GSM Radio Network Planning and OptimizationChapter 10 GPRS Radio Network Planning and Optimization For internal use only 1/28/2011 All rights reserved Page1 of 94 Table of Contents Chapter 10 GPRS Radio Network Planning and Optimization ................................................... 4 10.1 GPRS Basic Principles .................................................................................................... 4 10.1.1 Network Structure and Functional Entities ............................................................. 4 10.1.2 Service Function and Numbering Plan .................................................................. 9 10.1.3 Main Interfaces and Related Protocols ................................................................ 26 10.1.4 Radio Channels and Their Importance ................................................................ 34 10.1.5 System Information and Main Flows.................................................................... 43 10.1.6 Parameters and Application ................................................................................ 63 10.2 GPRS Network Planning ............................................................................................... 70 10.2.1 Capacity Planning ............................................................................................... 71 10.2.2 Coverage Planning ............................................................................................. 78 10.2.3 Frequency Planning ............................................................................................ 81 10.3 GPRS Network Optimization ......................................................................................... 82 10.3.1 GPRS Network Optimization Objectives and Principles ....................................... 83 10.3.2 Network Optimization Indexes ............................................................................. 84 10.3.3 Network Optimization Problem Analysis .............................................................. 91 List of Figures Figure 10-1 GPRS network structure ..................................................................................... 5 Figure 10-2 MM state transition model ................................................................................ 20 Figure 10-3 GPRS address and numbering diagram ........................................................... 21 Figure 10-4 NSAPI numbering diagram ............................................................................... 23 Figure 10-5 Main interfaces in the GPRS system ................................................................ 26 Figure 10-6 GPRS data transfer protocol platform ............................................................... 29 Figure 10-7 Structure of a radio channel .............................................................................. 35 Figure 10-8 Structure of the RLC/MAC data block ............................................................... 37 Figure 10-9 Structure of RLC/MAC block ............................................................................. 38 Figure 10-10 Uplink two phase access (acknowledged mode) flow on CCCH ...................... 46 GSM Radio Network Planning and Optimization Chapter 10 GPRS Radio Network Planning and Optimization For internal use only 1/28/2011 All rights reserved Page2 of 94 Figure 10-11 Downlink TBF establishment flow ................................................................... 49 Figure 10-12 Coordinated GPRS attach .............................................................................. 51 Figure 10-13 Update of the coordinated RA/LA within a SGSN. ........................................... 54 Figure 10-14 GPRS detach initiated by the MS.................................................................... 56 Figure 10-15 PDP context activation initiated by the MS ...................................................... 57 Figure 10-16 PDP context modification ............................................................................... 61 Figure 10-17 Recovery flow for GPRS suspended service ................................................... 62 Figure 10-18 Coverage corresponding to four GPRS channel coding schemes ................... 79 Figure 10-19 Relationship between C/I and distance ........................................................... 80 Figure 10-20 Relationship between C/I distribution probability and C/I ................................. 80 Figure 10-21 Frequency reuse clusters supported by GPRS channel coding schemes ........ 81 Figure 10-22 Relationship between C/I distribution probability and frequency reuse coefficient .................................................................................................................................... 82 List of Tables Table 10-1 Definition of the priority class in GPRS QoS ....................................................... 13 Table 10-2 Definition of the reliability class in GPRS QoS .................................................... 14 Table 10-3 Definition of the delay class in GPRS QoS ......................................................... 15 Table 10-4 Definition of the peak throughput class in GPRS QoS ........................................ 15 Table 10-5 Definition of the mean throughput in GPRS QoS ................................................ 16 Table 10-6 Application of the GPRS upper layer functions in network entities ....................... 17 Table 10-7 TID format .......................................................................................................... 24 Table 10-8 GPRS channel coding scheme ........................................................................... 36 Table 10-9 The size of the RLC/MAC data block .................................................................. 37 Table 10-10 Meaning of the SI bit ........................................................................................ 38 Table 10-11 Meaning of the S/P bit ...................................................................................... 38 Table 10-12 Meaning of the RRBP field ............................................................................... 39 Table 10-13 Meaning of the effective payload type ............................................................... 39 GSM Radio Network Planning and Optimization Chapter 10 GPRS Radio Network Planning and Optimization For internal use only 1/28/2011 All rights reserved Page3 of 94 Table 10-14 Meaning of the FBI bit ...................................................................................... 39 Table 10-15 MS multislot capability ...................................................................................... 40 Table 10-16 Coordination between GPRS NMO and paging ................................................ 48 Table 10-17 PDP context deactivation initiated by the MS .................................................... 60 Table 10-18 Meaning of the ALPHA ..................................................................................... 63 Table 10-19 Meaning of the INS_MEAS_CHANNEL_LIST_AVAIL........................................ 65 Table 10-20 GPRS subscriber’s average data rate .............................................................. 73 Table 10-21 Average available bandwidth of the PDCH under various configurations (a) ............... 76 Table 10-22 Average available bandwidth of the PDCH under various configurations (b) ............... 76 Table 10-23 Mapping relationship of GPRS channel coding scheme and C/I ........................ 78 Table 10-24 Percentage of voice coverage area relative to channel coding scheme ............. 79 Table 10-25 Relationship between grade of voice service and C/I ........................................ 81 Table 10-26 Definition of the GPRS PDCH number.............................................................. 85 Table 10-27 Definition of GPRS congestion rate .................................................................. 86 Table 10-28 Definition of GPRS call drop rate ...................................................................... 87 GSM Radio Network Planning and Optimization Chapter 10 GPRS Radio Network Planning and Optimization For internal use only 1/28/2011 All rights reserved Page4 of 94 Chapter 10 GPRS Radio Network Planning and Optimization 10.1 GPRS Basic Principles General Packet Radio Service (GPRS) is a kind of mobile packet data service developing from the existing GSM mobile communication network. GPRS introduces packet switching functional entities to the GSM digital mobile communication network. In this case, the data can be transmitted in terms of packet in a GPRS system. The GPRS system expands the services provided by the original GSM circuit switching system. Therefore, in a GPRS system, mobile users can use packet data mobile terminals to access the Internet or other packet data networks. The digital cellular mobile communication based on GSM and CDMA as and the packet data communication based on the Internet are the two industries enjoying the fastest growth in information area. Tendency shows that the two industries are coming to integration. The advent of the GPRS takes the first step towards the integration of the mobile communication and the packet data communication. Currently, while the voice service keeps developing, the 2G mobile communication gradually supports IP and high-speed data services. Moreover, the 3G mobile communication will be also characterized by IP and high data services. GPRS provides multiple data services, including PTP (Point-to-Point) service, PTM-M (Point to Multipoint Multicast) service, PTM-G (Point to Multipoint Group Call) service, and IP-M (Internet Protocol Multicast) service. GPRS can be applied in various areas, including E-mail, WWW browse, WAP service, electronic commerce, information query, remote supervisory, and so on. 10.1.1 Network Structure and Functional Entities The GPRS network supports packet switching and packet transmission, which enables the GSM network to efficiently support data services. As shown in Error! Reference source not found., the GPRS network is an overlay network of the existing GSM network. In the GPRS network, the functional entities, such as Service GPRS Support GSM Radio Network Planning and Optimization Chapter 10 GPRS Radio Network Planning and Optimization For internal use only 1/28/2011 All rights reserved Page5 of 94 Node (SGSN) and Gateway GPRS Support Node (GPSN) are added to the existing GSM network. The GPRS network and the existing GSM network share the same BSS system, but the corresponding hardware and software must be upgraded to meet the requirements of GPRS services. Meanwhile, the interfaces of the functional entities of the GPRS network and the GSM network must be properly defined. In addition, the MS must be required to support the GPRS services. The GPRS network can connect to PSPDN with the help of GGSN. Either the X.75 or X.25 can work as the interface protocol. Moreover, the GPRS network can connect to the IP network directly. Figure 10-1 GPRS network structure The following introduces the functions of the equipments related to the GPRS network in detail. I. GPRS mobile station Terminal equipment The terminal equipment (TE) is a computer terminal operated by users. In the GPRS system, it transmits and receives the packet data of the terminal users. The TE can be an independent computer, or can be integrated with the mobile terminal (MT). To some extent, all the functions provided by the GPRS network enable a packet data transmission path to be established to connect the TE and the external data networks. Mobile terminal GSM Radio Network Planning and Optimization Chapter 10 GPRS Radio Network Planning and Optimization For internal use only 1/28/2011 All rights reserved Page6 of 94 The mobile terminal can communicate with the TE. In addition, it can communicate with BTS through Um interface and establish logical links to SGSN. In a GPRS network, the ME can enjoy the services provided by the GPRS system only when it is configured with related GPRS functional software. During data communication, the MT connects the TE to the Modem in the GPRS system. The functions of the MT and TE can be integrated into one physical device. Mobile station The mobile station (MS) can be taken as the integration of the MT and TE. Physically, it can be either one entity or two entities (TE + MT). Three types of MSs are available, including type A, type B, and type C. The MSs of type A can perform packet switched service and packet circuit switched service simultaneously. The MSs of type B can be attached to the GPRS network and the existing GSM network, but they cannot perform packet switched service and packet circuit switched service simultaneously. The MSs of type C cannot be attached to the GPRS network and the existing GSM network. II. BTS The base station transceiver (BTS) is the wireless part in BSS system. It is controlled by base station controller (BSC) and serves one or more cells. The functions of the BTS are as follows: Realize radio transmission and related control function between the BTS and the MS through Um interface. Fulfill the functions of the Um interface at the first and second layers and transparently transmit the messages at the third layer. Help the BSC to fulfill the functions of the Um interface at the third layer. III. BSC BSC is the core controlling part in the BSS system of the GSM network and the GPRS network. For packet switched service, the BSC undertakes the following responsibilities: Configure packet radio channels Control the conversion of the radio channel between packet switched service and packet circuited service. Provide necessary packet call control support for the cells with no Packet Broadcast Control Channel (PBCCH). GSM Radio Network Planning and Optimization Chapter 10 GPRS Radio Network Planning and Optimization For internal use only 1/28/2011 All rights reserved Page7 of 94 IV. PCU Packet control unit (PCU) helps the BSS support the GPRS. Its functions are as follows: Manage the major part of the packet radio resources Control packet calls Transmit packet data Support Gb interface and Pb interface V. SGSN SGSN is a basic network element in the GPRS network. The SGSN is introduced to the GSM network to enable GPRS service. The main function of the SGSN is to forward the packet data for the MSs within the local SGSN service areas, which is similar to the function of the Visited Mobile Switching Center (VMSC) in the GSM circuit network. The specific functions of the SGSN are as follows: Forward the packet data and provide the route for all the GPRD MSs within the local SGSN service areas. Provide encryption and authentication Manage session Manage mobility Manage logical links Provide the interface with GPRS BSS, GGSN, HLR, SMS-GMSC, and SMS-IWMSC. Generate the output bills and collect the information of the utilized radio resources. In addition, the SGSN contains the function similar to that of the VLR in the GSM network. When subscribers are in GPRS attach state, the SGSN stores the information of the subscribers and their location. Similar to VLR, most information of the SGSN subscribers are obtained from the VLR when the subscribers perform location update. VI. GGSN The Gateway GPRS Support Node (GGSN) is introduced to the GSM network to support GPRS service. It provides the route and encapsulation for the data packets to be transmitted between the GPRS network and the external data networks. Which GGSN is selected as the gateway is decided according to subscribers’ subscription information and access point name (APN) during the PDP context activation. The GGSN provides the following functions: Provide the interface to the external data networks. Manage GPRS session and establish the communication between the MS and external networks. GSM Radio Network Planning and Optimization Chapter 10 GPRS Radio Network Planning and Optimization For internal use only 1/28/2011 All rights reserved Page8 of 94 Generate and output bills (it is mainly applied when subscribers use the external networks.) Note: The GGSN must provide the interface for the MS to access external packet data networks. From the perspective of external networks, the GGSN can be compared to the router of the IP of all the subscribers in the GPRS network, so it has to exchange the route information with external networks. VII. CG Charging gateway (CG) collects, combines, and preprocesses the GSN bills and keeps the communication at the interfaces between billing centers. This equipment does not exist in earlier GSM networks. The bills of the GPRS subscribers are generated from multiple network elements when the subscribers access the network once. Moreover, each network element will generate multiple bills. Therefore, the CG is introduced to combine and preprocess the bills before they are sent to billing center. As a result, the load of the billing center is eased. In addition, the SGSN and the GGSN do not have to provide the interface to billing center. VIII. RADIUS During non-transparent access, the network will authenticate the subscribers’ identities. The Remote Authentication Dial in User Service Server (RADIUS) stores the information of the authentication and authorization of the subscribers. This functional entity is not exclusive to the GPRS. IX. DNS Two types of Domain Name Servers (DNS) exist in the GPRS network. One connects the GGSN to external networks. Its main function is to resolve the domain name of the external networks, which is completely equivalent to the function of the general DNS fixed on the Internet. The other one is applied in the GPRS backbone network. It functions in two aspects. One is to resolve the IP address of the GGSN according to the determined APN during the PDP context activation. The other one is to resolve the IP address of the original SGSN according to the original routing area number during the routing area update. The DNS is not exclusive to GPRS. GSM Radio Network Planning and Optimization Chapter 10 GPRS Radio Network Planning and Optimization For internal use only 1/28/2011 All rights reserved Page9 of 94 X. BG In fact, the border gateway (BG) is a router. It provides the route between the SGSN and GGSN in the GPRS network and manages the security. It is not exclusive to the GPRS. XI. HLR The home location register (HLR) stores the permanent information of GPRS subscribers. It provides the required data of the subscribers to the SGSN. In addition, it can update the information of the subscribers if necessary and notify the update to the corresponding SGSN. The HLR has the following functions: Manage the data of GPRS subscribers Manage the information of the location of GPRS subscribers Authenticate subscribers’ identities Recover errors XII. MSC/VLR The Mobile Switching Center (MSC)/Visitor Location Register (VLR) can combine the GPRS service and the GSM service with the help of Gs interface. In this case, the MSC /VLR store both the information of the International Mobile Subscriber Identity (IMSI) of subscribers and the related SGSN numbers. The MSC/VLR have the following functions: Combine attachment and detachment Combine location update and route update Page circuit service Prompt non-GPRS in associated status Request subscriber information Indicate mobile information 10.1.2 Service Function and Numbering Plan In a Public Land Mobile Network (PLMN), the GPRS enables subscribers to transmit and receive data under end-to-end packet transfer mode. Two types of bearer services are defined in GPRS. They are PTP service and PTM service. Based on the standard network protocols supported by GPRS bearer services, GPRS carriers can support or provide subscribers with various telecommunication services. The application of the services provided by GPRS has the following characteristics: They are applicable in the transmission of the discontinuous non-periodic (burst) data. The occurrence interval of the burst data is far greater than its mean transmit delay. GSM Radio Network Planning and Optimization Chapter 10 GPRS Radio Network Planning and Optimization For internal use only 1/28/2011 All rights reserved Page10 of 94 They can be applied to process the data service shorter than 500 bytes. In this case, the data service can occur several times in each second and can be frequently transmitted. They can be applied to process the data service of thousands of bytes. In this case, the data service can occur several times in each hour and can be frequently transmitted. These characteristics prove that the GPRS is favorable to the application of the burst data services and can efficiently use the channel resources. However, the GPRS network must restrict the huge data services. The reasons are as follows: A small amount of data traffic is prescribed in the GPRS network. The GPRS network is developed from the existing GSM network. Currently, GSM networks mainly provide telephony service. The telephony subscribers are of great intensity and the traffic volume of great, but the intensity of the GPRS data subscribers is relatively low, so only a small number of channels can be applied to the GPRS service in a cell. The transmission rate of the data on radio channels is low. Currently, the CS-1 and CS-2 coding schemes are in general use. They can meet the requirement of carrier-to-interference ratio (C/I) is equal to or greater than 9 dB and ensure 100% (CS-1) and 90% (CS-2) of the GPRS coverage. In this case, however, the transmission rate of the data is only 9.05 Kbit/s (CS-1) and 13.4Kbit/s (CS-2) (including the RLC block header). The reason is that half of the bit rate (CS-1) and one third of the bit rate (CS-2) in the radio link control (RLC) blocks is applied to the forward error correction (FEC). Though this reduces the requirement of C/I, it reduces the transmission rate of the data. Though the transmission rate of the data under the CS-3 (15.6Kbit/s) and CS-4 (21.4Kbit/s) is relatively high (including the RLC block header), it is enhanced through reducing and canceling the error correction bits, so the CS-3 and CS-4 coding schemes require the C/I to be a greater value. In this case, the CS-3 and CS-4 are applicable in the areas with greater C/I value. In addition, the number of multislot channels supported by the MS is limited at present, so the GPRS network must restrict the huge data services. Generally, the high data is allowed to occur several times in each hour. When the GPRS service and GSM service share channels, the telephony service takes the higher priority if the channels are dynamically allocated. The two times of conversation gapping of any one dynamically allocated channel can be applied to the GPRS service. For the GPRS system, its packet data channel can be shared by multiple GPRS MSs. That is, multiple logical channels can be reused on one physical channel. Therefore, the GPRS can be particularly applied for the burst data. In this case, the utilization rate of the channels can be greatly enhanced. GSM Radio Network Planning and Optimization Chapter 10 GPRS Radio Network Planning and Optimization For internal use only 1/28/2011 All rights reserved Page11 of 94 I. GPRS bearer serivce 1) PTP data service The PTP service enables the transmission of one or more packets between two subscribers. Two types of PTP service are available. They are PTP Connectionless Network Service (PTP-CLNS) and PTP Connection Orientated Network Service (PTP-CONS). The PTP-CLNS belongs to the service type of data diagram. It is mainly applied in bursting non-interacting service and it is supported by the Connectionless Network Protocols (CLNP), such as the Internet Protocols (IP). PTP-CONS is applied in burst events and interacting application service. It is supported by the Connection Orientated Network Protocols (CONP), such as the X.25. 