Teknillinen Korkeakoulu Teletekniikan laboratorio S Teletekniikan erikoistyö. General Packet Radio Service

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1 Teknillinen Korkeakoulu Teletekniikan laboratorio S Teletekniikan erikoistyö General Packet Radio Service Tekijä: Jani Kokkonen 42916s Ohjaaja: Vesa Kosonen Jätetty:

2 I Table of Contest 1 GPRS GPRS NETWORK ARCHITECTURE BACKBONE NETWORKS GPRS MOBILES GPRS INTERFACES AND REFERENCE POINTS GPRS PROTOCOL ARCHITECTURE TRANSMISSION PLANE SIGNALLING PLANE PDP CONTEXT FUNCTIONS AND ADDRESSES QUALITY OF SERVICE PROVISION IN THE GPRS SUMMARY REFERENCES... 15

3 II List of Abbreviations BG Border Gateway BSC Base Station Controller BSS Base Station Sub-System BSSAP BSS Application Part BSSGP BSS GPRS Protocol BTS Base Transceiver Station CCITT Comité Consultatif International Télégraphique et Téléphonique EIR Equipment Identity Register ETSI European Telecommunications Standards Institute GGSN Gateway GPRS Support Node GMSC Gateway MSC GPRS General Packet Radio Service GSM Global System for Mobile Communications GSN GPRS Support Node GTP GPRS Tunnelling Protocol HLR Home Location Register HPLMN Home Public Land Mobile Network IMSI International Mobile Subscriber Identity IP Internet Protocol IPv4 Internet Protocol version 4 IPv6 Internet Protocol version 6 ISDN Integrated Services Digital Network ITU International Telecommunication Union ITU-T Telecommunication standardisation sector of ITU LIG Lawful Interception Gateway LLC Logical Link Control MAC Medium Access Control MAP Mobile Application Part ME Mobile Equipment MM Mobility Management MS Mobile Station MSC Mobile (services) Switching Center MT Mobile Termination NMS Network Management Subsystem NSS Network and Switching Subsystem PAD Packet Assembly/Disassembly PDP Packet Data Protocol PDN Packet Data Network PDU Protocol Data Unit PLMN Public Land Mobile Network PSPDN Packet-Switched Public Data Network PSTN Public-Switched Telephone Network PTM Point-To-Multipoint PTM-M PTM Multicast PTM-SC PTM Service Center

4 III PTP Point-To-Point QoS Quality of Service RLC Radio Link Control SIM Subscriber Identity Module SGSN Serving GPRS Support Node SM Short Message SM-SC Short Message Service Center SMS-GMSC Short Message Service Gateway MSC SNDCP Sub-Network Dependent Convergence Protocol SS7 Signalling System number 7 TCP Transmission Control Protocol TE Terminal Equipment TID Tunnel Identifier UDP User Datagram Protocol VLR Visitor Location Register VPLMN Visited Public Land Mobile Network

5 IV List of Figures Figure 2-1 GSM/GPRS network architecture [G_SYS00]... 2 Figure 2-2 Intra- and Inter-PLMN backbone networks [GSM0360]... 4 Figure 2-3 logical structure of GPRS system [GSM0360]... 5 Figure 3-1 GPRS transmission plane architecture [GSM0360]... 7 Figure 3-2 GPRS signalling plane architecture [GSM0360]... 9 Figure 4-1 PDP Context Activation procedure [GSM0960]... 11

