing customer signals Location Between OTMs Between OTM and OLA. the optical signals are simultaneously transmitted on working line and protection line. as shown in Fig. 1. 4.2-1. the OP board can implement 1+1 protection of OCH or OMS.2. the signals are permanently connected (bridged) with working line and protection at the transmitting end. 1+1 protection of OCH One OP board is used to protect a pair of bidirectional services. At the receiving end.WM_000_ E1 DWDM Basic Principles Table 4. 1+1 protection is implemented by the OP board.2 1+1 Protection In the 1+1 protection changeover. 4..2-2.2-2 1 +1 Protection of OMS When the OP board supervises the major optical channel. OTU OTU OTU OTU OTU OTU OTU OTU λ1 λ2 λ3 λn λ1 λ2 λ3 λ1 O M D OBA Line 1 in direction A Line 2 in direction A O P O P Line 1 in direction B Line 2 in direction B OTU OTU OTU OTU OTU OTU OTU OTU OPA O D U λ2 λ3 λn λ1 O D D OPA OBA O M U λ2 λ3 λn λn Line 1 is working channel and line 2 is protection channel..2-1 1+1 Protection of OCH 2. Fig. 63 . 4.. as shown in Fig.Chapter 4 Protection Principle of DWDM System Optical transponder Optical transponder Optical transponder λ1 λ2 λ3 λn O M U O U λ1 λ2 Optical transponder Optical transponder Optical transponder Protected channel Protection channel O P Protected channel D λ3 . changeover is implemented through optical switch inside the board when changeover condition is met. 4. Optical transponder Optical transponder .. 1 +1 protection of OMS The 1+1 protection of OMS is in segment-by-segment 1+1 protection mode. λn Optical transponder O P Optical transponder Optical transponder O D U O M U Optical transponder Protection channel Fig. O M U OTU O P OTU Working channel O D U O M U OTU O P OTU O D U O M U O D U Protection channe O D U O M U Site A Site B Fig. 1+1 protection of OCH The 1+1 protection of OCH can protect not only route but also devices. 4. We assume the ring network is as shown in Fig. the protection channel and protected channel reaches the receiving end through different routes. the protection can be divided into 1+1 protection of OCH and 1+1 protection of OMS.2-3 A Ring Networking The optical connection between Node A and Node B is shown in Fig. 4. 4.2-3. In the ring network.2-4 1+1 Protection of OCH (Ring Networking) 2. 1.2. 4. C Protection channel B D Working channel Fig. 1+1 protection of OMS 64 .2-4.WM_000_ E1 DWDM Basic Principles 4.2 Ring 1+1 Protection When the ring network uses 1+1 protection. the 1+1 protection of OMS protects the multiplexed signals. 5.2. Service can be transmitted Protection line Transmitting of working line 1 Transmitting of working line 2 Protection line Receiving of working line 1 Receiving of working line 2 This service is discarded. It is similar to the protection mode shown in Fig. 4. Signaling support is not required. Its working principle is shown in Fig. 4.Chapter 4 Protection Principle of DWDM System In the ring network. It can be implemented easily. 4. upon deterioration or failure of service signals on the working line. N working lines are bridged to the protection line at both ends.1 Working Principle In the 1:N protection changeover.3. Its bandwidth utilization ratio is low and cost is high. 4.3-1 Working Principle of 1:N Protection 65 .3 1: N Protection 4. the two nodes close to the broken points implement "loop-back" function. and thus protecting all the services.2-2. 4.3-1. multiplex working lines share one protection line. It can be used in any network structure (point-to-point. It is restorable protection without signaling support. This mode is called "transmitting-receiving changeover". Protection line is special and cannot be shared with other working lines. When the fiber is broken off. 4. ring or grid network). Protection line Transmitting of working line 1 Transmitting of working line 2 Protection line Receiving of working line 1 Receiving of working line 2 Changeover protocol Fig. 3. The protection function monitors and judges the received signal status. and changes over the services on this working line to the protection line.3 Features of 1+1 Protection 1. 