2) PTM data service The PTM data service enables single information to be sent to multiple subscribers. It includes the following three types of services. PTM-M data service This service enables the information to be sent to all the current subscribers in an area. It is a kind of one-way communication service, so not all subscribers can necessarily receive the information correctly. The time to provide the packet data and the quality of service (QoS) are decided according to the negotiation of the GPRS carriers and the PTM-M providers. PTM-G data service This service enables the information to be sent to current specific sub-group subscribers in an area. It provides both one-way communication and multi-way communication. The PTM-G data service is particularly used to provide the communication to group data subscribers, so it is mainly used in the areas, such as in the dispatching management of group subscribers, taxi dispatch, group classified information, and special news services. IP-M data service This service is one of the services defined in the Internet Protocols. The information of the data is transmitted among the participants of the IP-M data services. The subscribers in an IP group can be both fixed and mobile IP subscribers. The service areas of IP-M are not restricted in terms of geography, so the IP-M subscribers can be either a group of subscribers in a PLMN or in the Internet. GSM Radio Network Planning and Optimization Chapter 10 GPRS Radio Network Planning and Optimization For internal use only 1/28/2011 All rights reserved Page12 of 94 II. GPRS supplementary service According to the specifications defined in the ETSI GPRS in SMG#28 earlier, most of the supplementary services for the circuit switched service are inapplicable to GPRS. The supplementary services applicable to GPRS include: Call Forwarding Unconditional (CFU) Call Forwarding on Mobile Subscriber Not Reachable (CFNRc) Closed User Group (CUG) Advice of Charge (Information) (AoCI) Advice of Charge (Charging) (AoCC) In addition, specific GPRS supplementary services are applicable to GPRS subscribers. Currently, the specific GPRS supplementary service is the “barring GPRS interworking files” service. This service restricts subscribers from accessing external networks with the help of the interworking files activated by barring. The specifications in the ETSI GPRS in SMG#29 later clearly indicate that the supplementary services are not defined for GPRS. III. Other service relationships of GPRS and GSM 1) PTP SMS In the GPRS network, the MS can receive and send short messages. If the MS is in GPRS attach and IMSI detach state, the short message service (SMS) is provided by the GPRS channel. If the MS is in GPRS attach and IMSI attach state, short messages can be sent both on the GPRS channel and the CS control channel. In this case, the channel priority is decided by carriers. Generally, the radio resources will be more efficiently used if the short messages are sent on the GPRS channel. If the CS control channel is in use, the SGSN will page the MS, and the short messages are sent in this way. 2) Circuit switched service If both the SGSN and the MSC/VLR supports the Gs interface, and when the SGSN stores the corresponding VLR number and the VLR stores the corresponding SGSN number, a correlation will be established between the SGSN and the MSC/VLR. The correlation functions to coordinate the MSs in GPRS attach and IMSI attach state, and the operation mode is related to the operation mode of the network and the type of the MS. If a GPRS-attached MS enters the circuit switched mode (dedicated mode), it can request the network to suspend the GPRS service. After finishing the circuit switched service, however, it can recover the suspended GPRS service. GSM Radio Network Planning and Optimization Chapter 10 GPRS Radio Network Planning and Optimization For internal use only 1/28/2011 All rights reserved Page13 of 94 IV. GPRS QoS QoS stands for quality of service. Each PDP context has an independent QoS script related to itself. The GPRS QoS has several attributes, including priority class, delay class, reliability class, peak throughput class, and mean throughput class, and each attribute can be divided into multiple levels. These classes can form multiple GPRS script according to various combinations. A GPRS network can support only one subset of the QoS combinations. Upon subscription, the subscriber subscribes to the defaulted QoS script. During PDP context activation, the MS and the network side renegotiate the QoS script. The MS can request a different QoS from the subscribed one. Because all the attributes cannot be exclusive to the end-to-end transmission of the packet data, especially because many factors, such as the radio resources at the Um interface, the frame relay link resources at the Gb interface, and bandwidth of the GPRS backbone network, and the processing capability of various GPRS equipments, are related to the transmission, the best effort class is required to be the QoS at present. That is, the data must be transmitted as fast and accurate as possible while the most efficient utilization of the resources is ensured. Because the system resources needed by various services vary with the grade of service (GoS), the QoS enjoyed by subscribers varies. As a result, carriers can tell the classes of subscribers according to the subscribers’ GoS and adopt flexible charging strategies. And this is helpful for carriers to popularize the GPRS service. 1) Priority class The priority class ensures subscribers to enjoy the basic or important services in abnormal cases. When network resources are scarce or congested, the network side and the MS decide which data packet must be discarded and which data packet must be sent according to their priority classes. A priority class is a 3-bit binary code. Currently, three priority classes are defined. They are priority class 1, 2, and 3. For uplink transmission, the three priority classes maps the radio priority classes 2, 3, and 4. For the transmission of the signaling at radio interfaces, the high priority classes must be adopted. The priority class is defined in Table 10-1. Table 10-1 Definition of the priority class in GPRS QoS Coding Priority class Meaning Corresponding radio priority class for uplink transmission 001 1 Highest priority class 2 010 2 Normal priority class 3 GSM Radio Network Planning and Optimization Chapter 10 GPRS Radio Network Planning and Optimization For internal use only 1/28/2011 All rights reserved Page14 of 94 011 3 Lowest priority class 4 2) Reliability class The reliability class is defined by together by the GTP, LLC, and RLC transmission modes. The reliability class in the QoS script indicates the transmission features required by subscribers. The reliability class is selected according to the types of services. The reliability class is defined in Table 10-2. Table 10-2 Definition of the reliability class in GPRS QoS Priority class GTP mode LLC frame mode LLC data mode RLC mode Applicable service type 1 Acknowle dged Acknowle dged Protecte d Acknowle dged Non-real time service, great error sensitivity, applicable to the service that cannot process the loss of data 2 Unacknow ledged Acknowle dged Protecte d Protected Non-real time service, general error sensitivity, applicable to the service that can process a little loss the data 3 Unacknow ledged Unacknow ledged Protecte d Acknowle dged Non-real times service, poor error sensitivity, applicable to the service that can process the loss of the data and the GMM/SM service 4 Unacknow ledged Unacknow ledged Protecte d Unacknow ledged Real-time service, poor error sensitivity, applicable to the service that can process the loss of the data 5 Unacknow ledged Unacknow ledged Unprotec ted Unacknow ledged Real-time service, no error sensitivity, applicable to the service that can process the loss of the data Note: For real-time services, proper delay and throughput are required to be configured for QoS script. 3) Delay class The delay classes defined in the QoS script are the mean delay of the service data units and the maximum delay of the 99% of the service data units that are involved in the GSM Radio Network Planning and Optimization Chapter 10 GPRS Radio Network Planning and Optimization For internal use only 1/28/2011 All rights reserved Page15 of 94 end-to-end transmission of the data in the GPRS network. In the GPRS system, four delay classes are defined. The lowest requirement is that the network must support the delay class 4 (best effort). Currently, most carriers support the delay class 4 only. The delay class is defined in Table 10-3. Table 10-3 Definition of the delay class in GPRS QoS Coding Delay class Length of the service data unit 128 bytes 1024 bytes Mean delay/s 95% delay/s Mean delay/s 95% delay/s 001 1 (predicted) < 0.5 < 1.5 < 2 < 7 010 2 (predicted) < 5 < 25 < 15 < 75 011 3 (predicted) < 0.5 < 250 < 75 < 375 100 4 (best effort) Not defined 4) Peak throughput class The peak throughput class defines the maximum transmission rate that each PDP context can reach in the network. The peak throughput class is decided by the data transmission capability of the MS and the allocation of the radio resources. The peak throughput is measured at the Gi interface and the R reference point. The peak throughput class is defined in Table 10-4. Table 10-4 Definition of the peak throughput class in GPRS QoS Coding Peak throughput class Peak throughput (byte/s) 0001 1 1000 (8kbit/s) 0010 2 2000 (16kbit/s) 0011 3 4000 (32kbit/s) 0100 4 8000 (64kbit/s) 0101 5 16000 (128kbit/s) 0110 6 32000 (256kbit/s) 0111 7 64000 (512kbit/s) 1000 8 128000 (1024kbit/s) 1001 9 256000 (2048kbit/s) 5) Mean throughput class GSM Radio Network Planning and Optimization Chapter 10 GPRS Radio Network Planning and Optimization For internal use only 1/28/2011 All rights reserved Page16 of 94 The mean throughout class defines the mean transmission rate that the PDP context is expected to reach during the PDP context activation in the GPRS network. The mean throughput is measured at the Gi interface and the R reference point, and the measurement time does not include the data transmission time. If the mean throughput class is the best effort, it means that the network must provide subscribers with negotiable throughput class according to the requests of the subscribers and the allocation of the network resources. The mean throughput class is defined in Table 10-5. Table 10-5 Definition of the mean throughput in GPRS QoS Coding Mean throughput class Mean throughput (byte/h) 00001 1 100 (about 0.22bit/s) 00010 2 200 (about 0.44bit/s) 00011 3 500 (about 1.1bit/s) 00100 4 1000 (about 2.2bit/s) 00101 5 2000 (about 4.4bit/s) 00110 6 5000 (about 11bit/s) 00111 7 10000 (about 22bit/s) 01000 8 20000 (about 44bit/s) 01001 9 50000 (about 111bit/s) 01010 10 100000 (about 222bit/s) 01011 11 200000 (about 444bit/s) 01100 12 500000 (about 1.1kbit/s) 01101 13 1000000 (about 2.2kbit/s) 01110 14 2000000 (about 4.4kbit/s) 01111 15 5000000 (about 11kbit/s) 10000 16 10000000 (about 22kbit/s) 10001 17 20000000 (about 44kbit/s) 10010 18 50000000 (about 111kbit/s) 11111 31 Best effort V. GPRS upper layer function For the application of the GPRS upper layer functions in network entities, see Table 10-6. GSM Radio Network Planning and Optimization Chapter 10 GPRS Radio Network Planning and Optimization For internal use only 1/28/2011 All rights reserved Page17 of 94 Table 10-6 Application of the GPRS upper layer functions in network entities Function MS BSS SGSN GGSN HLR 1. Network access control function Registration X Authentication X X X License control X X X Message screening X Packet terminal adaptation X Charging Data Collection X X 2. Packet routing and transfer function Forward (relay) X X X X Routing X X X X Address translation and mapping X X X Encapsulation X X X Tunneling X X Compression X X Encryption X X X 3. Mobility management function X X X X 4. Logical link management function Logical link establishment X X Logical ink maintenance X X Logical link release X X 5. Radio resource management function Um management X X Cell reselection X X Um-Tranx X X Path management X X Note: X indicates that an entity has the function listed in the left column. The following introduces the network access control function, packet routing and transfer function, mobility management function, logical link management function, and radio resource management function respectively. GSM Radio Network Planning and Optimization Chapter 10 GPRS Radio Network Planning and Optimization For internal use only 1/28/2011 All rights reserved Page18 of 94 1) Network access control function This function controls the MS to access the network so as to make the MS use the related network resource to fulfill data service. For GPRS network, subscribers can access the network from the MT and the fixed network side (including the Internet and X.25). As for some special carriers, they can restrict subscribers from accessing the network or provide specific services to specific subscribers. The GPRS network access function consists of the following: Registration function Authentication function License function Message screening function Packet terminal adaptation function Charging Data Collection function 2) Packet routing and transfer function This function ensures the packet data to be sent to the destination according to the best path. This function consists of the following: Forward (relay) function Routing function Address translation and mapping function Encapsulation function Tunneling function Compression function Encryption function DNS function 3) Mobility management function This function is applied to monitor the current location of the MS in the PLMN. The mobility management function of GPRS is similar to that of GSM. That is, one or more cells constitute a routing area (RA) or a RA subset, and the SGSN provides services for several RAs. The mobility management (MM) of subscribers can be described according to three types of MM state. Each type of state describes a certain level of function and information allocation. All the information is stored in the MM context of the MS and the SGSN. The three types of MM state are idle state, standby state and ready state. The network monitors the MS location according to the MM state of the MS. If the MS is in standby GSM Radio Network Planning and Optimization Chapter 10 GPRS Radio Network Planning and Optimization For internal use only 1/28/2011 All rights reserved Page19 of 94 state, the network only knows the RA of the MS. When the MS is in ready state, the network can know which cell the MS is in. The following introduces the three types of state respectively. a) Idle state When the MS is in idle state, the MM context of the MS and the SGSN does not contain the valid location and routing information of the subscriber. The MS can receive the PTM-M service information, but it cannot carry out the PTM-G service. If the MM context is to be established between the MS and the SGSN, the MS must perform PLMN selection, cell selection and reselection, and GPRS attach program. b) Standby state When the MS is in standby state, the subscriber attaches the GPRS network, and MM context identified by the IMSI of the subscriber is established in the MS and the SGSN. The MS can receive both the PTM-M service data and the PTM-G service data. In addition, it can also receive the circuit switched pages passing the SGSN. However, the MS can neither receive the PTP service in this state, nor can it send the PTP service and the PTM-G data. The MS can select and reselect the GPRS RAs and cells. When the MS enters a new RA, it notifies its current location to the SGSN with the help of the MM procedure. When the MS is in standby state, it can initiate the procedures to activate and deactivate the PDP context. Before the receiving and sending data, the PDP context must be activated. When the network needs to send data or signaling to the standby MS, the SGSN will send a paging request in the RA where the MS is in. If the MS receives the paging request and makes a response, the MM state of the MS will change from standby state to ready state. Meanwhile, when the SGSN receives the response from the MS, the MM state of the SGSN will change from standby state to ready state. When the MS needs to send data and signaling to the network, the MM state of the MS will change from standby state to ready state after the MS sends the data and the signaling. When the SGSN receives the data and the signaling from the MS, its MM state will change from standby state to ready state. Either the MS or the SGSN can initiate the GPRS detach procedure to change the MM state to idle state. When the MS leaves the ready state for standby state, the SGSN will start the MS reachable timer. After receiving the PTP PDU from the MS, the SGSN will stop this timer. If this timer expires, the SGSN will initiates the implicit GPRS detach procedure. In this case, the MM context of the MS and the SGSN will be deleted and the MS will enter the idle state. GSM Radio Network Planning and Optimization Chapter 10 GPRS Radio Network Planning and Optimization For internal use only 1/28/2011 All rights reserved Page20 of 94 c) Ready state When the MS is in ready state, the information of the cell that the MS has camped on is added to the MM context corresponding to MS in the SGSN. The MS provides the information to the network through initiating the procedure to activate the PDP context. When the MS is in ready state, the MS can both receive and send PTP PDU. The network does not start the PS page for the MS, and the page for other services can be realized by the SGSN. Before the MM ready timer (T3314) expires, the MM state keeps in ready state regardless that the MS should be allocated with radio resource. After the T3314 expires, the MM state will change to standby state. If the MM state will change from standby state back to idle state, the MS must initiate the procedure to deactivate the PDP context. Figure 10-2 shows the transition of the three types of MM state. Figure 10-2 MM state transition model 4) Logical link management function The logical links indicate the links established between the MS and the GPRS network and needed for the transmission of packet data. The logical link management function keeps channel for the communication between the MS and the PLMN. After logical GSM Radio Network Planning and Optimization Chapter 10 GPRS Radio Network Planning and Optimization For internal use only 1/28/2011 All rights reserved Page21 of 94 links are established, the MSs and the logical links are one-to-one matched. The logical management function consists of the following: Logical link establishment function Logical ink maintenance function Logical link release function 5) Radio resource management function The radio resource management function involves the allocation and management of the radio communication channels. For the GPRS radio resource management function, it must enable the GPRS service and GSM service to share the radio channels. The radio resource management function consists of the following: Um management function Cell reselection function Um-tranx function 6) Network management function It is the operation and maintenance function of the GPRS system. The realization of this function varies with carriers. VI. GPRS numbering plan The distribution of the addresses, numbers, and identities involved in the GPRS are shown in Figure 10-3. Figure 10-3 GPRS address and numbering diagram In a GPRS backbone network, each SGSN had an internal IP address, which is used for the communication within the backbone network. In addition, the SGSN has a SGSN SS7 number, which is used for the communication with MSC/VLR, HLR, and EIR. Each GGSN also has an internal IP address, which is used for the communication within the backbone network. If the GGSN connects to the HLR through Gc interface, it should also have a GGSN SS7 number. Moreover, as the gateway connecting to external data networks, the GGSN has an address used for the connection with the external data networks. GSM Radio Network Planning and Optimization Chapter 10 GPRS Radio Network Planning and Optimization For internal use only 1/28/2011 All rights reserved Page22 of 94 The GPRS MS has an exclusive IMSI. When it is attached to the GPRS, the SGSN will allocate a temporary P-TMSI to the MS. If the MS intends to access an external PDN, it must have an address (it is called PDP address) corresponding to the PDN. For example, if the MS accesses the X.25/X.75 network, the type of the PDP address is X.121 address. If the MS accesses an IP network, the type of the PDP address is the IP address of the IP network. The IP address can be either a static address or a dynamic address, among which the dynamic address is allocated by the GGSN. When starting the packet data service, the MS must provide an APN to the SGSN; otherwise the network cannot know which external network the MS intends to access. When the GPRS packet data service is being processed, the logical link between the MS and the SGSN is identified by the TLLI only, and the logical link between the SGSN and the GGSN is identified by the TID only. 1) IMSI Compared with the original GSM subscribers, GPRS subscribers also have an IMSI except anonymous subscribers. The anonymous access means that a subscriber accesses the network anonymously without the IMSI or IMEI authentication. In this case, the call cost is paid by the called party. Carriers can decide whether to allow the anonymous access according to actual service needs. 2) P-TMSI The SGSN will allocate a P-TMSI used for packet call to the GPRS-attached subscribers. The P-TMSI is related to the subscriber IMSI. Both the P-TMSI and TMSI consist of are 32-bit codes, but the most significant bit of P-TMSI is 11, and the most significant bit of TMSI is 00, 01, and 10. 3) NSAPI As shown in Figure 10-4, the network layer service access point identifier (NSAPI) is the address used for application layer of the packet data protocol (PDP) to access the sub-network dependent convergence protocol (SNDCP). For the X.25 and IP, they have their own NSAPI. The value of NSAPI ranges from 0 to 15. Currently, the NSAPI whose value ranges from 5 to 15 are used for dynamic allocation. GSM Radio Network Planning and Optimization Chapter 10 GPRS Radio Network Planning and Optimization For internal use only 1/28/2011 All rights reserved Page23 of 94 Figure 10-4 NSAPI numbering diagram 4) TLLI The TLLI stands for temporary logical link identity. When the MS has just accessed the GPRS network and the GPRS network has not allocated the P-TMSI to the MS, the TLLI identifies the exchanged signaling between the MS and the SGSN. The TLLI identifies a solar logical link between the MS and the SGSN in an RA. If the MM context does not know which RA the TLLI belongs to, the TLLI and the routing area identifier (RAI) should be used together. In a RA, the TLLI and the IMSI are one-to-one matched. The TLLI consists of 32-bit codes. In the GPRS system, there are four types of TLLI, and they are introduced hereunder. a) Local TLLI The most significant bit of the local TLLI is 11. The other bits come from the P-TMSI allocated by the SGSN to the MS. The local TLLI is valid only in the RAs related to the P-TMSI. b) Foreign TLLI The most significant bit of the foreign TLLI is 10. The other bits come from the P-TMSI allocated by the other RA. When the subscriber performs RA update, the MS will report the RA update to the SGSN. c) Random TLLI If no P-TMSI is attached to the MS, the MS must provide a random TLLI. The most five significant bit of the random TLLI is 01111. The others are random bits. d) Assisted TLLI The assisted TLLI provides the identities to the anonymously-accessed MSs. The most five significant bit of the assisted TLLI is 01110. The others are allocated by the SGSN. e) NSAPI/TLLI pair The NSAPI/TLLI pair is used to select the route of the network layer. The TLLI can identify the logical links between the MS and the SGSN. The NSAPI and the TLLI are used as the route of the network layer. There is only one NSAPI/TLLI pair in a RA. f) PDP address The PDP address is the address of the packet protocol. The MS is identified by the IMSI. For different external networks, however, they must have the PDP address if they intend to fulfill the packet data function; because the PDP address helps the external data networks identify the PDP context of the MS. The PDP address can be divided into IP address (IP4 address or IP6 address) and X.121 address (for the X.25 service). GSM Radio Network Planning and Optimization Chapter 10 GPRS Radio Network Planning and Optimization For internal use only 1/28/2011 All rights reserved Page24 of 94 The previous types of addresses can be either statically or dynamically allocated. When they are statically allocated, the MS must be subscribe to the network first, and then the network will allocate a corresponding address to the MS. Meanwhile, the network will record the fixed address in the SIM card and the database of the subscriber. The type of PDP address must be made clear during subscription; otherwise the system will reject the PDP address that has not been subscribed. g) TID Tunnel identifier (TID) consists of IMSI and NSAPI. It lies in the GTP header and identifies the solar PDP context between GSNs (between SGSN and GGSN, or between SGSN and the original SGSN). The TID consists of 8 bytes. For the specific format, see Table 10-7. Table 10-7 TID format Bit 8 7 6 5 4 3 2 1 IMSI the 2 nd digit IMSI the 1 st digit IMSI the 4 th digit IMSI the 3 rd digit IMSI the 6 th digit IMSI the 5 th digit IMSI the 8 th digit IMSI the 7 th digit IMSI the 10 th digit IMSI the 9 th digit IMSI the 12 th digit IMSI the 11 th digit IMSI the 14 th digit IMSI the 13 th digit NSAPI IMSI the 15 th digit h) RAI The RAI stands for routing area identifier. The carriers define the RA. A RA can contain one or more cells, or it can be a location area, or a subset of a location area. A location area is controlled by a SGSN. As a kind of system information, the RA information will be broadcast on common control channels. RAI = MCC + MNC + LAC + RAC (The RAC has a maximum of 16 bits.) CGI = LAI + {RAC} + CI (If the cell supports GPRS, the CGI contains the RAC; otherwise it does not contain the RAC.) i) CI GSM Radio Network Planning and Optimization Chapter 10 GPRS Radio Network Planning and Optimization For internal use only 1/28/2011 All rights reserved Page25 of 94 The CI stands for cell identity. The CI of the GPRS network is the same as that of the original GSM network. j) GSN address and number To communicate with the GPRS backbone network and other GSNs, each SGSN and each GGSN have an IP address (IPv4/IPv6) of their own. These addressed are the internal addresses of the GPRS network, and each address has one or more corresponding domain names. The GSN address consists of address type (2 bits), address length (6 bits), and address (a maximum of 16 bits). When the address type is 0, the address is IPv4 address. When the address type is 1, the address is IPv6 address. To communicate with the HLR and the EIR, each SGSN must still have a SGSN SS7 number. If the GGSN connects to the HLR through Gc interface, it must have a GGSN SS7 number. k) APN The APN identifies the GGSN that is to be used in the GPRS backbone network. In the GGSN, the APN is used to characterize the external data networks. The APN consists of two parts. One is APN network identification. It is mandatory. The carriers allocate the APN network identification to the Internet service providers (ISP) and companies and it is the same as the fixed Internet domain name. The other one is APN carrier identification. It is optional and identifies the home network. It is expresses as “xxx.yyy.gprs”. For example, MNC.MCC.gprs. Generally, the APN network identification can be stored as the subscription data in the HLR. Subscribers can provide the APN to SGSN when initiating packet services, because the APN helps the SGSN select the GGSN that the subscribers should access and helps the GGSN judge which external network should be selected. In addition, the APN can be stored as a wildcard (*). In this case, the subscribers and the SGSN can choose to access an APN that is not stored in the HLR. The subscribers can select the GGSN according to different APNs. That is, the subscribers can activate multiple PDP contexts (each PDP context is related to one APN only). The reason for the subscribers to select different APNs is that they can select an external network to access through different GGSNs. The APN cannot obtain the real IP address of the GGSN or the external network node until the DNS successfully resolve their domain names. An APN consists of one or more labels, and each label consists of an octet indicating the length and multiple octets coded by the 8-bit ASCII. The maximum length of an APN GSM Radio Network Planning and Optimization Chapter 10 GPRS Radio Network Planning and Optimization For internal use only 1/28/2011 All rights reserved Page26 of 94 is 100 octets. The value of an octet can be expressed by English letters, Arabic numbers, and the mark “-“. The APN network identification contains at least one label, and the maximum length of the label is 63 octets. The label of the APN network identification cannot be a “*”, the first label cannot be “rac”, “lac”, and “sgsn”, and the last label cannot be “gprs”. The APN network identification is exclusively allocated by each GPRS PLMN. The APN carrier identification identifies one GPRS PLMN only, including three labels. They are “carrier name”, carrier group”, and “gprs” according to the front-to-back sequence. The defaulted APN carrier identification is “MNC.MCC.gprs”. 10.1.3 Main Interfaces and Related Protocols I. Main Interfaces Various interfaces exist among the entities in the GPRS network. Equipments of different carriers cooperate with each other if these interfaces are properly defined. Figure 10-5 shows the main interfaces in the GPRS system. Figure 10-5 Main interfaces in the GPRS system The following introduces the interfaces shown in Figure 10-5. 1) R reference point The R reference point is the interface connecting the TE to MT. Subscribers can perform packet data services at the TE. 2) Um interface GSM Radio Network Planning and Optimization Chapter 10 GPRS Radio Network Planning and Optimization For internal use only 1/28/2011 All rights reserved Page27 of 94 The Um interface connects the MS to the GPRS network side. It maintains the communication between the MS and the network side. In addition, it helps the realization of multiple functions, including the packet data transmission, mobility management, session management, and radio resource management, between the MS and the GPRS network side. The RF part of the Um interface utilizes the timeslot of the GSM carrier, but it must be configured with PDCH. The coding schemes on the PDCH can be divided into CS1, CS2, CS3, and CS4 corresponding to the data transfer rate. The PDCH supports multislot transmission. The number of transmitted timeslots is decided by the multislot level of the MS. 3) Gb interface The Gb interface connects the SGSN to the BSS. The SGSN maintains the communication with the BSS and the MS with the help of Gb interface so that the functions, including packet data transmission, mobile management, and session management, can be realized between them. Moreover, the Gb interface provides flow control function. This interface is a must in the GPRS networking. Currently, the standard GPRS protocols define that the Gb interface adopts frame relay as the transfer protocols at the lower layer. In addition, the standard GPRS protocols also define that either frame relay or PTP frame relay can be used for communication between the SGSN and the BSS. 4) Gi reference point The Gi reference point connects the GPRS network to the external packet data networks. It lies between the GGSN and the external packet data networks. The GPRS network connects communicates with various PDNs (such as the Internet) or ISDNs through the Gi reference point. The operations, including the encapsulation/decapsulation of the protocols, the conversion of the addresses (such as the conversion between IP address of a private network and that of a public network), the authentication and authorization of the subscriber access, must be performed at the Gi reference point. 5) Gn interface The Gn interface connects the GPRS support nodes (GSN) with each other. That is, the Gn interface connects the SGSNs to each other and connects the SGSN to the GGSN within the PLMN. Based on the TCP/UDP protocols, the Gn interface bears the GTP to carry out the communication and manage the packet data transmission and mobility. 6) Gs interface The Gs interface connects the SGSN to the MSC/VLR. It adopts the BSSAP+ protocols bearing on the No.7 signaling. The cooperation of the Gs interface and the MSC helps the SGSN manage the mobility of the MS. In addition, the Gs interface enables the SGSN to receive the circuit switched paging messages from the MSC and deliver them to the MS through PCU. If no Gs interface is provided, the SGSN cannot perform GSM Radio Network Planning and Optimization Chapter 10 GPRS Radio Network Planning and Optimization For internal use only 1/28/2011 All rights reserved Page28 of 94 paging coordination, so the network can work under the network mode of operation II (NMO II) and NMO III only, and this will prevent the enhancement of the call connected ratio. In addition, if no Gs interface is provided, the RAs of the combined locations cannot be updated, and this will results in heavy signaling load in the system. 7) Gr interface The Gr interface connects the SGSN to the HLR. It adopts the MA+ protocols bearing on the No. 7 signaling. The SGSN obtains the data relative to the MS through the Gr interface. The HLR stores the data and the routing information for the GPRS subscribers. When any RA updates in the SGSN, the SGSN will update the corresponding information in the HLR. When the data in the HLR changes, the HLR will notifies the change to the SGSN. 8) Gd interface The Gd interface connects the SGSN to the SMS-GMSC and SMS-IWMSC. The SGSN, SMS-GMSC, SMS-IWMSC, and SMC cooperate with each other at the Gd interface to fulfill the SMS in the GPRS system. The Gd interface enables the SGSN to receive short messages and forward them to the MS. If no Gd interface is provided, the Class C MSs cannot receive and send short messages when attached to the GPRS network. 9) Gp interface The Gp interface connects GPRS networks with each other, so it is used between the GSNs that belong to different PLMNs. The Gp interface is similar to the Gn interface in telecommunication protocols except that the border gateway and fire wall are added to the Gp interface. Because the BG provides the routing protocols for the border gateways, the Gp interface enables the GSNs belonging to different PLMNs to communicate with each other normally. 10) Gc interface The Gc interface connects the GGSN to the HLR. When the network side initiates the PDP context activation, the GGSN uses the IMSI to request the address of the current SGSN where the subscriber is in from the HLR. In current mobile data services, this kind of situation is seldom seen. 11) Gf interface The Gf interface connects the SGSN to EIR. Generally, the EIR is not configured with the networks at present, so the Gf interface cannot be necessarily emphasized. II. Related protocols The data and signaling in the GPRS system are transmitted in an integrated platform, as shown in Figure 10-6. GSM Radio Network Planning and Optimization Chapter 10 GPRS Radio Network Planning and Optimization For internal use only 1/28/2011 All rights reserved Page29 of 94 Figure 10-6 GPRS data transfer protocol platform The following introduces the protocols shown in Figure 10-6. 1) GTP The GTP stands for GPRS tunneling protocol. It is used for the tunneling transmission of the data and signaling between GSNs in the GPRS backbone network. The GTP is applicable to the Gn interface and the Gp interface. The GTP consists of GTP signaling and data transmission program. In the signaling platform, the GTP defines the tunneling control and management protocols for the MS to access the GPRS network. In the transfer platform, the GTP utilizes the tunnels established between the GSNs to transmit subscriber data. All PDP PDUs transmitted between GSNs must be encapsulated with the GTP header. The format of the GTP header is fixed to 20 bytes, the last 8 of which identify the TIDs for specific subscribers. In addition, the GTP provides the flow control function. 2) UDP/TCP The UDP/TCP is the transport layer protocol used to establish the reliable link from end-to-end. The TCP orients to connection. It can discard and protect the errors and controls flow. In addition, it ensures the data to be accurately transmitted and can used to bear the GTP PDUs that require reliable transmission. Compared with the TCP, the UDP orients to non-connection. It neither recovers errors nor cares about whether receives the messages correctly or not. Instead, it transmits and receives the data only. It is used to bear the GTP PDUs that do not require reliable transmission. 3) IP The IP stands for the Internet protocols. It is used to select the route for the subscriber data and control signaling in the GPRS backbone network. Currently the IPv4 is widely used, but it will gradually develop into IPv6. GSM Radio Network Planning and Optimization Chapter 10 GPRS Radio Network Planning and Optimization For internal use only 1/28/2011 All rights reserved Page30 of 94 4) BSSGP The BSSGP (BSS GPRS protocols) includes the functions of the network layer and parts of the transport layers. It provides a radio link that is used to transmit the unacknowledged data between the BSS and the SGSN. The primary function of the BSSGP is to explain the routing information and the QoS information. On downlinks, the SGSN provides the radio information to be used by the RLC/MAC to the BSS. On uplinks, the BSS provides the radio information obtained from the RLC/MAC from the SGSN. In addition, the BSSGP also manages and controls the nodes between the SGSN and the BSS. The BSSGP entity uses BSSGP virtual circuit (BVC) to transmit BSSGP packet data units (BSSGP PDU). The BVC consists of a group of network service virtual circuits (NS-VC) and is only identified by the BSSGP virtual circuit identification (BVCI) and network service entity identification (NSEI) in the SGSN. In the BSSGP layer, the BVCI identifies a cell. The BSSGP has three users. They are relay (RL), GPRS mobility management (GMM), and network management (NM). The function of flow control is available is the BSSGP layer. This function controls the load for the QoS delay class queues between the SGSN and the BSS so as to optimize the buffer area. Because the physical mediums and transmission protocols at the Gb interface are different from that at the Um interface, the data transfer rate at the Gb interface is higher that that at the Um interface. In addition, when the data is transferred on the downlink, the transfer will receive the restriction from multiple factors, such as the multislot capability of the MS, the radio quality, and the unavailability of the radio channels in cells, at the Um interface. Therefore, the data transmission rate is unstable. In this case, you must control the flow of the data transmitted on the downlink, while the control of the flow of the data transmitted on the uplink is unnecessary. The BSSGP layer manages two types of buffers. One buffer of the MS and the other is the buffer of the BVC. First the BSSGP puts each LLC-PDU, with TLLI as identification, received by each LLC-PDU into the buffer of the corresponding MS, and then put the data, with BVCI as identification, into the buffer of the BVC. When the cell works normally, the PCU must start the flow control procedure. In this case, the PCU reports the size and rate of the buckets in the cell to the SGSN periodically according to the radio packet channels. In addition, it must also report the size and the rate of the MS to the SGSN according to the radio resource seized by the MS. Upon receiving the reports, the SGSN adjusts the downlink data rate for the cell and the MS according to the reported parameters in needed time. And this is how the downlink data flow is controlled. The SGSN controls the flow according to one principle. GSM Radio Network Planning and Optimization Chapter 10 GPRS Radio Network Planning and Optimization For internal use only 1/28/2011 All rights reserved Page31 of 94 That is, when controlling the LLC-PDU flow, it controls the flow of the MS first, and then controls the flow of the BVC. Note: The bucket of the cell is the maximum of packet data that the cell can store. That is, it is the maximum amount of the packet data of that the buffer of the BVC can store. The bucket varies with the number of packet channel in the cell. The bucket of the MS is the maximum amount of the packet data that the MS can store. That is, it is the maximum amount of the packet data that the buffer of the MS can store. The bucket varies with the number of channels allocated to the MS. The bucket rate is the data transmission rate. 5) NS The NS stands for network service. The primary functions of the NS layer include the transmission of the network service packet data units (NS-PDU), the indication of network congestion, and the indication of NS layer state. The NS layer consists of the NS sub-network and the NS control part. Currently, the NS sub-network adopts the frame relay. The NS control part transmits the NS-PDU, shares the load, and manages the NS-VC. The NS-VC is the frame-relay permanent virtual circuit (PVC) and it is only identified by the NS-VCI in the SGSN. The NS-VCI is an end-to-end identification at the Gb interface. 6) SNDCP The SNDCP stands for Sub-Network Dependent Convergence Protocol. The SNDCP layer is the transition between the network layer and the link layer. It mainly contributes to the transparent transmission of the network layer PDU (N-PDU) and the enhancement of the channel utilization. The SNDCP can reuse the date (it is from various service source) to be transmitted. The PDP type of the data varies with its service source and the PDP context is identified by different NSAPIs. A type of PDP can contain several PDP contexts and NSAPIs and different PDPs can use a NSAPI. The SNDCP layer packet data unit (SN-PDU) includes header and data. One SN-PDU contains the data of one N-PDU only. The SN-PDU has two formats, SN-DATA PDU and SN-UNITDATA PDU, the first of which is used to transmit the data of the acknowledged mode, and the second of which is used to transmit the data of the unacknowledged mode. The SNDCP has two compression types, data header compression and data compression. GSM Radio Network Planning and Optimization Chapter 10 GPRS Radio Network Planning and Optimization For internal use only 1/28/2011 All rights reserved Page32 of 94 7) LLC The LLC stands for logical link control. The LLC protocol is the radio link protocol used at the transport layer and based on high-speed digital subscriber line (HDSL). It provides the logical link between the MS and the SGSN, with end-to-end encryption and no error. The LLC layer supports point-to-multipoint addressing, data retransmission and multiple QoS delay class. The intact LLC frame can be generated through adding the LLC address and frame field to the LLC based on the PDU. An LLC frame (LLC PDU) consists of address field (1 byte), control field (a maximum of 32 bytes), information field (a maximum of N201 bytes, and frame correction sequence (FCS) field (3 bytes). Under acknowledged mode, the parameter N201-I will indicate that the maximum valid bytes of SN-DATA PDU are 1520. Under unacknowledged mode, the parameter N201-U will indicate that the maximum valid bytes of the SN-UNITDATA PDU are 500. The LLC layer has set different service access points (SAP) for different upper layer subscribers. Each SAP is identified by one service access point identifier. A SAPI consists of 4 bytes and locates in the least-4 digits of the field address of the LLC frame. Currently 6 values are adopted, in which the SAPI = 1 matches the GPRS mobility management/session management (GMM/SM) service, the SAPI = 7 matches the short message service, and the SAPI = 3, 5, 9, and 11 match data service of the subscribers whose QoS is 1, 2, 3, and 4. 8) RLC The RLC protocol is applied between the link layer and the network layer. It controls the radio links. The primary function of RLC layer is to disassemble and assemble LLC-PDU. The application of the sliding window mechanism enables the RLC layer to ensure the data transmission between the MS and the BSS through adopting the acknowledged mode or unacknowledged mode. The size of the GPRS RLC sliding window is 64. a) RLC acknowledged mode Under this mode, each RLC data blocks sent by the sending end must be acknowledged by the receiving end; otherwise the sending end must select the automatic repeat request (ARQ) mechanism to resend the data. Only after all the data are sent and acknowledged by the receiving end, the temporary block flow (TBF) can be released. The TBF is a kind of physical connection used for the transmission of the data between the MS RR entity and the BSS RR entity. It exists during data transmission only. b) RLC unacknowledged mode GSM Radio Network Planning and Optimization Chapter 10 GPRS Radio Network Planning and Optimization For internal use only 1/28/2011 All rights reserved Page33 of 94 Under unacknowledged mode, the data sent by the sending end do not have to be acknowledged by the receiving end and the discarded data blocks do not have to be resent either. The TBF can be released after the data is completely sent. Note: At present, all GPRS networks adopt the RLC acknowledged mode. 9) MAC The MAC stands for medium access control. The MAC protocol is applied to the link layer. Its primary functions are to define the GPRS logical channels at the Um interface and make coordination to enable multiple MSs to share these channels or enable one MS to use the physical channels of different timeslots. Therefore, the MAC must fulfill the following tasks: Fully reuse the uplink and downlink Perform the contention and decision during uplink access Transmit the downlink data according to access attempt sequence Process the radio priority class Match the LLC-PDU to the corresponding physical channels There are three MAC modes, including fixed allocation, dynamic allocation, and extended dynamic allocation, which are detailed in the following. a) Fixed allocation Under this mode, the BSS allocates the radio blocks to be used by the MS in advance. If there is still data needs to be transmitted after all the radio blocks have been used, the BSS will reallocates radio blocks to the MS. b) Dynamic allocation Under this mode, the BSS allocates the radio blocks to be used by the MS when MS needs them. When assigning radio resources to the MS, the BSS will assign several radio channels and the corresponding uplink state flag (USF) for the MS. Upon receiving an assignment message, the MS begins to listen the downlink radio blocks in the assigned channel to obtain the USF. If this USF is the same as the assigned USF, the MS will transmit the data on the corresponding uplink radio block. c) Extended dynamic allocation Under this mode, the resource allocation mechanism is the same as that under the dynamic allocation mode except that greater uplink throughput is provided under the dynamic allocation mode. According to extended dynamic allocation, the multislots are allocated to the MS according to its multislot capability. Upon receiving a USF on one GSM Radio Network Planning and Optimization Chapter 10 GPRS Radio Network Planning and Optimization For internal use only 1/28/2011 All rights reserved Page34 of 94 PDCH, the MS can transmit the data on both the PDCH and the PDCHs with a greater number. 10.1.4 Radio Channels and Their Importance I. Radio Chnnels 1) Types of packet data logical channels Packet data logical channels can be divided into four types. They are packet data traffic channel (PDTCH), packet broadcast control channel (PBCCH), packet common control channel (PCCCH), and packet dedicated control channel (PDCCH). The following details the four types of channels respectively. a) PDTCH The PDCH is used to transmit the subscriber data under packet transfer mode and the transfer rate ranges from 0 to 22.8 kbit/s. All the PDCCHs are unidirectional. The PDTCH uplink (PDTCH/U) helps the MS to transmit the data to the GPRS network. The PDTCH downlink (PDTCH/D) helps the GPRS network to transmit the data to the MS. b) PBCCH The PBCCH broadcasts the parameters needed by the MS to access the network for packet service. In addition, it also broadcasts the parameters used for circuit switched service. The GPRS-attached MSs monitor the PBCCH only. If a cell has the PBCCH, corresponding prompt is present in the BCCH. That is, the MS will be told that this cell is configured with the PBCCH through SI13. If there is no PBCCH is the cell, the BCCH will broadcast the parameters used for packet service. c) PCCCH The PCCH can be divided into the following types: Packet paging channel (PPCH) It is applied to the downlink only and used to page the MS. Packet random access channel (PRACH) It is applied to the uplink only and used to request one or more PDTCH for the MS. Packet access grant channel (PAGCH) It is applied to the downlink only and used to allocate one or more PDTCH. Packet notification channel (PNCH) It is applied to the downlink only and used to notify the MS that the PTM-M call exists. If there is no PCCCH in a cell, the information of the packet service is transmitted on the CCCH. If there is PCCCH, the information of the packet service is transmitted on the GSM Radio Network Planning and Optimization Chapter 10 GPRS Radio Network Planning and Optimization For internal use only 1/28/2011 All rights reserved Page35 of 94 PCCCH, and the information of the circuit switched service can also be transmitted on the PCCCH. d) PDCCH The PDCCH can be divided into the following types: Packet associated control channel (PACCH) It is bi-directional and used to transmit the packet signaling. Packet timing advance control channel uplink (PTCCH/U) It is used to transmit the random access burst so that the BSS side can estimate the timing advance for the MS performing the packet service. Packet timing advance control channel downlink (PTCCH/D) It is used to provide the information of timing advance for multiple MSs. A PTCCH/D matches multiple PTCH/Us. 2) Combination of the packet data logical channels There are the following combinations: PBCCH + PCCCH + PDTCH + PACCH + PTCCH PCCCH + PDTCH + PACCH + PTCCH PDTCH + PACCH + PTCCH If the cell needs the PBCCH, the first combination is used. A cell needs only one group of the combination only. If a great number of MSs are present in a cell, one or more groups of the second combination can be configured when the PCCCH is busy. The second combination cannot be configured with a cell unless the first combination is configured. The third combination is used to transmit the uplink and downlink packet data only. A cell can be configured with one or more groups of this combination. 