2. it notifies the SWE boards at the transmitting end and receiving end through protocols. When multiple channels of services are faulty at the same time.WM_000_ E1 DWDM Basic Principles 4. 4. 4.3 Features of 1:N Protection 1. input ports 1 ~ N of the SWE respectively receive the signals from OTU. We take the protection implemented by the Electrical Switching Board (SWE) as an example. ZTE DWDM can provide 1:N protection for OCH. The implementation process is relatively complicated. It can be used in ring and grid networks. The protection line is shared by multiple working lines.3. the service in high priority will take priority of protection. 4. The SWE implements changeover in electrical switching mode. At the transmitting end. 66 . At the receiving end. 1 1 1 Incoming SDH optical signal 16 SWE OTU OTU 1 SWE Outgoing OTU optical signal 16 Outgoing OTU optical signal OMU 17 OTU ODU Incoming SDH optical signal OTU 17 Fig. and then output to OTU through output ports 1 ~ N of the SWE.3-2 Functional Block Diagram of 1+1 Protection of OCH If any channel in the N channels of services becomes faulty. N channels of service signals are input to input ports 1 ~ N of the SWE. 3. once the receiving end detects the faulty service. The protection function is shown in Fig. 2. Its bandwidth utilization ratio is high but protection reliability is low. Protection is restorable. and then the receiving/transmitting end changes this channel of service to the port N + 1 to protect the service.3.2 Protection Implementation At present. Signaling support is required. 5. 4. and output ports 1 ~ N of the SWE output such signals to the user terminal. The protection priority is set in the NM.3-2. 4. Meanwhile the two changeover switches at the receiving end start operations.4 Bidirectional Optical Channel Protection 1. Besides channel protection of the ring network. T1（λ1） A B C Reserved wavelength channel (λ1) in Internal ring F E D T2（λ1） Changeover node T3（λ1） Adding channel. and services are transmitted along the protection route. the OPCS board also controls the adding status of adding protection wavelength through connecting with the 67 . and thus the access switch starts operation at the service transmitting end.4-1 Principle of 2-Fiber Bidirectional Channel Shared Protection 2. Working principle In the 2-fiber bidirectional channel shared protection ring. ZTE DWDM. when a cross-section fiber is faulty (× means faulty).4-1. dropping channel T4（λ1） Changeover node Fig. and the protection channel shares protection of all services on the working channel. and services are received from the protection route. As shown in Fig. 4.Chapter 4 Protection Principle of DWDM System 4. The working channel allows wavelength multiplexing of multiple unidirectional services. λ1 of the external ring forms the working channel. implements bidirectional channel shared protection through the Optical Channel Shared Protection (OPCS) board. and λ1 of the internal ring forms the protection channel. the services passing this span are damaged. service protection is implemented. Implementation mode of OPCS board The ZXMP M800. In this way. and λ21 of internal ring serves as the protection wavelength of external ring λ21. to avoid conflict of multiple services that use the same working wavelength on the protection ring. and service from B to A is borne by λ22 (internal ring). 4. the required service wavelength is differential-wavelength transmission. But service bidirectional feature and differential-wavelength of working wavelengths must be guaranteed. 68 . 4. First of all. H A λ21(B→A) B λ21(B→A) B λ22(A→B) λ22(A→B) G F λ22(E→F) E D λ22(E→F) λ21(F→E) λ21(F→E) Fig. In this way. the default principle is allocating wavelengths by adjacent odd and even wavelengths.4-2 shows a networking example. For the convenience of project debugging and maintenance. The wavelength allocation can be flexibly adjusted. Service from A to B is borne by λ21 (external ring). In the configuration.WM_000_ E1 DWDM Basic Principles optical switch. 