3) Mapping transformation between logical channels and physical channels The GPRS channel adopts 52-multiframe structure. Each packet channel has 52 multiframes in total, four of which form a radio block. Therefore, a radio block consists of 12 radio blocks and 4 idle frames. Figure 10-7 shows the structure. B0 B1 B2 X B3 B4 B5 X B6 B7 B8 X B9 B10 B11 X B0-B11: 12 radio blocks X: idle frame Figure 10-7 Structure of a radio channel In a GPRS system, the packet logical channel has a mapping relationship with the PDCH physical channel according to the radio block sequence. That is, B0, B6, B3, B9, B1, B7, B4, B10, B2, B8, B5, B1. GSM Radio Network Planning and Optimization Chapter 10 GPRS Radio Network Planning and Optimization For internal use only 1/28/2011 All rights reserved Page36 of 94 For the PBCCH, it can be mapped to the B0, B3, B6, B9, and so on. The specific numbe of radio blocks are decided by the parameter BS_PBCCH_BLKS according to how busy the PBCCH is. For the PCCCH, the PAGCH and the PPCH can be mapped to any radio block of the downlink channels except the radio blocked seized by the PBCCH. The uplink radio blocks seized by the PRACH correspond to the radio blocks seized by the PBCCH, PAGCH, and PPCH. For the PDTCH, it can be mapped to all the radio blocks and is used to transmit the packet data. For the PACCH, it can be mapped to all the radio blocks and is used to send the control message. For the PTCCH, the 12 th and the 38 th uplink frames of each 52-multiframe form a PPCCH/U channel, and the 12 th and the 38 th downlink frames of two neighboring 52-multiframe form a PTCCH/D channel. II. Channel coding A GPRS radio block can only bear 456 bits. Four coding schemes are available for the GPRS channel. They are CS-1, CS-2, CS-3, and CS-4. When coding the channel according to CS-1, first add the USF to the data bit as information bit, and then add the block check sequence (BSC) to detect and correct the codes. After that, add the tail bits and convolute all the bits by half. Finally, the 456-bit channel code is obtained. When coding the channel according to CS-2 and CS-3, first add the USF to the data bit as information bit, and then add the BCS to detect and correct the codes. After pre-coding the USF, add the tail bit and convolute all the bits by half using the chopping technologies. Finally, the 456-bit channel code is obtained. When coding the channel according to CS-4, first add the USF to the data bit as information bit, and then add the BCS to detect and correct the codes. After that, pre-code the USF. Finally, the 456-bit channel code is obtained. Note that the CS-4 does not adopt convolutional codes. For details, see Table 10-8. Table 10-8 GPRS channel coding scheme Channel coding scheme Data bit USF BCS Tail bit Data rate (kbit/s) CS-1 181 3 40 4 9.05 CS-2 268 6 16 4 13.4 CS-3 312 6 16 4 15.6 GSM Radio Network Planning and Optimization Chapter 10 GPRS Radio Network Planning and Optimization For internal use only 1/28/2011 All rights reserved Page37 of 94 CS-4 428 12 16 0 21.4 The data transmission rate and the requirement on transmission quality vary with the CS. The higher the data transmission rate, the higher the transmission quality is required. When transmitting the data, the BSS can adjust the channel CS according to the variation of the radio quality. In this case, the radio resource can be fully used and the data transmission rate can be enhanced based on the assurance of the transmission quality. III. RLC/MAC block structure When used for packet data transmission, the structure of the RLC/MAC block is different from that when it is used for control message transmsion. 1) RLC/MAC data block The RLC/MAC data block consists of MAC header and RLC data block, the later of which consists of RLC header, RLC data unit, and reserved bit. And the RLC data unit contains the octets that are fragmented from one or more LLC PDU. Figure 10-8 shows the structure of the RLC/MAC data block RLC/MAC block MAC header RLC data unit RLC header RLC data unit Reserved bit Figure 10-8 Structure of the RLC/MAC data block The size of the RLC/MAC data block varies with the CS type. For details, see Table 10-9. Table 10-9 The size of the RLC/MAC data block CS type The size of RLC/MAC data block when reserved bit is not accounted (byte) Reserved bits The size of RLC/MAC data block (byte) CS1 22 0 22 CS2 32 7 32 7/8 CS3 38 3 38 3/8 CS4 52 7 52 7/8 2) RLC/MAC control block The RLC/MAC block used for the control message transmission consists of MAC header and RLC/MAC control block. Figure 10-9 shows the structure of the RLC/MAC block. RLC/MAC block MAC header RLC/MAC control block GSM Radio Network Planning and Optimization Chapter 10 GPRS Radio Network Planning and Optimization For internal use only 1/28/2011 All rights reserved Page38 of 94 Figure 10-9 Structure of RLC/MAC block 3) Field remark The following provides the remarks on the main fields. a) USF field The USF field is transmitted in all uplink RLC/MAC blocks. It indicates the owner of the downlink block or the user of the next radio block of the same timeslot. When performing dynamic allocation, the network side uses the USF to allocate the uplink radio resources to the MS. The USF field stands for the eight different values composed of the binary codes. On the PCCCH, the value of the USF is 111, indicating that the corresponding uplink block is the uplink random access block. b) SI bit The SI bit stands for suspend indication bit. In the window mechanism, the SI bit indicates whether the RLC transmit window of the MS needs the suspension. The MS must set the SI bit in all uplink RLC data blocks. For the meaning of the SI bit, see Table 10-10. Table 10-10 Meaning of the SI bit Value Meaning 0 The RLC transmit window of the MS does not need suspension. 1 The RLC transmit window of the MS needs suspension. c) S/P bit The S/P bit stands for the supplementary/polling bit. It is used to indicate whether the relative reserved block period (RRBP) field is valid or not. For the meaning of the S/P bit, see Table 10-11. Table 10-11 Meaning of the S/P bit Value Meaning 0 The RRBP field is invalid. 1 The RRBP field is valid. d) RRBP field GSM Radio Network Planning and Optimization Chapter 10 GPRS Radio Network Planning and Optimization For internal use only 1/28/2011 All rights reserved Page39 of 94 The RRBP stands for relative reserved block period. This field indicates the location of the uplink blocks where the network side sends the PACKET CONTROL ACK message or the PACKET DL ACK/NACK message to. The value of the RRBP bit ranges from 0 to 3. For the meaning of the RRBP field, see Table 10-12. Table 10-12 Meaning of the RRBP field Value Meaning 00 TDMA frame number = (N+3) mod 2715648 uplink block 01 TDMA frame number = (N+17 or N+18) mod 2715648 uplink block 10 TDMA frame number = (N+21 or N+22) mod 2715648 uplink block 11 TDMA frame number = (N+26) mod 2715648 uplink block e) Effective payload type This field indicates the range of the effective data contained in the RLC/MAC block. For the meaning of this field, see Table 10-13. Table 10-13 Meaning of the effective payload type Value Meaning 00 The RLC/MAC block contains one RLC data block. 01 The RLC/MAC block contains one RLC control block but not contains the optional bytes in the RLC/MAC control header. 10 On the downlink, the RCL/MAC block contains both one RLC control block and the first optional byte in the RLC/MAC control header. 11 Reserved. f) FBI bit The FBI bit stands for the final block indicator bit, indicating the final downlink RLC data block in the downlink TBF. For the meaning of the FBI bit, see Table 10-14. Table 10-14 Meaning of the FBI bit Value Meaning 0 The current block is not the final RLC data block in the TBF. 1 The current block is the final RLC data block in the TBF. GSM Radio Network Planning and Optimization Chapter 10 GPRS Radio Network Planning and Optimization For internal use only 1/28/2011 All rights reserved Page40 of 94 g) RTI field The RTI stands for the radio transaction identifier. This field is used to number a group of the RLC/MAC blocks constituting a RLC control message. A RTI consists of 5 bits, and its value ranges from 1 to 31. h) TFI field The TFI field stands for the temporary block flow identifier field. It identifies all the TBFs affiliated to the RLC. Different channels can share the same TFI, which can identify the same TBF and different TBFs. The TBI of a channel is affiliated to an uplink or downlink TBF only at any time. The same MS can use either the same TFI or different TBIs during TBF transmission. Both the uplink TFI and downlink TFI are 5-bit binary codes, and their value ranges from 0 to 31. i) BSN field The BSN field stands for the block sequence number field. It is the number of each of the RLC data blocks in the TBF. The BSN is the 7-bit binary code, and its value ranges from 0 to 127. IV. MS multislot capability The MS multislot capability is divided into 27 classes. The number of the packet channels can be used by the MS simultaneously varies with the multislot capability class. The MS reports its multisolt capability class in the packet resource request message. When allocating the radio resource to the MS, the BSS must consider multiple factors, such as the amount of the data to be transmitted, the required QoS, and the number of available radio channels. If the radio resource can be fully used, the multislot capability class of the MS should be ensured to the maximum. Table 10-15 lists the multislot capability of the MS. Table 10-15 MS multislot capability Multislot capability class Maximum timeslots Multislot capability class Maximum timeslots Rx Tx Sum Rx Tx Sum 1 1 1 2 16 6 6 NA 2 2 1 3 17 7 7 NA 3 2 2 3 18 8 8 NA 4 3 1 4 19 6 2 NA 5 2 2 4 20 6 3 NA 6 3 2 4 21 6 4 NA 7 3 3 4 22 6 4 NA 8 4 1 5 23 6 6 NA 9 3 2 5 24 8 2 NA 10 4 2 5 25 8 3 NA GSM Radio Network Planning and Optimization Chapter 10 GPRS Radio Network Planning and Optimization For internal use only 1/28/2011 All rights reserved Page41 of 94 11 4 3 5 26 8 4 NA 12 4 4 5 27 8 4 NA 13 3 3 NA 28 8 6 NA 14 4 4 NA 29 8 8 NA 15 5 5 NA Note: Rx indicates the maximum timeslots used by the MS for downlink transmission in a TDMA frame. Tx indicates the maximum timeslots used by the MS for uplink transmission in a TDMA frame. Sum indicates the sum of the timeslots used by the MS for data transmission in a TDMA frame. That is, the number of the timeslots used by the MS for uplink and downlink transmission must be equal to or less than the value of the Sum. According to the multislot capability class configured with the MS, the MS can be divided into two types. That is, type 1 and type 2. In the 29 multislot capability classes, the MSs whose multislot capability classes are 1-12 and 19-29 belong to type 1, and the MSs whose multislot capability classes are 13-18 belong to type 2. The MSs of type 1 cannot transmit and receive data simultaneously, while the MSs of type 2 can. V. DRX The DRX stands for discontinuous reception. To reduce the power consumption in idle state, the MS needs to adopt the DRX model. Under this model, the MS receives the related paging messages on the paging channels only regardless of the paging type. When attaching to the GPRS, the MS needs to notify the GPRS/GSM network whether it supports the DRX or not. If yes, the MS works according to the DRX parameters received from the network. For the current network, it determines the paging group that the MS belongs to according to the BS_PA_MFRMS received on the CCCH. VI. Power control Currently, the GPRS system provides the algorithms for uplink open loop power control. The open power control is based on the assumption that the uplink and downlink have the same path loss. Therefore, the MS can adjust the output power according to the level of the received signals. The power control parameters are delivered in the SI13. In the GPRS system, the algorithm for uplink power control is expressed as follows: P CH = min (I 0 - I CH - o * (C + 48), PMAX) Here, P CH is the output power of the MS during uplink power control I 0 is the maximum transmit power of the MS I CH is the power control parameter sent by the network side to the MS. o is the power control parameter sent by the network side to the MS GSM Radio Network Planning and Optimization Chapter 10 GPRS Radio Network Planning and Optimization For internal use only 1/28/2011 All rights reserved Page42 of 94 C is the normalized level of the received signals. It is the average level of the bursts of 4 TDMA frames in a radio blocks. PMAX is the maximum transit power of the MS allowed by the cell, and it is delivered in the system information. For open loop power control, the o = 1.0, so P CH = min (I 0 - I CH - C - 48), PMAX). When the MS performs the uplink open loop power control, the values of I 0, C, and PMAX are known for the MS. The value of the I CH is calculated by the network side according to the BTS receiving level, BTS transmit power, and the maximum MS transmit power. In addition, the value of the I CH can be sent to the MS in the RLC/MAC control message. When the MS performs the downlink open loop power control, it uses the same power to transmit the 4 TDMA frames in a radio block. When the MS has reselected and entered a new cell, it must transmit the data with the maximum output before receiving the power control parameters from the cell. VII. Timing advance control Because the packet data is discontinuously transmitted, it is hard for the network side to obtain the timing advance (TA) of the MS. As a result, the downlink data may not be normally received. Therefore, the GPRS system introduces a new timing advance algorithm, which is divided into initial timing advance control algorithm and continuous timing advance control algorithm. In this case, the valid TA can always be obtained. 1) Initial timing advance control algorithm When the MS accesses the network, the BTS uses this algorithm to calculate the TA of the MS according to the access burst received on the PACH (the MS sends the access burst). After that, the BTS delivers the BTS to the MS in a Packet Assignment message. If a TBF is established during uplink or downlink data transmission, the Packet Downlink Assignment message may not include TA. In this case, the network side can set the polling bits in the Packet Downlink Assignment message. After receiving the message, the MS will send a Packet Control Ack/Nack message to the network side to help it to obtain the TA. In addition, when the MS has sent a Packet Channel Request message, the network cannot necessary deliver a Packet Assignment message to the MS due to no available resource. In this case, to obtain the TA, the network side can enable the packet polling procedure to send a Packet Polling message to make the MS send 4 Packet Ack/Nack messages in the form of bursts. If the Packet Assignment message does not include TA, the MS can obtain the TA from the Packet Power Control/Timing Advance message or the timing advance update only. GSM Radio Network Planning and Optimization Chapter 10 GPRS Radio Network Planning and Optimization For internal use only 1/28/2011 All rights reserved Page43 of 94 2) Continuous timing advance control algorithm This algorithm is applied to the packet transfer mode. The MS ascertains the PTCCH allocated to it the according to the TAI sent in the Packet Assignment message and sends the access bursts on the corresponding PTCCH. (The TAI stands for timing advance index, and its value ranges from 0 to 15.). The network side calculates the new TA of the MS according to the received access bursts. After that, it tells the TA to the MS through the corresponding PTCCH, or the Packet Power Control/Timing Advance message and the Packet Uplink ack/Nack message. 10.1.5 System Information and Main Flows I. Packet system information The packet system information (PSI) is used to broadcast the parameters needed by the MS when the MS intends to access the network. If the cell supports GPRS service, the SI13 must be added to the BCCH. The SI13 includes the information on GPRS cell access control, GPRS cell selection and reselection, and GPRS power control. The PBCCH can be either configured or not configured in a cell. The SI13 will tell the MS whether the PBCCH is present in the cell or not. On the PBCCH mainly the GPRS dedicates PSI is broadcast, including PSI1, PSI2, PSI3, PSI3bis, PSI4, PSI5, and PSI13. The following lists them respectively. PSI1 contains the information on cell selection, PRACH control, control channel description, and power control parameters. PSI2 contains the information on frequency list, cell allocation table, GPRS mobile allocation table, and PCCCH description. PSI3 contains the information on neighbor cell BCCH allocation table and the service cell/non-service cell selection parameters. PSI3bis contains the information on neighbor cell BCCH allocation table and non-service cell selection parameters. PSI4 contains the PDCH lists used by the MS to make measurement in service cells. PSI5 contains the information on measurement report and network control cell reselection. PSI13 is the same as the SI13 broadcast on the BCCH. It contains the information relative to the access of the specific GPRS cells. The PSI1, PSI2, PSI3, and PSI4 can be broadcast either on the PBCCH or PACCH. The PSI5 can only be broadcast on PBCCH. The PBCCH can only be broadcast on PACCH. If there is a PBCCH in the cell, it does not send the PSI13 and the PSI1 is broadcast periodically on the PACCH. If there is no PBCCH in a cell, the PSI13 is broadcast periodically on the PACCH. GSM Radio Network Planning and Optimization Chapter 10 GPRS Radio Network Planning and Optimization For internal use only 1/28/2011 All rights reserved Page44 of 94 II. Cell reselection The GPRS cell reselection is independent of the GSM cell reselection. Handover is not used in the GPRS system. In either packet transfer mode or packet idle mode, the service cell is selected according to cell reselection program in the GPRS system. In the GPRS system, the cell reselection can be performed by the MS automatically or controlled by the network side. 1) MS automatically-started cell reselection The MS always monitors the PBCCH/BCCH carrier of the neighbor cells. It chooses to attach the best cell according to the signal strength and the base station color code (BCC) of the carriers. Meanwhile, it starts the RA update procedure to notify its RA to the network. If no PBCCH is present in service cell where the MS is in, the MS will monitor the SI broadcast on the BCCH and reselects the cell using the C1/C2 rules that are used in the GSM. For the parameter CRH, however, it is processed according to different situations. When the MS is in packet idle mode, if the RA of the service cell is different from that of the reselected new cell, the CRH must be used. When the MS is in ready state, if the MS reselects a new cell, the CRH must be used. If the PBCCH is configured with the service cell where the MS is in, the MS needs to monitor the SI broadcast on the PBCCH only. In this case, the MS can reselect the cell using the C31/C32 rules used in the GPRS. (For details, see protocols GSM05.08.) Generally, no PBCCH is configured with the current GPRS system, so the MS reselect the cell using the C1/C2 rules only. 2) Network-controlled cell reselection The network side can also control the cell reselection. In this case, the MS will submit the measurement report to the network side periodically according to the parameters broadcast in the SI. The network will send the packet cell change order to assign the cell that the MS must attach according to the factors, such as the measurement report from the MS and the load of the neighbor cells. Upon receiving the packet cell change order, the MS will stop transmitting the data on the uplink immediately and decoding the data on the downlink. Meanwhile, the MS starts the timer T3174 and begins to enter the reselected cell. This process is controlled by the parameter NETWORK_CONTROL-ORDER, which is a network control mode parameter. There three types of network control modes, including NC0, NC1, and NC2. They are defined as follows. a) NC0 GSM Radio Network Planning and Optimization Chapter 10 GPRS Radio Network Planning and Optimization For internal use only 1/28/2011 All rights reserved Page45 of 94 This is for general MSs. In this mode, the MS will reselect the cell automatically without sending the measurement report to the network side. b) NC1 In this mode, the MS will send the measurement report to the network side while reselect the cell automatically. c) NC2 This is the network control mode. In this mode, the MS will send the measurement report to the network side but not reselect the cell automatically. Instead, it accepts the network-controlled cell reselection. The NC1 and NC2 are applicable to the MSs in ready state only. The NC0 is applicable to the MSs both in ready state and standby state. That is, the MSs in standby state can only use the NC0 mode. In the current GPRS phase, only the NC0 mode is used for cell reselection. That is, the network-controlled cell reselection has not been used. III. Uplink TBF establishment When there is data to be transmitted on the upper layer of the MS, the RLC/MAC layer of the MS will initiate packet access procedure. The packet access of the MS can be divided into short access, one phase access, two phase access, paging response, cell update, and mobility management. They are detailed as follows: If the data blocks to be transmitted are less than 8 RLC blocks, the channel request type of the MS is short access. The number of data packets is calculated according to CS-1. If the data blocks to be transmitted are more than 8 TLC blocks and the RLC mode is required to be the acknowledged mode, the channel request of the MS is one phase access or two phase access. If the measurement report of the MS is to be transmitted, the channel request type of the MS is the first phase of the two phase access. If the paging response, cell update, or mobility management works as the channel request type, it is generally be treated as one phase access or two phase access. The same procedure is used for short access or one phase access. That is, the radio resource (such as the TFI and dynamically-allocated USF, or the radio block bit table) is allocated to the MS once, and then the MS begins to transmit the data. For the two phase access channel request, the network side allocates one radio block to the MS only for the first time. The MS sends the Packet Resource Request message on the single allocated radio block, and then the network side allocates the radio GSM Radio Network Planning and Optimization Chapter 10 GPRS Radio Network Planning and Optimization For internal use only 1/28/2011 All rights reserved Page46 of 94 resource (including the TFI, USF, or radio block bit table) to the MS for the second time). A packet channel request is an 8-bit or 11-bit access burst, which carries only a little information, but a packet resource request is a RLC/MAC control block coded according to CS-1, which carries more information, including the TLLI of the MS, the multisolt capability of the MS, and the radio priority class), so this is helpful for the network to allocate more suitable resource to the MS. In the current GPRS system, most carriers adopt the two phase access, so the following mainly introduces the two phase access procedure. Figure 10-10 shows the uplink two phase access (acknowledged mode) flow on CCCH. Figure 10-10 Uplink two phase access (acknowledged mode) flow on CCCH The following details the flow: The MS sends a Channel Request message to the network side on RACH. a) Upon receiving the Channel Request message, the network side will send an Immediate Assignment message, which carries an assigned uplink block, on the corresponding AGCH. b) The MS sends a Packet Resource Request message, which carries TLLI, in the assigned uplink block. c) The network side allocates the uplink channel to the MS according to the packet resource request and receives the Packet Uplink Assignment message, which GSM Radio Network Planning and Optimization Chapter 10 GPRS Radio Network Planning and Optimization For internal use only 1/28/2011 All rights reserved Page47 of 94 carries TLLI, delivered on the PDCH. In this case, the two phase access contention and decision are completed. d) For dynamic allocation, the network side sets the USF on the assigned uplink packet channel, and the MS sends the RLC data blocks, which carries no TLLI, on the corresponding uplink blocks. To acknowledge that the RLC data blocks have been received, the network side sends back a Packet Uplink Ack/Nack message to the MS after receiving multiple uplink RLC data blocks according to the range and the data transmission delay of the sliding window. In addition, the network side sends back the Packet Uplink Ack/Nack message to the MS upon receiving an uplink RLC data block whose SI field is 1. e) When the data is being transmitted, the network will acknowledge the data. After that, the MS will consider retransmitting the data according to the acknowledgement. f) When the data is transmitted to the last BS_CV_MAX (it is a parameter broadcast in the SI) data block, the MS will start the countdown flow. In the last uplink RLC data block, the MS will se the CV field to 0. g) Upon receiving the data block whose CV field is 0, the network side will send a Packet Uplink Ack/Nack message to the MS. If the network side has received all the RLC data blocks, it will set the FAI field of the Packet Uplink Ack message to 1. If the network has not received all the network RLC data block, it will set the FAI to 0. In this case, the MS must retransmit the unacknowledged data block. h) Upon receiving a Packet Uplink Ack/Nack message whose FAI is 1, the MS will send a packet control ack message to the network side, and then release the TBF. The data transmission flow in the uplink unacknowledged mode is almost the same as that in the uplink acknowledged mode except that the data do not have to be retransmitted in the former mode. IV. Downlink TBF establishment If the network side needs to send data to the MS in packet idle mode, it must establish the downlink TBF through packet paging. 1) Paging coordination In the GPRS system, the network side can page the circuit service through the paging coordination as well as packet paging. For the GPRS-attached and IMSI-attached MS, the MSC/VLR can page the circuit service when the network works in network of operation mode I (NMOI). In this case, the connection between the SGSN and MSC GSM Radio Network Planning and Optimization Chapter 10 GPRS Radio Network Planning and Optimization For internal use only 1/28/2011 All rights reserved Page48 of 94 must support the Gs interface. If the MS is in standby state, the page is performed in the RA. If the MS is in ready state, the page is performed in the cell. Under NMOI, if a PDCH is allocated to the MS, the system sends the paging message on the PACCH. If no PDCH is allocated but a PCCCH is configured with the system, the system sends the paging message on the PPCH. If the PCCCH is not configured with the system, the system sends the paging message on the PCH through Pb interface. If no Gs interface is provided between the SGSN and MSC, the GPRS/GSM system can works in NMOII and NMOIII. In this case, the system can send the Packet Paging message on the CCCH only. Upon receiving the Packet Paging message, the MS accesses the RACH and then starts to connect the circuit. If the MS is performing the GPRS service, the MS will starts the GPRS-suspended procedure to suspend the GPRS service unless the circuit connection is released. After that, the network side starts the recovery procedure to recover the GPRS service. For the coordination between GPRS NMO and paging, see Table 10-16. Table 10-16 Coordination between GPRS NMO and paging NMO Channel for circuit paging Channel for GPRS paging Paging coordination I PPCH PPCH The paging coordination function is available and the Gs interface should be selected. For the GPRS-attached MSs, the packet paging channel assigned by the network functions the same as the circuit paging channel, so the MSs have to monitor one type of the channels only. If having allocated a PDCH to the MS, the network delivers the circuit paging information to the MS on the PDCH. PCH PCH PACCH – II PCH PCH The paging coordination function is not available. All paging messages are delivered on PCH and the MS has to monitor the PCH only, even if it has been assigned the PDCH. III PCH PPCH The paging coordination function is not available. The network delivers the Circuit Paging message on the PCH. If the cell is configured with the PCCCH, the network delivers the Packet Paging messages on the PPCH; otherwise it is sent on the PCH. PCH PCH 2) Downlink TBF establishment on CCCH Figure 10-11 shows the downlink TBF establishment flow. GSM Radio Network Planning and Optimization Chapter 10 GPRS Radio Network Planning and Optimization For internal use only 1/28/2011 All rights reserved Page49 of 94 Figure 10-11 Downlink TBF establishment flow The following detail the flow: i) Where there is downlink data to be transmitted to the MS, the SGSN must send a Packet Paging message. j) The SGSN sends the Packet Paging message to the PCU through Gb interface, and then the PCU converts the message into a Packet Paging Request message at the Um interface and sends it to the MS. If the BSS system is configured with the PCCCH, this message is sent on the PPCH. If no PCCCH is configured for the BSS system, the PCU delivers this message to the BSC through Pb interface, and the BSC sends it on the PCH. k) Upon receiving the Packet Paging message, the MS will initiate the TBF establishment flow, and then send the message in the form of RLC data block to the PCU at the Um interface through packet response packets. The RLC data block carries the TLLI. l) The PCU sends the paging response data block in the form of uplink PDU to the SGSN. GSM Radio Network Planning and Optimization Chapter 10 GPRS Radio Network Planning and Optimization For internal use only 1/28/2011 All rights reserved Page50 of 94 m) Upon receiving the paging response, the SGSN make the response to the PCU in the form of downlink PDU. n) Upon receiving the downlink PDU from the SGSN, the PCU delivers an Immediate Assignment message on the CCCH through Pb interface. The Immediate Assignment message carries the assigned downlink packet channels and the starting time of TFI and TBF. o) During the downlink TBF establishment, to enable the MS to obtain the valid TA, the network side can deliver the Packet Polling Request message with TFI as the identification on the downlink PACCH. p) Upon receiving the Packet Polling Request message, the MS will responds a Packet Control Acknowledgement message whose type is access burst (AB) on an immediate assignment channel. q) The network side extracts the TA from the AB, and then notifies the TA to the MS through a Packet Power Control/Timing Advance message. After that, the network assigns the downlink channel to the MS by delivering a Packet Downlink Assignment message. r) The network side fragments the PDU into several downlink RLC data blocks and sends them to the MS through the downlink packet channel. According to the range of the sliding window, the network side sets the RRBP field in the sent RLC data blocks. According to the indication of the RRBP, the network side acknowledges the received the RLC data block by sending the Packet Downlink Ack/Nack message on the corresponding PACCH. s) The network side will set the FBI to 1 in the last downlink data block. If the MS has receives all the RLC data blocks, it will send the packet downlink ack/nack whose FAI is 1; otherwise it will send the packet downlink ack/nack whose FAI is 0 to request the network to resend the data blocks that it has not received. t) Upon receiving the Packet Downlink Ack/Nack message whose FAI is 1, the network side will release the TBF. V. Mobility management The GPRS mobility management includes many aspects; including GPRS attach/detach and cell/RA update, and coordinated RA/LA update. When the MS completes the GPRS attach, the SGSN will establish mobility management for the MS, stores the current location and state of the subscriber. After that, when the MS roams between cells or RAs, it will start the cell/RA update automatically. The SGSN will monitor the MS and store the latest location of the MS if the MS performs the cell/RA update successfully. When the MS performs the coordinated GPRS attach/IMSI attach GSM Radio Network Planning and Optimization Chapter 10 GPRS Radio Network Planning and Optimization For internal use only 1/28/2011 All rights reserved Page51 of 94 and coordinated cell/RA update, the SGSN will exchange the location information of the MS with the MSC through the Gs interface. The following introduces the GPRS attach, cell update, location area (LA) update, and GPRS detach respectively. 1) GPRS attach The GPRS attach is divided into normal GPRS attach and coordinated GPRS attach. The normal GPRS attach attaches the IMSI of the MS to the GPRS service and the coordinated GPRS attach attaches the IMSI of the MS to both the GPRS service and non-GPRS service. The GPRS attach is applied to most of the current networks. Figure 10-12 shows the coordinated GPRS attach. 7d. Cancel Location Ack 7c. Cancel Location 7b. Update Location 7g. Update Location Ack 7e. Insert Subscriber Data 7f. Insert Subscriber Data Ack 6d. Insert Subscriber Data 6c. Cancel Location Ack 6b. Cancel Location 3. Identity Response 2. Identification Response 2. Identification Request 1. Attach Request 5. IMEI Check 3. Identity Request 4. Authentication 6a. Update Location 7a. Location Update Request 7h. Location Update Accept 6f. Update Location Ack 6e. Insert Subscriber Data Ack MS BSS new SGSN old SGSN GGSN HLR EIR old MSC/VLR new MSC/VLR 9. Attach Complete 8. Attach Accept 10. TMSI Reallocation Complete Figure 10-12 Coordinated GPRS attach The following details the flow: GSM Radio Network Planning and Optimization Chapter 10 GPRS Radio Network Planning and Optimization For internal use only 1/28/2011 All rights reserved Page52 of 94 a) The MS sends an Attach Request message to the SGSN. The message carries the IMSI (or P-TMSI and the old RAI), MS classmark, attach type, DRX parameter, and the old P-TMSI signature (it is used when the P-TMSI is used). Whether to use the IMSI or not depends on whether the current P-TMSI is valid or not. The P-TMSI and the old RAI must appear at the same time. The attach type indicates the current attach types, including GPRS attach, IMSI attach, and coordinated GPRS/IMSI attach. b) If the MS is identified by the P-TMSI and the it has moved to another SGSN service area when it is in detach state, the SGSN obtains the IMSI of the MS through sending an identification request message (it contains the P-TMSI, the old RAI, and the old T-PMSI) to the old SGSN. The old SGSN will send back an identification response message, which contains the IMSI and authentication triplet. c) If both the new and the old MS do not know the MS, the new SGSN will send an identity request message to the MS, with ISMI as the identification type. The MS will send back an identity response message, which carries the IMSI. d) If the MM context does not exist at the network side, the network side will enforce the authentication procedure. If the P-TMSI allocation is necessary, the network side must support encryption. e) The flow checking the IMEI is optional. Generally, it can be omitted. f) If the SGSN codes change during GPRS detach, the new SGSN must send an update location message, which includes the SGSN codes, SGSN address, and IMSI, to the HLR. After that, the HLR will send a cancel location message to the old SGSN. Upon receiving the message, the old SGSN will sends back a Cancel Location Ack message. Then the HLR will send an Insert Subscriber Data message, which contains the IMSI and the data of GPRS subscribers, to the new SGSN. After acknowledging that the MS is in a new RA, it constructs the MM context of the MS and sends back an Insert Subscriber Data Ack message, which carries the IMSI, to the HLR. After canceling the old MM context and inserting the new MM context, the HLR will send back an Update Location Ack message to the new SGSN. g) If the attach type of the MS is IMSI attach or GPRS/IMSI attach, upon receiving the first Insert Subscriber Data message from the HLR, the new SGSN will start the location update to the new MSC/VLR. The VLR codes are from the LA information. This operation makes the VLR identifies the MS as GPRS attach. h) The new SGSN sends an Attach Accept message to the MS. The message contains the P-TMSI, VLR TMSI, P-TMSI, and SMS priority. GSM Radio Network Planning and Optimization Chapter 10 GPRS Radio Network Planning and Optimization For internal use only 1/28/2011 All rights reserved Page53 of 94 i) If the received P-TMSI and the VLR TMSI have been updated, to acknowledge that the TMSI has been received, the MS will send back an Attach Complete message. j) If the received VLR TMSI has been updates, to acknowledge that the VLR TMSI is reallocated, the new SGSN will send a TMSI Reallocation Complete message to the VLR. k) If the attach request has not been accepted, the new SGSN will send back an Attach Reject message to the MS. 2) Cell update When the MS in ready state enters the new cell that has the same RA with the old one, the MS will perform cell update. The following details the cell update flow: a) The MS sends an uplink LLC-PDU of any type that carries the identification of the MS to the BSS. b) At the Gb interface, the BSS sends the received PPC-PDU and the CGI of the new cell (the CGI includes the RAC and LAC) to the SGSN in the form of BSSGP PDU. c) The BSSGP PDUs that contain the identification of the new cell are correctly received by the SGSN. d) The SGSN records the results of the cell update of the MS. The network sends the subsequent service to the MS through the new cell. 3) LA update The LA update includes the update of the LAs within the SGSN, the update of the LAs between SGSNs, the update of the coordinated RA/LA within the SGSN, and the update of the coordinated RA/LA between RA/LAs. Generally, a SGSN contains one or more LA, and each LA contains one or more RA. The IMSI-attached and GPRS-attached MS can perform the coordinated RA/LA update when entering the cells of NMOI. If the MS enters the a RA of other NMO that does not support paging coordination, it is meaningless for the MS to initiate the RA/LA update. In addition, the ways the MSs supporting the RA update vary with the types of the MSs. For the MSs of chass A, they support the RA update only when performing the circuit service. For the MSs of class B, they do not support any update when performing the circuit update. For the MSs of class C, they do not support the coordinated RA/LA update. GSM Radio Network Planning and Optimization Chapter 10 GPRS Radio Network Planning and Optimization For internal use only 1/28/2011 All rights reserved Page54 of 94 For the current GPRS networks, its data traffic is not great, so the RA and the LA are set in the same way. Figure 10-13 shows the update flow of the RA/LA within a SGSN. Figure 10-13 Update of the coordinated RA/LA within a SGSN. The following details the flow: a) The MS sends a Routing Area Update Request message, which contains the old RAI, the old P-TMSI signature, and the update type) to the SGSN. The update type indicates whether the update is the coordinated RA/LA update or the IMSI-attached RA/LA update. Before sending the message to the SGSN, the BSS joins the CGI (including the RAC and LAC) of the cell where the message belongs to. b) The security function is optional. c) If the Gs interface is present and has connected the SGSN to the VLR, when the update type indicates that the coordinated RA/LA attached to the IMSI updates or the LA changes during LA update, the SGSN will send a Location Update Request message, which includes the new LAI, IMSI, SGSN number, and LA update type, to the VLR. The location update type indicates whether the location update is IMSI attach or it is the normal update. The VLR number can be GSM Radio Network Planning and Optimization Chapter 10 GPRS Radio Network Planning and Optimization For internal use only 1/28/2011 All rights reserved Page55 of 94 obtained from the RAI listed in the data table of the SGSN. The VLR creates or updates its connection with the SGSN through storing the SGSN number. d) If the VLR discovers that the data of a subscriber has not been identified by the HLR, the new VLR must notify it to the HLR. Upon receiving the notification, the HLR must cancel the data of the subscriber in the old VLR and insert the data to the new VLR. e) The new VLR allocates a new VLR TMSI to the subscriber and sends back a Location Update Accept message (VLR TMSI) to the SGSN. If the VLR has not changed, this message does not contain the VLR TMSI. f) If the MS is not allowed to attach to the new LA or the IMEI (optional) detection fails, the SGSN will rejects the Location Update Request. If the MS is allowed to attach to the new LA and the IMEI detection (optional) succeeds, the SGSN will update the MM context of the MS and allocates a new P-TMSI to the MS. Meanwhile, the SGSN will send back a Routing Area Update Accept message, which carries the new P-TMSI, new VLR TMSI, and P-TMSI, to the MS. g) If the P-TMSI or the VLR TMSI is reallocated successfully, the MS will send back a Routing Area Update Complete message, which carries the new P-TMSI and new VLR TMSI, to the SGSN. h) If the MS has acknowledged the VLR TMSI, the SGSN will send a TMSI Reallocation Complete message (VLR TMSI) to the VLR. 4) GPRS detach The GPRS detach can be initiated by either the MS or the network side, in which the GPRS detach initiated by the network side can be initiated by either the SGSN or the HLR. The purpose for the HLR to initiate the GPRS detach is to cancel the subscriber MM and PDP contexts stored in the SGSN. And the GPRS detach initiated by the SGSN can help the MS reattach to the GPRS or reactivate the old PDP contexts. Figure 10-14 shows the GPRS detach initiated by the MS. 3. IMSI Detach Indication 2. Delete PDP Context Response 1. Detach Request 2. Delete PDP Context Request 5. Detach Accept MS BSS GGSN SGSN MSC/VLR 4. GPRS Detach Indication GSM Radio Network Planning and Optimization Chapter 10 GPRS Radio Network Planning and Optimization For internal use only 1/28/2011 All rights reserved Page56 of 94 Figure 10-14 GPRS detach initiated by the MS The following details the flow: a) The MS sends a Detach Request message that contains the detach type and switch off indication to the SGSN. The detach type includes GPRS detach, IMSI detach, and coordinated IMSI/GPRS detach. The switch off indication indicates whether the detach is cause by the switch off or not. b) If the detach type is GPRS detach, the SGSN will send a Delete PDP Context Request message to the GGSN to activate the PDP context of the MS in the GGSN. Meanwhile, the GGSN must confirm the message by sending back a Delete PDP Context Response message to the SGSN. c) If the detach type is IMSI detach, the SGSN will send an IMSI Detach Indication message to the VLR. d) If the MS attached to both the IMSI and GPRS performs GPRS detach only, the SGSN will send a GPRS Detach Indication message to the VLR. Upon receiving this message, the VLR will cancel the information related to the SGSN and not deliver any paging message or location update message through this SGSN. e) If the switch off indication indicates that the GPRS detach is not caused by the switch off, the SGSN will send a Detach Accept message to the MS. VI. Session management The GPRS session management includes PDP context activation and deactivation, PDP context correction, and short message sending and reception. In addition, the GPRS session management can be also applied to suspend and recover the circuit switched service under GPRS packet transfer mode. The MS can be connected to many other external PDNs through the GPRS network. As a result, to tell the packet data service from each PDN, the GPRS network will allocate a PDP address to the each PDN, and each PDP address matches a PDP context. The PDP context describes the PDP state and the information related to the current services, including PDP type, PDP address, PDP state, APN, NSAPI, and QoS. The PDP state includes PDP activation and PDP deactivation. When the MS attaches to the GPRS network, it must activate the PDP context if it intends to transmit packet data. When transmitting the data, the MS can modify the PDP context according to the status of the current service. After finishing transmitting the data, the MS breaks the connection with the external PDNs by deactivating the PDP context. GSM Radio Network Planning and Optimization Chapter 10 GPRS Radio Network Planning and Optimization For internal use only 1/28/2011 All rights reserved Page57 of 94 1) PDP context activation Figure 10-15 shows the PDP context activation initiated by the MS. Figure 10-15 PDP context activation initiated by the MS The following details the activation flow: a) The MS sends an Activate PDP Context Request message to the SGSN. This message contains the NSAPI, TI, PDP type, PDP address, APN, requested QoS, PDP configuration options. The NSAPI tells the multiple PDP contexts simultaneously constructed by the MS from each other. The network uses the IMSI and the NSAPI together to distinguish different PDP contexts, so the NSAPIs of the PDP contexts of the MS must be different with each other. The TI and PDP type indicate the type of the PDP address, which is divided into the IP address and the X.121 address. The PDP address indicates that is the dynamic PDP address or the static PDP address the MS adopted. If the MS adopts the dynamic PDP address, the field is vacant. The APN is the logical name of an interface of an external PDN that the MS intends to connect. The MS can choose a reference point in the external networks with the help of APN, and the APN corresponds to one physical or logical interface in the GGSN. The requested QoS indicates the QoS script that the MS expects to use. The PDP configuration options contain the protocol options necessary for the data transmitted between the GGSN and the MS. The PDP configuration options are transparently transmitted through the SGSN. GSM Radio Network Planning and Optimization Chapter 10 GPRS Radio Network Planning and Optimization For internal use only 1/28/2011 All rights reserved Page58 of 94 b) After receiving the request message, the SGSN judges whether to perform the authentication and encryption and reallocate the P-TMSI or not. For anonymous access, the SGSN does not perform the authentication and encryption. c) After completing the encryption, the SGSN provides the MS with the PDP context and the PDP address (if it is static address) and authenticate the APN through the information recorded in the PDP context. First the SGSN must authenticate whether the end of the APN address is marked with GPRS. If yes, the SGSN must remove the identification part (such as .MNC.MCC.GPRS) from the APN and compare the network name with the subscription information. If the subscription record exists, the SGSN must continue authenticating other records concerning the MS. After that, the SGSN sends the complete APN to the DNS and obtains the corresponding address of the GGSN from the APN. If the SGSN cannot resolve the address of the GGSN from the APN or the activation request is invalid, the SGSN will reject the PDP activation request. If the SGSN resolve the address of the GGSN from the APN successfully, it creates a TID (IMSI + NSAPI) (The IMSI is stored in the MM context of the SGSN, and the NSAPI is from the Activate PDP Context Request message), and then sends a Create PDP Context Request message to the GGSN. This message contains the PDP type, PDP address, APN, the negotiated QoS, TID, MSISDN, selection model, and PDP configuration options. If the MS requests the dynamic address, the PDP address is vacant, so the SGSN has to request the GGSN to allocate a dynamic address to the MS. The GGSN can find out an external network with the help of APN. The SGSN can restrict the properties of the requested QoS according to its capability, current load and the subscribed QoS script. The selection model indicates which type of APN is used, including the subscribed APN, the non-subscribed APN sent by the MS, and the non-subscribed APN selected by the SGSN used. d) The GGSN can decide to accept or reject the PDP context activation according to the selection model. For example, if the subscribed APN is required, the GGSN only accepts the PDP context activation for the subscribed APN indicated by the SGSN selection model. If the GGSN accepts the PDP context activation, it will create a new record that will develop into a billing identification in the PDP context list. This record allows the PDP PDU to be transmitted between the GGSN, SGSN, and external networks. Meanwhile, the billing begins. GSM Radio Network Planning and Optimization Chapter 10 GPRS Radio Network Planning and Optimization For internal use only 1/28/2011 All rights reserved Page59 of 94 The GGSN can further restrict the negotiated QoS according to its capability and current load. After that, the GGSN will send back a Create PDP Context Response to the SGSN. This message contains the TID, PDP address, re-sequencing request, PDP configuration options, negotiated QoS, billing identification, and cause value. If a PDP address is allocated in the GGSN, the Create PDP Context Response message must contain the PDP address unit. The re-sequencing request indicates whether to re-sequence the N-PCUs before the SGSN sends them to the MS. The PDP configuration options contain the optional PDP parameters that the GGSN can send to the MS. These PDP parameters are transparently transmitted through the SGSN. The Create PDP Context message traverses the whole GPRS backbone network. If the negotiated QoS received by the SGSN is incompatible with the PDP context to be activated, that is, the reliability class cannot support the PDP type; the GGSN will reject this Create PDP Context Request message and sends back a Reject Create PDP Context Request message to the SGSN. The compatible script is configured by the GGSN operator. e) If the SGSN receives the Create PDP Context Response message from the GGSN, it will insert the NSAPI, GGSN address, dynamic PDP address into the PDP context. In addition, it will select the radio priority class according to the negotiated QoS. After that, it will send back an Activate PDP Context Accept message to the MS. This message includes the PDP type, PDP address, TI, negotiated QoS, radio priority, and the PDP configuration options. In this case, the SGSN can transmit the PDP PDU between the MS and the GGSN and the billing begins. Different QoS scripts can be applied for the each PDP address. For example, some PDP address related to the application of the E-mail can stand a long delay, while the PDP address related to other applications, such as the interactive application, cannot stand the delay and requires great throughput. These requirements are reflected at the QoS script. If such requirements is beyond the PLMN capability, the PLMN-negotiated QoS script must as near to the requested QoS as possible. For the MS, it has to choices. One is accept the negotiated QoS script, and the other is to activate the PDP context. f) After receiving an Activate PDP Context Accept message from the MS, the SGSN enters the PDP activation state. At this time, the route for the packet data transmission between the MS and the GGSN is established. If the PDP context activation fails or the SGSN sends back an Activate PDP Context Reject message, with the PDP Configuration Options as the cause, the MS can attempt GSM Radio Network Planning and Optimization Chapter 10 GPRS Radio Network Planning and Optimization For internal use only 1/28/2011 All rights reserved Page60 of 94 to activate the same APN for up to 5 times. If the MS still fails to activate it for the fifth time, the MS will stop activating the PDP context. 