3. Fig. we install the OPCS board at Sites A and B and connect fibers. Similarly.4-2 Wavelength Configuration of Channel Shared Protection We assume that a pair of bidirectional services between Site A and Site B need protection. 1) Application features It is used for loop protection. and thus implementing shared protection of multiple services in the ring network. the working wavelength formed by λ21 and λ22 can be repeatedly used between other nodes in the ring network. λ22 wavelength serves as the protection wavelength of internal ring λ22. and the scheme of mutual protection can be employed for the 8 wavelengths of the internal and external rings. The first 16 wavelengths of the external ring serve as protection wavelengths. 4. For example. Only 8 wavelengths of the 32-wavelength system can be protected as well. The wavelengths are complementarily distributed. the actual working wavelengths of the system are 24 wavelengths.5 Bidirectional OMS Protection 1. 4. 69 . for the 32-wavelength system.5-1 shows the schematic diagram of MS protection for mutual protection of the wavelengths in the internal and external rings. The solid lines indicate working routes. Changeover depends on quality of the signals in a channel leaving the loop. 4) In the changeover. and the dotted lines indicate protection routes of the external ring in case of fault between D and E. 3) In the loop. 5) Wavelength allocation is flexible. with 16 working wavelengths.Chapter 4 Protection Principle of DWDM System 2) Service protection is based on channels. Working principle In the 2-fiber bidirectional MS protection. changeover is implemented in adding channel node and dropping channel node of the service. The resource utilization ratio is high. and the last 16 wavelengths serve as working wavelengths. and the last 16 wavelengths serve as protection wavelengths. the system uses the same wavelength in internal ring and external ring for mutual protection. The working wavelengths usually transmit services while the protection wavelengths usually not. the transport directions of node receiving information and node transmitting information are two reverse directions. that is. Fig. the first 16 wavelengths of the internal ring serve as working wavelengths. 5-2 Wavelength Configuration of MS Shared Protection We assume that a pair of bidirectional services between Site A and Site B need protection. λ21(B→A) B λ21(B→A) H A B λ43(A→B) λ43(A→B) G F λ43(E→F) E D λ43(E→F) λ21(F→E) λ21(F→E) Fig.5-1 Schematic Principle Diagram of 2-Fiber Bidirectional MS Shared Protection 2. implements bidirectional OMS shared protection through the Optical MS Shared Protection (OPMS) board.WM_000_ E1 DWDM Basic Principles B A H G λ17 C D E λ17 F Adding channel. In the configuration. dropping channel Fig. the service wavelengths are in differential-wavelength transmission mode. Implementation mode of OPMS board The ZXMP M800.5-2 shows a networking example. ZTE DWDM. and both working bands and protection bands of internal/external ring are distributed symmetrically. 4. we install the OPMS board at Sites A and B and connect fibers. For example. 4. First of all. 16 70 . dropping channel λ1 λ1 Adding channel. Fig. 4. 3 THz ~ 196. In the configuration of MS shared protection. at least one Optical MS Shared Protection board (with preventing resonance switch) is configured in the loop. 3) In the loop. changeover is executed between adjacent nodes of faulty span. Similarly. 4) 5) Upon fault. to avoid self-stimulation of the loop. the transport directions of node receiving information and node transmitting information are two reverse directions. and service from B to A is borne by λ43 (internal ring). the working wavelength formed by λ21 and λ43 can be repeatedly used between other nodes in the ring network. and λ21 of internal ring serves as the protection wavelength of external ring λ21.8 THz) of the external ring serve as working wavelengths of external ring. and thus implementing shared protection of multiple services in the ring network. Changeover depends on quality of the MS signals between adjacent nodes. 1) 2) Application features It is used for loop protection. λ43 wavelength of external ring serves as the protection wavelength of internal ring λ43.0 THz) of the internal ring serve as working wavelengths of internal ring. Service protection is based on MS. and 16 wavelengths (194.1 THz ~ 193. 71 . The resource utilization ratio is high. In this way.Chapter 4 Protection Principle of DWDM System wavelengths (192. We assume that service from A to B is borne by λ21 (external ring). 3. . Appendix A Abbreviations Abbreviation AFR AFEC AGENT AIS APR APS APSD APSF ASE AWG BER BLSR BSHR CDR CMI CODEC CPU CRC DBMS DCC DCF DCG DCN DCM DCF DDI DFB-LD DSF DGD DTMF DWDM DXC EAM ECC EDFA EFEC Absolute Frequency Reference Advanced FEC Alarm Indication Signal Automatic Power Reduction Automatic Protection Switching Automatic Power Shutdown Automatic Protection Switching for Fast Ethernet Amplified Spontaneous Emission Array Waveguide Grating Bit Error Ratio Bidirectional Line Switching Ring Bidirectional Self-Healing Ring Clock and Data Recovery Code Mark Inversion Code and Decode Center Process Unit Cyclic Redundancy Check Database Management System Data Communications Channel Dispersion Compensation Fiber Dispersion Compensation Grating Data Communications Network Dispersion Compensation Module Dispersion Compensating Fiber Double Defect Indication Distributed Feedback Laser Diode Dispersion Shifted Fiber Differential Group Delay Dual Tone Multi Frequency Dense Wavelength Division Multiplexing Digital Cross-connect Electrical Absorption Modulation Embedded Control Channel Erbium Doped Fiber Amplifier Enhanced FEC 73 Full Name . WM_000_ E1 DWDM Basic Principles Abbreviation EX FDI FEC FPDC FWM GbE GUI IP LD LOF LOS MANAGER MDI MCU MOADM MBOTU MQW MSP MST NCP NDSF NE NNI NMCC NRZ NT NZDSF OA OADM OBA Och ODF ODU OGMD OHP OHPF OLA OLT OMU ONU OP OPA Extinction Ratio Forward Defection Indication Forward Error Correction Full Name Fiber Passive Dispersion Compensator Four Wave Mixing Gigabits Ethernet Graphical User Interfaces Internet Protocol Laser Diode Loss of Frame Loss of Signal Multiple Document Interface Management and Control Unit Metro Optical Add Drop Multiplexer Equipment Sub-rack backplane for OTU Multiple Quantum Well Multiplex Section Protection Multiplex Section Termination Net Control Processor None Dispersion Shift Fiber Network Element Network Node Interface Network Manage Control Center Non Return to Zero Network Termination Non-Zero Dispersion Shifted Fiber Optical Amplifier Optical Add/Drop Multiplexer Optical Booster Amplifier Optical Channel Optical fiber Distribution Frame Optical Demultiplexer Unit Optical Group Mux/DeMux Board Order wire Overhead Processing Board for Fast Ethernet Optical Line Amplifier Optical Line Termination Optical Multiplexer Unit Optical Network Unit Optical Protection Unit Optical Preamplifier Amplifier 74 . Appendix A Abbreviations Abbreviation OPM OPMSN OPMSS OSC OSCF OSNR OTM OTN OTU OXC PDC PMD PDL RZ SBS SDH SDM SEF SES SFP SLIC SMCC SMT SNMP SPM SRS STM SWE TCP TFF TMN VOA WDM XPM Optical Performance Monitor Full Name Optical Protect for Mux Section (without preventing resonance switch) Optical Protect for Mux Section (with preventing resonance switch) Optical Supervisory Channel Optical Supervision channel for Fast Ethernet Optical Signal-Noise Ratio Optical Terminal Optical Transport Network Optical Transponder Unit Optical Cross-connect Passive Dispersion Compensator Polarization Mode Dispersion Polarization Dependent Loss Return to Zero Stimulated Brillouin Scattering Synchronous Digital Hierarchy Supervision add/drop multiplexing board Severely Errored Frame Severely Errored Block Second Small Form Factor Pluggable Subscriber Line Interface Circuit Sub-network Management Control Center Surface Mount Simple Network Management Protocol Self-Phase Modulation Stimulated Raman Scattering Synchronous Transfer Mode Electrical Switching Board Transmission Control Protocol Thin Film Filter Telecommunications Management Network Variable Optical Attenuator Wavelength Division Multiplexing Cross-Phase Modulation 75 .