2) PDP context deactivation Table 10-17 shows the PDP deactivation initiated by the MS. Table 10-17 PDP context deactivation initiated by the MS The following details the deactivation flow: a) The MS sends a Deactivate PDP Context Request message that carries the TI needed in the PDP context deactivation, to the SGSN. b) The security functions are optional. c) The SGSN sends a Delete PDP Context Request message to the GGSN. Upon receiving it, the GGSN deletes the PDP context and sends back a Delete PDP Context Response (TID) to the SGSN. If the MS is using the dynamic address, the GGSN releases the PDP address and marks this address for subsequent use. The Delete PDP Context message is transmitted through the whole GPRS backbone network. d) The SGSN sends back a Deactivate PDP Context Accept (TI) message to the MS. 3) PDP context modification Figure 10-16 shows the PDP context modification. GSM Radio Network Planning and Optimization Chapter 10 GPRS Radio Network Planning and Optimization For internal use only 1/28/2011 All rights reserved Page61 of 94 Figure 10-16 PDP context modification The following details the modification flow: a) The SGSN sends an Update PDP Context Request message that contains the TID, negotiated QoS, and the programs used to start the PDP context modification to the GGSN. b) Upon receiving the Update PDP Context Request message, if the GGSN discovers that the negotiated QoS does not match the PDP context to be modified, it will reject this message. The GGSN can restrict the negotiated QoS according to its capability and the current load. The GGSN stores the negotiated QoS and sends back an Update PDP Context Response message that carries the TID and negotiated QoS to the SGSN. c) Upon receiving the Modify PDP Context Request message, the SGSN will send a Modify PDP Context Request message that carries the TI, negotiated QoS, and radio priority class to the MS. d) Upon receiving the Modify PDP Context Request message, the MS will send back a Modify PDP Context Accept message to the SGSN if accepting the modification request; otherwise the MS will deactivate the PDP context. If the SGSN does not receive the response from the MS on time, the SGSN can resend the Modify PDP Context Request message up to four times. 4) GPRS service suspension and recovery If the MS in GPRS packet transfer mode starts the GSM circuit service and the MS cannot perform the GPRS service and the GSM circuit service at the same time, the MS request the network side to suspend the GPRS service. When the GSM service completes, the network side will notify the MS to recover the suspended GPRS service. Figure 10-17 shows the flow. GSM Radio Network Planning and Optimization Chapter 10 GPRS Radio Network Planning and Optimization For internal use only 1/28/2011 All rights reserved Page62 of 94 2. Suspend 6. Routeing Area Update Request 1. Dedicated Mode Entered MS BSS SGSN MSC/VLR 3. Suspend 4. Resume 5. Channel Release 3. Suspend Ack 4. Resume Ack Figure 10-17 Recovery flow for GPRS suspended service The following details the flow: u) The MS in packet transfer mode must enter the dedicated mode. v) The MS sends a RR Suspend message that carries the TLLI and RAI to the PCU through the BSC to indicate to suspend the current packet data service and begins the dedicated service. Upon receiving the message, the PCU can suspend all the GPRS services. w) The PCU sends a Suspend message that carries the TLLI and RAI to notify the SGSN to suspend the current packet data service. Upon receiving the message, the SGSN stops sending the PDU to the MS and stores the status of the current packet data service, and then sends back a Suspend Ack message to the PCU. x) Upon receiving the Suspend Ack message, the PCU notifies the BSC to begin the dedicated service and stores the TLLI and RAI of the MS. In this case, the MS can still request the SGSN to recover the GPRS service after leaving the dedicated mode. y) Once detecting that the dedicated service completes, the BSC prepares for releasing the circuit service channel and notifies the current location information of the MS to the PCU. If the PCU can request the SGSN to recover the GPRS service, it will send a Resume message that carries the TLLI and RAI to the SGSN to recover the suspended packet data service. Meanwhile, the PCU report the current location information of the MS. z) The SGSN sends back a Resume Ack message to the PCU to confirm that the packet data transmission is recovered and begins to transmit the PDU. GSM Radio Network Planning and Optimization Chapter 10 GPRS Radio Network Planning and Optimization For internal use only 1/28/2011 All rights reserved Page63 of 94 aa) The PCU notifies the BSC to recover the suspended packet data service. After that, the BSC sends a Channel Release message (Resume) to the MS and releases the resource seized in the dedicated mode. In addition, the PCU notifies the MS whether to recover the suspended data service in this message. The Resume indicates that whether the BSS has requested the SGSN to recover the GPRS service for the MS successfully or not. That is, whether the BSS has received a Resume Ack message before sending the Channel Release message or not. bb) When the MS leaves the dedicated mode for the packet transfer mode, it begins to transmit and receive the packet data. cc) If the MS has not received any Channel Release (Resume) message before leaving the dedicated mode, or the message indicates that the BSS has not requested the SGSN to recover the GPRS service for the MS successfully, the MS will recover the GPRS service by sending a Location Update Request message to the SGSN. 10.1.6 Parameters and Application I. Cell reselection parameters Currently, the cell reselection algorithm for most GPRS systems is the same as that for the GSM system, so the parameters concerned are the same. These parameters have been introduced in the previous chapters, so here we will not focus on detailing them. II. Power control parameters Generally, power control parameters are delivered in the system information (SI). Upon receiving these parameters, the MS judges its TX class according to the environment at the radio interfaces. Hereunder is the description of the parameters. 1) ALPHA This parameter is used by the MS to calculate the value of its output power (P CH ) on the uplink PDCH. This parameter consists of 4-bit codes. For the meaning, see Table 10-18. Table 10-18 Meaning of the ALPHA Value Code Corresponding value 0 0000 0.0 1 0001 0.1 GSM Radio Network Planning and Optimization Chapter 10 GPRS Radio Network Planning and Optimization For internal use only 1/28/2011 All rights reserved Page64 of 94 2 0010 0.2 3 0011 0.3 4 0100 0.4 5 0101 0.5 6 0110 0.6 7 0111 0.7 8 1000 0.8 9 1001 0.9 10 1010 1.0 For the open loop power control, this parameter must be set to 1.0. Because the current GPRS systems adopt the open loop power control only, this parameter is always set to 1.0. 2) GAMMA This parameter is the class of the initial power class of the MS according to the GPRS power control algorithm. Its value ranges from 0 to 31, in the unit of dB. 3) Pb This parameter sets the BTS output power reduction (relative to the output power used on BCCH) used on the PBCCH. If a cell is not configured with the PBCCH, this parameter makes nonsense. Because current GPRS systems are not configured with the PBCCH, this parameter is not used at present. The values of this parameter can be 0, -2 dB, -4 dB… and -30 dB. 4) T_AVG_W This parameter sets the signal strength filter period for the MS when it is in packet idle mode. The MS uses this parameter to measure the downlink signal strength in packet idle mode and calculate C value. Here C stands for the level of the signals received by the MS under certain algorithm. Its values range from 0 to 25. 5) T_AVG_T This parameter sets the signal strength filter period for the MS when it is in packet transfer mode. The MS uses this parameter to measure the downlink signal strength in packet idle mode and calculate C value. 6) N_AVG_I This parameter sets the filtering constant of the collision signal strength for the power control. Its values range from 0 to 15. 7) PC_MEAS_CHANNEL GSM Radio Network Planning and Optimization Chapter 10 GPRS Radio Network Planning and Optimization For internal use only 1/28/2011 All rights reserved Page65 of 94 This parameter sets which channel is used by the MS to measure the received power class. If is set to BCCH, the MS will measure the level of the signals received on the downlink BCCH. If it is set to PDCH, the MS will measure the level of the signals received on the downlink PDCH. Generally, this parameter is set to BCCH. The value of this parameter can be either the BCCH or PDCH according to actual conditions. 8) INS_MEAS_CHANNEL_LIST_AVAIL This parameter indicates whether the information of the INT_MEAS_CHANNEL_LIST is broadcast in the PSI4 for the cell. Currently, this parameter is set to 0 for the network. This parameter is 1-bit code. For the meaning, see Table 10-19 Meaning of the INS_MEAS_CHANNEL_LIST_AVAIL Code Meaning 0 The information of the INT_MEAS_CHANNEL_LIST is not broadcast in the PSI4. 1 The information of the INT_MEAS_CHANNEL_LIST is broadcast in the PSI4. III. Radio link control parameters Generally, the reservation and release of the radio links are controlled by related timers and counters that are delivered in the SI. When the MS is in a different radio link status, it will start the corresponding timer and counter. The following details these timers and counters respectively. 1) T3168 This timer sets the time of the MS waiting for the Packet Uplink Assignment message. If the PBCCH does not exist, this timer is broadcast in SI13. After sending a Packet Resource Request message or a Packet Control Acknowledgement message, the MS starts this timer to wait for the Packet Uplink Assignment message. Upon receiving the Packet Uplink Assignment message, the MS stops the timer. When the timer expires, the MS will restart the packet access. The value of this timer can be set to 500ms, 1000ms, 1500ms, 2000ms, 2500ms, or 3000ms. When setting the value for this timer, you must consider the effect of the radio environment. When the radio environment is favorable, the timer can be set to a smaller value to ensure the utilization of radio resource; otherwise the timer must be set to a bigger value to enhance the success rate of TBF establishment. Generally, this timer is set to 1000ms. GSM Radio Network Planning and Optimization Chapter 10 GPRS Radio Network Planning and Optimization For internal use only 1/28/2011 All rights reserved Page66 of 94 2) T3192 This timer sets the time of an MS waiting for the TBF release after the MS receives the last data block. The MS starts this timer when sending a “Packet Downlink Ack/Nack” message (FAI = 1) to the network, or sending a “Packet Control Ack” message as a response to the last data block in the unacknowledged mode. Upon receiving the Packet Downlink Assignment message or the Packet Timeslot Reconfigure message delivered by the MS, the MS will stop this timer. If this timer expires, the MS will release the resource relative to the TBF and begin to monitor the paging channel. The value of this parameter can be set to 500ms, 1000ms, 1500ms, 0ms, 80ms, 120ms, 160ms, or 200ms. If it is set to a too small value, the MS will return to idle state immediately after receiving the data. In this case, new TBF blocks must be established if there is new data to be sent to the MS, and this will result lengthen the data transmission time. If it is set to a too big value, however, it will waste the radio resource. Generally, this timer is recommended to 500ms. 3) DRX_TIMER_MAX This parameter sets the maximum time for the MS to work in the non-DRX mode when the MS leaves the packet transfer mode for the packet idle mode. This parameter is broadcast in the cell broadcast. When leaving the packet transfer mode for the packet idle mode, the MS must stay in the non-DRX mode for a while. When the MS stays in the non-DRX mode, it will monitor all CCCH blocks and the PCU will reserve the related contexts of the MS. The reservation time is decided by the minimum value of the Non_DRX_Timer and DRX_Timer_Max. This parameter is represented in 3-bit binary. The parameter value is represented as 2 (k - 1), (Here k = DRX_TIMER_MAX, ranging from 0 to 7. When k = 0, this parameter value is 0.). The value of the parameter can be set to one of the 0s, 1s, 2s, 4s, 8s…64s. The “Immediate Assignment Command” message is sent faster in the non-DRX mode that in DRX mode, so the TBF establishment takes shorter time in the non-DRX mode, but power consumption of the MS is great in the non-DRX mode. Generally, this parameter can be set to 3, that is, the MS enters the DRX mode 4s later. 4) PAN In radio link failure control, the PAN parameters and the N3102 used at the MS side are used together. The PAN consists of three parameters, including PAN_DEC, PAN_INC, and PAN_MAX. Each time the MS reselects a new cell, it will set the value for N3102 according to the parameter PAN_MAX. When the MS receives a Packet Ack/Nack message, the N3102 GSM Radio Network Planning and Optimization Chapter 10 GPRS Radio Network Planning and Optimization For internal use only 1/28/2011 All rights reserved Page67 of 94 is increased to the PAN_INC, but it cannot excel the PAN_MAX. If the sending window is full, the MS will stop the T3182. If the MS does not receive the Packet Ack/Nack message until the T3182 expires, the MS will subtract the PAN_DEC from the N3102. When the N3102 is equal or less than 0, the MS will perform abnormal TBF release and triggers the cell reselection. The value of PAN_DEC ranges from 0 to 7. The value of the PAN_INC ranges from 0 to 7. And the value of the PAN_MAX can be one of the 0, 4, 8, 12, 16, 20, 24, 28, and 32. If the values of the previous three parameters are all set to 0, the N3102 is invalid. 5) BS_CV_MAX This parameter is used by the MS to calculate the Countdown Value (CV). If the PBCCH does not exist, it is broadcast in the SI13. During the countdown period, the MS sets the CV in each uplink RLC data block and notifies the BSN’ of the RLC data blocks to be transmitted on the uplinks to the network to ensure that the last RLC data block is transmitted just when the CV is 0. The maximum CV can be 15. Suppose that the number of the RLC block to be transmitted is the last but X one, if X ≤ BS_CV_MAX, the CV = X; otherwise the CV = 15. The value of the BS_CV_MAX ranges from 0 to 15. IV. Mobility management parameters For the mobility management in the GPRS system, corresponding GMM timers must be used to control the waiting time or period for some operations in case of abnormal conditions. In fact, the initial values of the needed GMM timers are defined during system initialization. Users can view and modify the initial value of a timer. The following introduces some important parameters. 1) T3312 This parameter designates the setting of the length of the periodical route update timer that is used at the MS side. The network sends this timer parameter to the MS in an Attach Accept or Routing Area Update Accept message during MS attach or route update. The MS updates the location areas periodically according to the period defined by this timer. The MSs of Class B do have to update their location areas periodically when performing circuit switched telecommunication, but other GPRS-attached MSs must update their location area periodically. The flow of periodical route update is the almost the same as the location area update in the SGSN except the update type. For the MSs attached to both the IMSI and GPRS, their periodical route update type is defined by the network mode of operation (NMO). GSM Radio Network Planning and Optimization Chapter 10 GPRS Radio Network Planning and Optimization For internal use only 1/28/2011 All rights reserved Page68 of 94 Under the NMOI mode, the MS performs the periodical routing area update only. In this case, the MSC/VLR takes the GPRS-attached MS as implicit detach, and the periodical location area update is completed by the SGSN. If the SGSN receives no message on the periodical location area update and the SGSN has detached the MS, it will send an IMSI Detach Indication message to the MSC/VLR. Under the NMOII or NMOIII mode, the MS must perform the periodical routing area update and the periodical location area update respectively, in which the former is realized through the Gb interface and the later is realized through the Abis interface. When the MS is in ready state, it will stop T3312 when sending the LLC-PDU each time. When the MS returns to the standby state, it will restart the T3312. If the MS cannot complete the periodical routing area update in the GPRS network, it must wait for a compensation time that is equivalent to the T3312 to the restart the periodical routing area update. If the T3312 is set to a too small value, huge signaling flow will be generated in the system and the power of the MS will be consumed quickly though the service quality of the network can be enhanced. Therefore, the values of the T3312 mainly depend on the traffic flow of the system and the system’s capability to process the traffic flow. Generally, the T3312 must be set to a relatively bigger value for the networks whose traffic flow is great. The T3312 consists of 8-bit codes, ranging from 0 to 255, and the step length is 6 minutes. Here, “0” indicates that the periodical routing area update is unnecessary and “1” indicates 6 minutes. The defaulted value is 54 minutes. 2) T3314 This timer, a READY timer, maintains the READY timers in the MS and SGSN. If the T3314 is enabled, great memory consumption caused by the infinite accumulation of the MM contexts and PDP contexts can be avoided in the SGSN. The READY timer can control the time for the MS and the MM context in the SGSN to keep in MM READY state. After sending the LLC-PDU, the MS will restart the T3312. Once receiving the correct LLC-PDU, the SGSN will also restart the T3314. When the MS is in READY state and the T3312 is working, the MS must perform the cell update after each cell reselection. (If the new cell and the service cell belong to different routing areas, the MS must perform the routing area update.). Once the T3314 expires, the MS and the MM context in the SGSN must transit to the STANDBY state. In this case, the MS does not perform cell update after each cell reselection. (If the new cell and the service cell belong to different routing areas, the MS still must perform the routing area update.) GSM Radio Network Planning and Optimization Chapter 10 GPRS Radio Network Planning and Optimization For internal use only 1/28/2011 All rights reserved Page69 of 94 The length of the T3314 in the MS and SGSN is the same, and it is controlled by the SGSN through the Attach Accept message and the Routing Update Accept message. If it is not provided in the Attach Accept message and the Routing Update Accept message, the length of the READY timer recommended by the MS should be used. If the MS does not provide the length either, the defaulted value should be used. The T3314 consists of 8-bit codes, in which the least 5 significant bits indicate the values of the timer, and the most 3 significant bits indicate the step length of the value. The value of the T3314 ranges from 0 to ∞. For details, see The most 3 significant bits The least 5 significant bits Value of the timer Remark 000 00000 0s The step length is 2s. 000 00001 2s – – – 000 11111 62s 001 00000 0 The step length is 1min. 001 00001 1min – – – 001 11111 31min 010 00000 0 The step length is 6min. 010 00001 6min – – – 010 11111 ∞ 111 – Timer barred Not activated Note: When the value of the T3314 is set to 0, the MS must return to STANDBY state. The T3314 is defaulted to 44s. V. Other parameters Other parameters relative to radio network planning and optimization are the parameters concerning PDCH support capability and the channel coding conversion. When setting these parameters, you must consider the operation of the current networks and the cooperation between equipments. Because the setting of these parameters varies with carriers, they are not detailed in this book. GSM Radio Network Planning and Optimization Chapter 10 GPRS Radio Network Planning and Optimization For internal use only 1/28/2011 All rights reserved Page70 of 94 10.2 GPRS Network Planning When planning the GPRS network, you must ensure the QoS of the existing voice service and take measures to reduce the negative effect of the GPRS services imposing against the voice service. In addition, you must also fully use the radio network equipments and radio resources to save the investment and cost. Based on that, you must ensure the quality of the GPRS network. The introduction of the GPRS network has the following effects against the existing GSM network: Currently, the GPRS system does not provide the uplink power control and the duty ratio (activation factor) of the radio signals on the PDCH nearly reaches 100%, so the introduction of the GPRS will bring new interference into the GSM network. To some extent, it will result in the decrease of the voice quality and the rise of the call drop rate. In this case, the voice service area of the GSM network will shrink. The frequency hopping has no apparent advantage against the GPRS service. For CS-1, the frequency hopping can improve the performance of the GPRS network. For CS-2 or CS-3, the frequency hopping makes little sense. For CS-4, the frequency hopping will reduce the performance of the GPRS network. At the current stage, the GPRS does not support the network-controlled cell reselection, and the GPRS MS and GSM MS use the same cell selection and reselection parameters. Therefore, if you intend to ensure the QoS of the GPRS service, you must well plan the addresses, capacity, and the coverage for the GSM network and the GPRS network. With the development of the GPRS service, it may affect the channel load of the GSM network, so you need to make an overall re-planning. For example, if the CCCH is used as the access channel of the GPRS service, the load on the CCCH will greatly increase. The GPRS introduces the dynamic PDCH allocation strategy, which complicates the dispatch of radio resources, so it has certain effect on the channel allocation and occupation for the GSM network. Though the previous effects are present, you can take the following measures to ease the negative effects. The BCCH carrier must also transmits at the full power, so the introduction of the GPRS service will not add new interference on the downlink direction. As a result, it has no effect on the service area of the GSM network. Therefore, you can configure the PDCH on the BCCH carrier. When the GPRS service grows up, you can divide a location area into several routing areas according to the geographic distribution and the GPRS service distribution within this location area. GSM Radio Network Planning and Optimization Chapter 10 GPRS Radio Network Planning and Optimization For internal use only 1/28/2011 All rights reserved Page71 of 94 During the early stage after the GPRS service is introduced, to avoid complicating the network planning, you would better not change the existed frequency hopping parameters. Meanwhile, to fully use the advantages of GPRS coding technology while ensure the existing voice quality, you can consider planning an independent channel for the GPRS service. In addition, you can also consider introducing new frequencies into the network or planning the GPRS capability on the frequencies that where the traffic is not great. Furthermore, you can still consider making out a more encoding rate solution. Enable the uplink power control in the GPRS system. Do not frequently change the channel coding schemes in the GPRS system. 10.2.1 Capacity Planning The capacity planning for the GPRS network includes the traffic channel planning and signaling channel planning, the former of which helps you configure a suitable number of PDCHs to provide the GPRS service to the expected GPRS subscribers, and the later of which helps you judge whether to expand the capacity for the signaling channels or not according to the network configuration and the rise of the signaling load caused by the application of the GPRS. I. Traffic channel capacity calculation Plan the capacity for the traffic channels according to the following steps: a) Calculate the number of GPRS subscribers according to the GPRS permeation rate and the number of GSM subscribers. b) Calculate the mean throughput of each subscriber according to the traffic model of the GPRS subscribers and convert the mean throughput into the bandwidth requirement (bps) and the traffic (erl). c) Calculate the number of PDCHs the system must provide according to the GoS of the packet service. d) Calculate the number of the static PDCHs and dynamic PDCHs the system must provide according to the PDCH utilization rate. II. Calculating the number of GPRS subscribers Based on the number of GSM subscriber in a cell and the permeation rate of the GPRS subscribers, you can calculate the number of the GPRS subscribers in the cell. The formula is as follows: The number of GPRS subscribers = the number of GSM subscribers * the permeation rate of the GPRS subscribers. GSM Radio Network Planning and Optimization Chapter 10 GPRS Radio Network Planning and Optimization For internal use only 1/28/2011 All rights reserved Page72 of 94 III. Equivalent busy-hour traffic calculation Due to the features of packet data services, such as delay and call queuing, the ErlangB cannot be directly applied to calculating the traffic of GPRS subscribers, which is different from the application of this formula in the calculation of the congestion in speech services. Generally, some methods based on fixed IP networks can be used to estimate the capacity of a GPRS system. According to the traffic model of the fixed IP network and the characteristics of GPRS protocols, you can calculate the average traffic of each subscriber. Then according to the bearer capability of the physical channels at the wireless interfaces, you can calculate the equivalent traffic of each GPRS subscriber. 1) Calculating the GPRS subscribers’ average access rate In actual planning, you need to estimate the busy-hour average traffic of each subscriber. Before the estimation, however, you need to estimate the subscriber’s average access rate. Generally, the average access rate is decided together by the channel coding schemes, subscriber’s multislot capability, block error rate (BLER), header, and average load factor. You need to make the following assumptions before calculating the average access rate: There is no SNDCP compression and decompression, and no SNDCP segmentation and reassembly. (An IP packet like this is transmitted by one LLC PDU at the LLC layer.) The LLC uses unacknowledged mode to transmit data, and the RLC uses the acknowledged mode (a retransmission rate of 10% must be considered). LLC frames= LLC header (9 bytes) + SNDCP header (4 bytes) + IP data + FCS (3 bytes), and each packet seizes a RLC indication byte in length. The average length of each IP packet is 200 bytes. The IP data flows continuously within at least 10 IP packets. If the data is transmitted in RLC acknowledged mode, the transmission of each LLC PDU means once TBE setup and release. Generally, from the TBF setup to TBF release, the RLC/MAC control block overheads account for 20% of the total radio blocks. The known conditions are as follows: A radio block is transmitted every 20ms. The BLER is 10%. The RLC/MAC header seizes 3 bytes. Therefore, if the tail bit is not considered, under the CS-1, CS-2, CS-3, and CS-4, the LLC PDU transmitted in each RLC data block is 20 bytes, 30 bytes, 36 bytes, and 50 bytes. GSM Radio Network Planning and Optimization Chapter 10 GPRS Radio Network Planning and Optimization For internal use only 1/28/2011 All rights reserved Page73 of 94 The sum of the bytes of the head of the FR, NS, BSSGP, LLC, and SNDCP at the Gb interface is 53 bytes. Based on the previous assumptions and known conditions, the following formulas can be used to calculate the average access rate under each CS at the IP layer. ( B A M / 1 = ¸ ¸ ¸ ¸ | | 02 . 0 1 . 0 2 . 0 × × + × + = M M M T T A V IP / 2 = V Gb = V IP × (150 + 53)/150 = 1.327 V IP Here, “M” indicates the minimum number of RLC data blocks needed for transmitting N* LLC-PDUs. “A 1 “indicates the total bytes of the N*LLC-PDUs. “A 2 “indicates the total bytes of N*IP packets. “B” indicates the bytes of LLC-PDU that each RLC data block can bear. “T” indicates the time for transmitting N*LLC-PDUs (N*IP packets). “V IP “indicates the estimated bearer rate of each PDCH at the IP layer. “V Gb ” indicates the estimated bearer rate needed by each PDCH at the IP layer. Table 10-20 lists the GPRS subscriber’s average data rate. Table 10-20 GPRS subscriber’s average data rate CS-1 (kbps) CS-2 (kbps) CS-3 (kbps) CS-3 (kbps) Physical layer rate at Um interface 9.05 13.4 15.6 21.4 Bearer rate at IP layer 5.42 8.14 9.77 13.63 Bearer rate needed at Abis physical layer 16 16 32 32 Bearer rate needed at Gb physical layer 7.19 10.79 12.96 18.09 For the GPRS networks where the conversion among various channel coding rates is available, the average access rate of each timeslot at the IP layer can be calculated by the following formula: V IPa = V CS1 *R CS1 + V CS2 *R CS2 + V CS3 *R CS3 + V CS4 *R CS4 Here, GSM Radio Network Planning and Optimization Chapter 10 GPRS Radio Network Planning and Optimization For internal use only 1/28/2011 All rights reserved Page74 of 94 “V IPa ” indicates the average bearer rate of each timeslot at the IP layer. “V CS1 , V CS2 , V CS3 , and V CS4 ” indicates the average bearer rates of CS-1 to CS-4 at the IP layer respectively. “R CS1 , R CS2 , R CS3 , and R CS4 ” indicates the percentage of the CS-1 to CS-4 respectively. Based on the bearer rate at IP layer listed in Table 10-20, the average access rate of each subscriber can be calculated by the following formula when multiple timeslots are bound: Va = V Ipa *N*R Here, “Va” indicates the average access rate of each subscriber. “V Ipa “indicates the average bearer rate of each timeslot at the IP layer. “N” indicates the maximum number of bound timeslots allowed by the multislot capability of the MS. “R” indicates the multislot capability utilization of an MS. If the ratio of CS-1 to CS-2 is 1:9, the average access rate of each timeslot at the IP layer in the network = 5.42 x 10% + 8.14 x 90% = 7.868kbit/s. If the multislot capability of an MS is 3 + 1, that is, up to 3 downlink timeslots and 1 uplink timeslot can be bound, the multislot capability utilization of the MS can reach 60%, so the average access rate of each subscriber is = 7.868 x 3 x 60% = 14.162kbit/s. 2) Calculating GPRS subscriber’s average traffic Currently, GPRS subscriber’s average traffic is calculated according to the following method. Based on the model (it is from ChinaNet) applied in fixed IP areas and the characteristics of mobile data, each GPRS subscriber’s traffic can be calculated according to the following two formulas: S = r1 × r2 × (A × n × T × r3 × R/3600) S/A = r1 × r2 × n × T × r3 × R/3600 Here, “S” indicates the traffic of the local GPRS network. “V” indicates the busy-hour average traffic of the GPRS subscriber. “A” indicates the number of local GPRS subscribers. “n” indicates the average times for each subscriber to access the network. “T” indicates the average length of each conversation. GSM Radio Network Planning and Optimization Chapter 10 GPRS Radio Network Planning and Optimization For internal use only 1/28/2011 All rights reserved Page75 of 94 “r1” indicates busy-day concentration coefficient, which indicates the ratio of the traffic on the busiest day in a month. This coefficient has little relation with the traffic types. Generally, it can be 1/20. “r2” indicates the busy-hour concentration coefficient, which indicates the ratio of the traffic at the busiest hour in a day. It varies greatly according to different traffic types. “r2” indicates the duty ratio, which indicates the ratio of the actual time for a subscriber to download data to the total time for the subscriber to stay in the network. Generally, it is 1/4. “R” indicates the average access rate of a subscriber. According to the previous methods, China Mobile estimated the average traffic for GRPS subscribers in 2001-2002, and the busy-hour average traffic of each subscriber is 180bit/s. After calculating the GRPS subscriber’s average traffic, you can plan the capacity for the GPRS network. 3) Calculating equivalent busy-hour traffic of each GPRS subscriber In actual conditions, subscribers are quite sensitive to the dynamic bandwidth used during the transmission. Considering the convenience and flexibility of the calculation, you can convert the busy-hour average traffic of a subscriber into the traffic required by the average bandwidth and the busy-hour traffic The average bandwidth of GPRS subscriber is equal to the GSM physical channel rate or the GPRS subscriber’s average access. Because the GSM physical channel rate is 16 kbps, the average bandwidth of GPRS subscriber is 1 KByte/s if the previous assumptions are present. If the ErlangB can calculate the equivalent traffic of a GPRS subscriber, you can abstract the behavior of the GPRS subscriber as “packet call”. If each packet call seizes one “GPRS channel”, the bandwidth of the GPRS channel can be taken as the average bandwidth of a GPRS subscriber. In this case, busy-hour equivalent traffic of a GPRS subscriber = busy-hour average traffic/ channel width = busy-hour average traffic/average bandwidth. IV. PDCH number calculation After obtaining the busy-hour equivalent traffic of a GPRS subscriber, you can estimate the capacity requirement for the GPRS network according to ERLANGB. The corresponding formulas are as follows: The number of GPRS channels = ERLANGB (GPRS traffic and GoS) GSM Radio Network Planning and Optimization Chapter 10 GPRS Radio Network Planning and Optimization For internal use only 1/28/2011 All rights reserved Page76 of 94 GPRS traffic = the number of GPRS subscribers * busy-hour equivalent traffic of a GPRS subscriber The number of PDCHs = the number of GPRS channels * GPRS channel bandwith/IP layer bandwidth of each PDCH Here, the GPRS channel bandwidth is the average bandwidth of the GPRS subscriber, and the IP layer bandwidth is the IP layer rate at each timeslot. V. Static and dynamic PDCH configuraiton calculation Through calculating the probability of the TCHs to be seized using the Poisson probability distribution formula, you can obtain the average available bandwidth of the dynamic PDCH under various configurations. Hereunder are two examples. If the static PDCH = 1, and the dynamic PDCH = 1, 2, and 3, the average available bandwidth of the PDCH under various configurations are listed in the Table 10-21. Table 10-21 Average available bandwidth of the PDCH under various configurations (a) Number of TRXs Available TCH/PDCH timeslot Circuit service traffic Probability for the number of idle TCHs is N Number of average available PDCHs when the number of dynamic PDCHs is M N=0 N=1 N=2 N=3 M=1 M=2 M=3 1 7 2.9 4.68% 9.68% 16.69% 23.03% 1.95 2.81 3.50 2 14 8.2 3.48% 5.52% 8.08% 10.84% 1.97 2.88 3.70 3 22 14.85 2.95% 4.18% 5.63% 7.20% 1.97 2.90 3.77 4 29 21 2.77% 3.70% 4.75% 5.89% 1.97 2.91 3.80 5 37 28.25 2.67% 3.40% 4.22% 5.07% 1.97 2.91 3.81 6 45 35.6 2.57% 3.18% 3.84% 4.53% 1.97 2.92 3.82 7 52 42.1 2.51% 3.03% 3.60% 4.19% 1.97 2.92 3.83 8 60 49.6 2.44% 2.91% 3.40% 3.91% 1.98 2.92 3.83 If the static PDCH = 0, and the dynamic PDCH = 1, 2, and 3, the average available bandwidth of the PDCH under various configurations are listed in the Table 10-22. Table 10-22 Average available bandwidth of the PDCH under various configurations (b) Number of TRXs Available TCH/PDCH timeslot Circuit service traffic Probability for the number of idle TCHs is N Number of average available PDCHs when the number of dynamic PDCHs is M N=0 N=1 N=2 N=3 M=1 M=2 M=3 1 7 2.9 1.90% 4.59% 9.50% 16.38% 0.98 1.92 2.76 2 14 8.2 2.00% 3.41% 5.41% 7.92% 0.98 1.93 2.82 GSM Radio Network Planning and Optimization Chapter 10 GPRS Radio Network Planning and Optimization For internal use only 1/28/2011 All rights reserved Page77 of 94 3 22 14.85 1.95% 2.90% 4.10% 5.52% 0.98 1.93 2.84 4 29 21 1.97% 2.72% 3.63% 4.66% 0.98 1.93 2.85 5 37 28.25 2.00% 2.62% 3.33% 4.13% 0.98 1.93 2.85 6 45 35.6 2.00% 2.52% 3.12% 3.77% 0.98 1.93 2.86 7 52 42.1 1.99% 2.46% 2.97% 3.53% 0.98 1.94 2.86 8 60 49.6 1.98% 2.40% 2.85% 3.33% 0.98 1.94 2.86 When the number of the dynamic PDCHs is M, the number of the average available PDCHs is the sum of the followings: The defined number of the static PDCHs If only one dynamic PDCH is available, the probability is the figure in the previous table when N = 1. If only two dynamic PDCHs are available, the probability is the figure in the previous table when N = 2. If M * dynamic PDCHs are available, the probability is the sum of figures in the previous table when N ≥ M. VI. Signaling channel planning The steps to plan the signaling channels are as follows: a) Calculate the capacity of the current signaling channel according to the configuration of the existing network. b) Calculate the signaling load of the unit subscribers of various service types according to the GPRS service model. c) Calculate the total signaling load of the each service type according to the capacity planning result. d) Judge whether to expand the signaling channel capacity according to the signaling load. Currently, most of the existing GSM networks adopt non-combined CCCH configuration. During the early GPRS network construction stage, the network size and the number of subscribers are not great, so you do not have to expand the CCCH after introducing the GPRS service to the existing GSM network. With the development of the GPRS network, however, you must increase the number of CCCHs in accordance with the rise of the network load. GSM Radio Network Planning and Optimization Chapter 10 GPRS Radio Network Planning and Optimization For internal use only 1/28/2011 All rights reserved Page78 of 94 The size of the RA can be planned the same as that of the LA at the planning stage. As the number of GPRS subscribers grows, however, to reduce the load on the PCH, you must scale down the RA to reduce packet pages. Considering the operation of the previous GPRS network, you do not have to expand the capacity for the RACH and the PCH, but you need to configure the extended-BCCH for the AGCH based on actual conditions in case of AGCH congestion. You can ease the signaling channel capacity using the following methods: Configure multiple non-combined CCCHs Enable the PCCCH as early as possible Support the Gs interface 10.2.2 Coverage Planning The coverage area of the GPRS network depends greatly upon the channel coding schemes. For a certain coverage area, the E/N is the restriction factor. For the interference-restricted area, the C/I is the main restriction factor. The following lists the coverage indexes required for the current GPRS system: The BLER is equal to or less than 10%. Under the CS-1, the area covered by the GPRS service must be the same with the area covered by the voice service provided by the GSM network. Under the CS-2, the area covered by the GPRS service must equal to 80% or more of the area covered by the voice service provided by the GSM network. Table 10-23 lists the values of carrier-to-interference ratio (C/I) corresponding to each GPRS channel coding scheme. Table 10-23 Mapping relationship of GPRS channel coding scheme and C/I Channel coding scheme C/I (frequency hopping is used) C/I(frequency hopping is not used) CS-1 7.1 dB 10.8 dB CS-2 11.5 dB 12.8 dB CS-3 13.6 dB 13.7 dB CS-4 20.8 dB 17.2 dB Note: The data in this table is calculated when the BLER is 10%. GSM Radio Network Planning and Optimization Chapter 10 GPRS Radio Network Planning and Optimization For internal use only 1/28/2011 All rights reserved Page79 of 94 In the environment where interference is present, the coverage area percentage of the GPRS network relative to the voice coverage area varies with the channel coding schemes, as listed in Table 10-24 Percentage of voice coverage area relative to channel coding scheme Channel coding scheme Okumura-Hata (%) Walfish-Ikegami (%) CS-1 79 80 CS-2 61 63 CS-3 54 57 CS-4 34 37 Note: The data in this table is obtained based on the following assumptions: The interference in the service area is constant. The frequency hopping is not used. The C/I of the voice service is 9 dB. For the coverage corresponding to the four channel coding schemes, see Figure 10-18. cs-2 cs-1 cs-3 cs-4 Figure 10-18 Coverage corresponding to four GPRS channel coding schemes In actual GPRS network planning, you must define the coverage and continuous converge based on the areas covered by the voice service of the existing GSM network. Currently, you can obtain the corresponding curve indicating the performance of the GPRS network with the help of emulation tools. As shown in Figure 10-19, under normal conditions, the C/I at the cell edge is 9 dB when the GSM traffic is busy. Once the GPRS service is introduced, the C/I decrease as the GPRS load increases. When the GPRS load is 100%, the area where the C/I is greater than 9 dB accounts for only 88% of the original area. GSM Radio Network Planning and Optimization Chapter 10 GPRS Radio Network Planning and Optimization For internal use only 1/28/2011 All rights reserved Page80 of 94 Figure 10-19 Relationship between C/I and distance Figure 10-20 shows the relationship between the C/I distribution probability and C/I. As the curve shows, the C/I also decreases as the GPRS load increases. Suppose that the C/I at the cell edge is equal or greater than 9 dB, the coverage ratio is 90% when the GPRS service is not introduced, but the coverage ratio reduces to 86% when the GPRS load reaches 100%. Figure 10-20 Relationship between C/I distribution probability and C/I GSM Radio Network Planning and Optimization Chapter 10 GPRS Radio Network Planning and Optimization For internal use only 1/28/2011 All rights reserved Page81 of 94 According to related protocols, a mapping relationship exists between the grade of voice service and the C/I, as listed in Table 10-25. Table 10-25 Relationship between grade of voice service and C/I Grade of voice service 0 1 2 3 4 5 6 7 C/I (dB) 23 19 17 15 13 11 8 4 Generally, the BCCH carrier is of 4*3 reuse and the C/I cannot be greater than 13 dB, so the requirement on the GPRS coverage can be ensured. The reuse level of the non-BCCH carrier is higher than that of the BCCH carrier, so the C/I must be planned greater than 9 dB. Therefore, it is recommended to configure the PDCH on the BCCH carrier. 10.2.3 Frequency Planning When planning the frequency for the GPRS network, you must consider the data rate of the network, because the data rate varies with channel coding schemes. As introduced before, the requirement on C/1 is different under different channel coding schemes, while the frequency reuse pattern depends largely on the requirement on C/I. Figure 10-21 lists the frequency reuse clusters supported by the GPRS channel coding schemes. Figure 10-21 Frequency reuse clusters supported by GPRS channel coding schemes Channel coding scheme C/I threshold (dB) Frequency reuse clusters Voice 9 7～9 CS-1 10.8 9 CS-2 12.8 12 CS-3 13.7 13 CS-4 17.2 >19 For the GSM network, the voice communication has relatively lower requirement on the bit error rate. To enhance spectral frequency, you can use aggressive frequency reuse technology. However, the GSM data communication has high requirements on the bit error rate, so aggressive frequency reuse technology cannot ensure the data transmission. Generally, voice service and data service exist within the same cell simultaneously. Because the 4*3 reuse pattern is used in the frequency planning for GSM BCCH carrier, the C/I of the channels on the BCCH carriers can meet the requirements of GPRS GSM Radio Network Planning and Optimization Chapter 10 GPRS Radio Network Planning and Optimization For internal use only 1/28/2011 All rights reserved Page82 of 94 service. As a result, if configuring the PDCH on the BCCH carriers, you do not have to consider the aggressive reuse pattern, frequency hopping, and power control. For the static PDCH, it must be configured on the BCCH carrier or on the TCH carrier where the reuse distance meet the C/I requirement. For the dynamic PDCH, it must be configured on the carriers where the C/I requirement is met. For the cells with various frequency reuse patterns, you must begin to configure them on the carriers with further reuse distance according to the GPRS channel selection mechanism. In addition, different GPRS load has different effect on the C/I. The larger the frequency reuse coefficient is, the looser the frequency reuse, so the network can bear heavier GPRS load. Figure 10-22 shows the relationship between the C/I distribution probability and frequency reuse coefficient. Figure 10-22 Relationship between C/I distribution probability and frequency reuse coefficient For the areas where the GPRS load is heavy, to ensure the QoS of the GPRS network, you must choose looser frequency reuse pattern. This conclusion, however, contradicts to the frequency reuse solution used by the existing network. Therefore, you can consider introducing new bands to make an independent frequency plan for the GPRS network. 10.3 GPRS Network Optimization As the GPRS service grows, the QoS of the current GPRS networks cannot meet the users’ requirements, so you must optimize these GPRS networks. The challenges you may meet in the network optimization are listed the following: GSM Radio Network Planning and Optimization Chapter 10 GPRS Radio Network Planning and Optimization For internal use only 1/28/2011 All rights reserved Page83 of 94 The channel coding schemes adopted for the most of the current GPRS networks are CS-1 and CS-2, and the multislot capability of the GPRS MSs are 3+1 or 4+1. Therefore, the limit rate for the data to be transmitted is 40.2 kbit/s or 53.6 kbit/s. That is, 13.4*(3/4) kbit/s = (40.2/53.6) kbit/s. However, the rate reduction during the transmission is not considered into this limit rate. Because the activation factor on the GPRS channel is near 100%, theoretically the interference against the GPRS service is 3 dB more than that against the voice service. As a result, the C/I in the areas where the GPRS traffic is huge will become small and the service area will shrink. In this case, the access success ratio of the GPRS service will be reduced. In addition, the reduction of the C/I will result in the decrease of the data transmission rate. As the voice service of the GSM network grows, the radio channel resource will become scarce, so only a small number of PDCHs can be provided to the GPRS service. Meanwhile, owing to the improper configuration of the static channels and dynamic channels, the channel utilization ratio in some areas is low or even no channel is available to the data service in some areas. In addition, to avoid network congestion, you need to adopt the traffic guide strategies different from that used for the GSM service for the GPRS service according to the characteristics of the data service. The operation of the GPRS system will add the signaling load on the PCH and AGCH. For the GPRS networks with great load, you must reduce the load on the PCH and AGCH through reducing the scope of the routing area. In this case, you must re-plan the routing area. Currently, the GPRS system does not support the network-controlled cell reselection, but the GPRS-attached MSs still reselect the cell following the same procedure used in the GSM network. Therefore, if the network layout and the parameters are not properly set, the MSs will reselect the cell frequently. Though frequent cell reselection has little effect on the GSM network, it will greatly reduce the GPRS data transmission rate. 10.3.1 GPRS Network Optimization Objectives and Principles I. GPRS network optimization objectives The objectives to optimize the GPRS network are listed in the following: Establish and research mobile data service model. To achieve this objective, you must collect and analyze the data on the system performance, resource configuration, network resource, data service, and subscribers’ behavior. Optimize the configuration of the parameters and radio resources for the radio access network. To achieve this objective, you must understand the operation of the GPRS network in combination with the market position. GSM Radio Network Planning and Optimization Chapter 10 GPRS Radio Network Planning and Optimization For internal use only 1/28/2011 All rights reserved Page84 of 94 Accumulate the experiences on GPRS network optimization for future use (3G service). II. GPRS network optimization principles The principles to optimize the GPRS network are listed in the following: Mining the existing equipment resources to the maximum to meet the growing services. Enhance the investment efficiency. Maximize the spectrum resource utilization. Improve theh QoS of the GPRS network as much as possible while ensuring the GSM circuit switched service. Adopt different adjustment strategies to optimize the network at each stage according to the development of the GPRS network. The work flow applied to the GPRS network optimization is similar to that applied to the GSM network optimization, and it is introduced in the previous chapters. Therefore, this chapter aims to analyze the problems hit during the network optimization and provides some beneficial suggestions. 10.3.2 Network Optimization Indexes Because the GPRS system enable the data to access the network in the form of packets, the indexes used for the GPRS network performance are different from that used for the GSM network. In addition, the GPRS service will affect the GSM service to some extent, so you must consider the related indexes used for the GSM network if you intend to have an overall evaluation of the GPRS network. It must be emphasized that the GPRS has not developed to a high level at present, and the traffic model used is relatively conservative, but the GPRS service may change rapidly if new services are introduced. Therefore, to prevent the rapid change of GPRS traffic model from impacting the network planning, you must keep monitoring the key indexes used for the GPRS network. The GPRS network indexes are divided into traffic statistics index and test index. The former is registered by the OMC and collected by the PCU or SGSN. The later is obtained from the results of the items tested by means of CQT and DT. For traffic statistics indexes, this section introduces the indexes that are measured at the PCU side and indicates the performance of the radio interfaces. For test indexes, this section introduces parts of the indexes relative to the current GPRS service. GSM Radio Network Planning and Optimization Chapter 10 GPRS Radio Network Planning and Optimization For internal use only 1/28/2011 All rights reserved Page85 of 94 I. Traffic statistics indexes The analysis of the traffic statistics indexes can help you optimize the network, handle failures, and establish packet traffic model. Therefore, the traffic statistics indexes needed to be emphasized during the routing maintenance can be divided into the following three categories: System performance indexes These indexes indicate the processing capability and the data throughput capability of the system. Through analyzing these indexes, you can know more information on the network operation and plan and optimize the network accordingly. Maintenance indexes These indexes indicate abnormities occurred in the network. Through analyzing them, you can find out the where the failure lies and optimize the accordingly. Reference indexes These indexes are related to traffic models. Through analyzing these indexes, you can accumulate more data and experiences on network operation for the future network optimization and re-planning. The following details the three categories of indexes respectively. 1) System performance indexes The system performance indexes includes PDCH number, network congestion ratio, and call drop rate. They are detailed in following. a) PDCH number This index indicates whether the system has overloaded or not. For the definition of the related PDCH traffic statistics indexes, see Table 10-26. Table 10-26 Definition of the GPRS PDCH number Index Definition Available PDCH average number During a measurement period, the average number of PDCHs available under the BSC. Seized PDCH average number During a measurement period, the average number of the PDCHs available under the BSC. Currently, the PDCH is divided into dynamic PDCH and static PDCH. The traffic data is an average value, so if the average number of the PDHC available excels the average value, the maximum value is probably beyond the capability of the current PCU GSM Radio Network Planning and Optimization Chapter 10 GPRS Radio Network Planning and Optimization For internal use only 1/28/2011 All rights reserved Page86 of 94 equipments to process the PDCH. In this case, the service in some cells may be affected. When checking the average number of the available PDCHs, you must check the average number of the seized PDCHs. If the average number of the seized PDCHs is small, it means that the actual traffic volume is not great probably due to too many static PDCHs have been configured for the cell. In this case, the channel utilization ratio is low. To solve this problem, you must reduce the number of static PDCHs configured for the cell where the traffic volume is low. If the average number of the seized PDCHs is large, it means that the actual traffic volume is great. In this case, you must expand the capacity for the hardware equipments. b) Network congestion ratio The network congestion ratio originally means that the ratio of the MSs who cannot access the network due to network congestion. In the GPRS system, the access flow used for the MS is different from that in the GSM system, so the meanings of this index are different. In the GPRS system, the network congestion ratio refers to the rise of the TBF setup failures resulted from the decrease of the packet immediate assignment success ratio due to multiple factors, such as data configuration, network capacity, channel quality, circuit service busyness, packet service busyness, and so on. The GPRS congestion ratio is recorded on the uplink and downlink as follows: Uplink TBF congestion rate = times of uplink TBF setup failure/times of uplink TBF setup request = (times uplink TBF setup request – times of uplink TBF setup success)/times of uplink TBF setup request. Downlink TBF congestion rate = times of downlink TBF setup failure/times of downlink TBF setup request = (times of downlink TBF setup request – times of downlink TBF setup success)/times of downlink TBF setup request. For more information on the GPRS congestion rate, see Table 10-27. Table 10-27 Definition of GPRS congestion rate Index Definition Times of uplink TBF setup request The times for the PCU to receive the Uplink TBF Setup Request message. Times of uplink TBF setup success The times for the PCU to successfully receive the uplink data blocks from the MS after sending the Uplink Assignment message. Times of downlink TBF setup request The times for the PCU to send the Downlink Assignment message. Times of downlink TBF setup success The times for the PCU to send the Downlink Assignment Ack message to the MS. GSM Radio Network Planning and Optimization Chapter 10 GPRS Radio Network Planning and Optimization For internal use only 1/28/2011 All rights reserved Page87 of 94 According to the GPRS flow, assignment messages are retransmitted many times, so an assignment failure has little effect on the MS. As a result, it is still acceptable that the congestion ratio can be a little higher. In the GPRS network, high uplink congestion ratio is probably resulted from the scarcity of the network resource or poor quality of the radio links. If the downlink congestion ratio is high, it is probably because of the poor quality of the radio links. c) Call drop rate In the GPRS system, call drop rate refers to the TBF abnormal releases during data transmission. It is also recorded on the uplinks and downlinks as follows: Uplink TBF call drop rate = times of uplink TBF abnormal release/ times of uplink TBF setup success = (times of uplink TBF setup success – times of uplink TBF normal release)/times of uplink TBF setup success. Downlink TBF call drop rate = times of downlink TBF abnormal release/timed of downlink TBF setup success = (times of downlink TBF setup success – times of downlink TBF normal release)/times of downlink TBF setup success For more information on the GPRS call drop rate, see Table 10-28. Table 10-28 Definition of GPRS call drop rate Index Definition Times of uplink TBF normal release The times for the PCU to receive the Packet Control Ack message indicating stopping the TBF transmission from the MS. Times of uplink TBF setup success The times for the PCU to successfully receive the uplink data blocks of the MS after sending the downlink acknowledged or control message indicating stopping the TBF transmission from the MS. Times of downlink TBF normal release The times for the PCU to receive the Downlink Times of downlink TBF setup success The times for the PCU to send the Downlink Assignment Ack message to the MS. For current GPRS networks, the TBF transmission time is short (2 to 3 seconds in average), so the call drop rate is higher than that in the GSM network, but it has little effect on the service. Similar to the congestion rate, high call drop rate is probably resulted from the poor quality of the radio links. In addition, frequent cell reselection will result in more TBF abnormal releases. 2) Maintenance indexes GSM Radio Network Planning and Optimization Chapter 10 GPRS Radio Network Planning and Optimization For internal use only 1/28/2011 All rights reserved Page88 of 94 These indexes include the indexes related TBF setup and release performance, RLC data transmission performance, LLC data transmission performance, and radio channel performance. a) TBF setup and release performance The TBF setup and release performance is closely related to the radio quality, traffic volume, and the MS behaviors. The indexes related to the TBF setup and release performance are listed in the following: Uplink TBF abnormal releases caused by N3101 overflow Uplink TBF abnormal releases caused by N3103 overflow Downlink TBF abnormal releases cause by N3105 overflow Uplink or downlink TBF abnormal releases caused by SUSPEND Uplink or downlink TBF abnormal releases caused by FLUSH Uplink or downlink TBF abnormal releases due no available radio resource. b) LLC data transmission performance The indexes related to the LLC data transmission performance include the uplink or downlink throughput rate at the LLC layer and the times for the LLC-PDU to be discarded. c) Radio channel performance The indexes related to the radio channel performance checks the utilization of the PDCH and its logical channels and the cell reselection of the MS, including actual average PDCH utilization rate, uplink PDTCH/ACCH utilization rate, and the times of Pingpong cell reselection. 3) Reference indexes The reference index include the traffic statistics at the LLC layer, TBF data flow measurement, PDCH seizure measurement, and MS behavior measurement. These indexes check the traffic properties of the LLC layer, RLC/MAC layer, physical layer, and the MS respectively. II. Test indexes This section details the test indexes. 1) GPRS attach test The following details the indexes related to the GPRS attach test, and they are tested according to the multiple login of the GPRS network. GRPS attach success rate GSM Radio Network Planning and Optimization Chapter 10 GPRS Radio Network Planning and Optimization For internal use only 1/28/2011 All rights reserved Page89 of 94 GPRS attach success rate = times of GPRS success attach/times of GPRS attach attempt * 100%. The GPRS attach success means that the MS receives the GPRS attach accept message within 15 seconds after sending the Attach Request message. Average attach time Average attach time = the sum of each GPRS attach success time/times of GPRS attach success. After sending the first Attach Request message, the MS will receive the GPRS attach message, and the delay between the two messages is defined as the GPRS attach success time. 2) WAP test The WAP test includes the WAP website login test and page update test. During WAP website login test, the buffers must be cleared from the MS. During the page update test, multiple pages must be visited. The indexes related to the WAP test are introduced as follows: PDP activation success rate PDP activation success rate = times of PDP activation success/total times of PDP activation attempt * 100%. The PDP activation success means that the MS receives the Activate PDP Context Accept message within 15 seconds after sending the Activate PDP Context Request message. Average PDP activation time Average PDP activation time = the sum of each PDP avtivaton success time/times of PDP activation success. After sending the first Activate PDP Context Request message, the MS will receive the Activate PDP Context Accept message, and the delay between the two messages is defined as the PDP activation success time. WAP website login success rate WAP website login success rate = times of success login/times of login attempt. The WAP website login success rate means that the web page displays completely within 60 seconds after the MS applies to visit the WAP website. If the MS fails to log in to the system, you must record the login failure messages of the MS and divide them into certain types. Average WAP first page display time Average WAP first page display time = the sum of each WAP first page display time/times of WAP first page display success. After the MS request to visit the WAP website, it takes a time for the first page of the website to completely display, and this time is defined as the average WAP first page display time. WAP page update success rate WAP page update success rate = times of WAP upate success/times of WAP update attempt. The successfully updated WAP pages refer to the pages to be browsed are completely and correctly displays in 60 seconds. GSM Radio Network Planning and Optimization Chapter 10 GPRS Radio Network Planning and Optimization For internal use only 1/28/2011 All rights reserved Page90 of 94 3) Ping test According to the ping test, the tailored length of the data packet is used to ping the sites in the GGSN local area network. The indexes related to the ping test are introduced as follows. Ping success rate Ping success rate = times of ping success/times of ping attempts *100%. Ping average delay Ping average delay = the sum of each ping success/times of ping success. 4) PTF application layer download rate test According to this test, the files with the tailored length are downloaded from the designated server. The main test indexes are introduced as follows. PTF application layer download rate test PTF application layer download rate = actual downloaded quantity (byte)/actual downloaded time (s) Average throughout of FTP download RLC layer This index is recorded by dedicated test software. 5) RAU (Routing Area Update) test Average RAU interval Average RAU interval time = total test time (s)/total RAU times. Average RAU distance Average RAU distance = total test distance (km)/totoal RAU times. 6) Cell reselection test Average interval for cell reselection Average interval for cell reselection = total test time (s)/total times of cell reselection. Average distance for cell reselection Average distance for cell reselection = total test distance (Km)/total times of cell reselection. 7) Coverage test This index is measured by coverage ratio per kilometer, which is equal to the ratio of the total test distance (Km) to the times for the MS not within the GPRS coverage. No GPRS coverage occurs in the following situations: During the coverage test, the MS transits from READY state or STANDBY state to IDLE state. The MS stays in READY STATE or STANDBY state, but its level stays lower than -94dBm in 5 seconds or more. GSM Radio Network Planning and Optimization Chapter 10 GPRS Radio Network Planning and Optimization For internal use only 1/28/2011 All rights reserved Page91 of 94 The MS receives the out-of-service message. 8) Call drop test It is measured according to the call drop rate per kilometer (kilometer/time), which is equal to the rate of the total test distance/total times of download call drop. The call drop is defined as follows: The PPP connection between the computer and the MS is interrupted. That is, the computer prompts that the dialing connection is interrupted. The PPP connection is normal, but the data cannot be transmitted for 3 minutes or more when the signals in the network are normal. 10.3.3 Network Optimization Problem Analysis I. Attach problems 1) Attach failure The subscriber attach failure is divided into individual failure, area failure, and total failure. If any failure occurs, first you should analyze the MS. If it is the area failure, you should analyze the cell and PCU. If it is the total failure, you should analyze the core network. The individual failure is caused by the following factors: The properties of the APN or the MOSEN of the MS are wrongly set. The MS cannot attach the network in time due to frequent cell reselection. The MS does not subscribe to the GPRS service. The visit class control restricts the MS to access the network. The area failure is caused by the following factors: The GPRS function is not enabled in the cell. The GPRS resource cannot be allocated to the MS due to voice channel congestion and no static PDCH available. The PCU uplink and downlink flow is abnormal as a result of the PCU hardware and software failures. The MS cannot attach the network due to poor network coverage. The total failure is caused by the following factors: The data is not or wrongly transmitted at the signaling network. The signaling interface plane fails. The addressing format of the SGSN equipment and that of the HLR equipment is incompatible with each other. The network interfaces or links are abnormal. The subscriber data in the HLR is abnormal. 2) Slow attach GSM Radio Network Planning and Optimization Chapter 10 GPRS Radio Network Planning and Optimization For internal use only 1/28/2011 All rights reserved Page92 of 94 The MS may attach the network at a low speed due to the following factors: The relative configuration of the GSN and the No.7 signaling network is incorrect. The poor performance of the SGSN slows down the processing speed. The signaling load of the network is too high. 3) Cross-SGSN routing area update failure The cross-SGSN routing area update failure is caused by the following factors: The data for the adjacent SGSNs are wrongly configured. The RAI in the DNS is incorrectly translated in the SGSN. The Gn interface or Gr interface fails. II. Access problems 1) Low random access success rate The random access occurs when the subscriber sends a channel request on the RACH but the network cannot decode the request due to radio interference or equipment failure. To solve this problem, you must fully consider the interference band and channel seizure. Generally, if both the random access rate and the immediate assignment success rate are quite low, the reasons are probably that the channels are unavailable and the system is unstable. 2) High TBF setup failure rate Currently, the main reasons for the high TBF setup failure rate are that the MS and the network equipments are incompatible with each other and the related parameters are not properly set. The other reasons are listed in the following: The effect from the radio environment and system resource. The MS does not support the assigned frequency. The MS cannot seize a radio channel. The parameters PAN_DEC, PAN_INC, and PAN_MAX are not properly set. The timers, such as T3168 and T3192 are not properly set. III. Paging problems Paging problems are closely related to low paging success rate. If the PCU cannot receive the access request fed back by the MS after the system sends the packet paging message, paging problems will occur. The reasons for the low packet paging success rate are listed in the following: There are dead zones in the coverage area. The parameters related to the paging channels and the routing areas, including CCCH number, BS_AG_BLKS_RES, and BS_PA_MFRMS, are not properly set, which cause PCH congestion and AGCH congestion. GSM Radio Network Planning and Optimization Chapter 10 GPRS Radio Network Planning and Optimization For internal use only 1/28/2011 All rights reserved Page93 of 94 There is no response to the delivered paging messages due to frequent location update and cell reselection. The interference present in the radio environment prevents the MS from correctly decoding the information. The unreasonable paging strategies causes paging overload. The paging process cannot be completed due to heavy load on the traffic channel. (Generally, in the GPRS system, the first LLC PDU received by the SGSN marks the success page.) IV. PDP activation problems 1) Low PDP activation success rate The PDP activation success rate is low when there is no response to the MS or the MS is rejected during the PDP activation after the MS attaches the GPRS network. The possible reasons are listed in the following: The APN of the MS is wrongly set. The MS mismatches the QoS. The IP address of the MS is wrongly set. The subscriber operation is wrong. The PCU fails. The DNS resolution is abnormal. The GTP between the SGSN and the GGSN is abnormal. The firewall is in abnormal work. The problems are present in the PDN. The router fails. 2) Low PDP activation speed The possible reasons for low PDP activation speed are as follows: The APN of the SGSN is wrongly configured. The local DNS or roaming DNS failure causes low APN resolution speed. The improper configuration of the DHCP or RADIUS server causes the GGSN to process the data and signaling at a low speed. V. Abnormal PDP deactivation The abnormal PDP deactivation indicates that the PDP context is deactivated abnormally. If the PDP context is deactivated abnormally, the connection between the subscribers and the external data network will be interrupted. In this case, the PDP context must be reactivated if the subscriber intends to access the network. The possible reasons for the abnormal PDP deactivation are listed in the following: The poor radio quality causes the MS to fail to make a response to the network. Improper network operation causes abnormal PDP deactivation. GSM Radio Network Planning and Optimization Chapter 10 GPRS Radio Network Planning and Optimization For internal use only 1/28/2011 All rights reserved Page94 of 94 The MS cannot respond to the network-activated PDP context due to inadequate processing capability. The MS fails to get the authentication from the HLR during location update. VI. Data transmission problems 1) Low GPRS data transmission rate The possible reasons for the low GPRS data transmission rate are listed in the following: Frequent cell reselection is caused by improper setting of the parameters and addressing layout. The C/I fails to meet the requirement to retransmit the RLC. The C/I cannot ensure the adoption of higher channel coding scheme. The system bottleneck is caused by the improper setting of the channels at the Um interface and the parameters at the Gb interface. 2) Abnormal TBF disconnection If the TBF is abnormally released after being released, the corresponding data needs to be retransmitted but the PDP context needs not to be re-setup, and this is defined as the abnormal TBF disconnection. The possible reasons for abnormal TBF disconnection are listed in the following: The dynamic packet resource is seized by force due to high traffic load. The cell reselection happens frequently. The radio environment is poor. The MS fails.