6 1 1 GPRS Up to now, the main application of the most mobile systems has been mobile telephony. However while wireless data usage doubles every year in the advanced markets, it became evident that the circuit switched bearer services provided by existing GSM can t satisfy the market needs. In the GSM user data is sent on the normal circuit switched traffic channel (TCH) that suffers from long set up time (~20s in modem usage) and low bit rate (9.6 kbit/s). In addition, the user charging is based on connection time instead of data send. Therefore standardisation of packet switched General Packet Radio Service (GPRS) network started in the European Telecommunications Standards Institute (ETSI) in The purpose of GPRS is to efficiently accommodate data sources that are bursty in nature. Another important goal is to provide methods that allow GSM operators share physical resources efficiently between packet switched services and circuit switched services. The introduction of packet radio technology demands new protocol stack to the radio interface and to the IP based core network together with the new network elements. In the GPRS radio channels are allocated dynamically according to resources available and user needs: maximum peak rate can be 171,2 kbps per user by utilising all eight channels without error correction. The GPRS maintains also strict separation between the radio subsystem and the network subsystem, which allows changing radio access interface in the future. Charging in the GPRS systems is based on amount of data transmitted and the quality of service negotiated. The first GSM/GPRS networks should be running end of this year. The GPRS is also essential stepping stone towards the third generation UMTS networks. [CONSE97][GENPA7]

7 2 2 GPRS Network Architecture The GPRS introduces new network nodes in the existing GSM PLMN. The most important ones are the Serving GPRS Support Node (SGSN) and the Gateway GPRS Support Node (GGSN). The others are: Point-to-Multipoint Service Center (PTM-SC), Border Gateway (BG), Charging Gateway (CG) and Legal Interception Gateway (LIG). The Inter-PLMN and Intra-PLMN networks can also be considered as new network elements. The following figure gives an overview of the GPRS network architecture: R/S Um BTS BSC Packet network PSTN Packet Inter-PLMN Backbone network network Serving GPRS Support Node (SGSN) Border Gateway (BG) Gp Firewall Point-To- Multipoint Service Center (PTM SC) Gb Gn Intra-PLMN backbone network (IP based) Gn Gr Gd Gs GPRS INFRASTRUCTURE Gateway GPRS Support Node (GGSN) MSC Gi.IP Gi.X.25 Gs Firewall Gr Packet SS7 network Network MAP-F Data Packet network (Internet) HLR/AuC EIR Router Corporate 1 Server Local area network SMS-GMSC Gd Corporate 2 Server Firewall Data Packet network (X.25) Router Local area network Figure 2-1 GSM/GPRS network architecture [G_SYS00] The Serving GPRS Support Node is the main component of the GPRS network and it is at the same hierarchical level as the MSC. The SGSN keeps track location of the individual MS and it performs security and user access control functions. The SGSN performs also relay functions between the MS and the relevant GGSN. The connection between the SGSN and the BSC is based on Frame Relay.

8 3 The Gateway GPRS Support Node provides interworking capability with external packet data networks, like X.25 and IP networks. The GGSN contains routing information for attached GPRS users. Therefore, from external network point of view, the GGSN is like a router to the GPRS subnetwork. Routing information is used to tunnel PDUs to the MSs current point of attachment. The GGSN is connected to SGSN by private IP-based network. [G_SYS00] The Border Gateway (BG) is a symbolic name associated with an inter-plmn backbone link. The BG is a secured link that carries GTP traffic and DNS lookups over the IP protocol. The BG is used if the GGSN being able to serve specific PDP context is located to other GPRS network than MSs current location. [W_PAC98] Both the SGSN and the GGSN send CDR s corresponding to activated PDP contexts to a Charging Gateway (CG). The CG consolidates the CDR information s and send it for further processing to external Customer Care and Billing system. The Lawful Interception Gateway (LIG) provides to law enforcement authorities access to see packet data traffic from the selected mobile station. The LIG is able to sniff the GTP traffic and interrogate the SGSN to achieve location information of the specific user. The Point-To-Multipoint Service Center (PTM-SC) is similar to the Short Message Service Center in the GSM, with the exception that it uses the GPRS transport to access mobile stations. The PTM-SC can reach users according to certain geographic area or to members of a named group. [W_PAC98]

9 4 2.1 Backbone Networks The GPRS consist of two kinds of IP-based backbone networks: intra-plmn and inter- PLMN. The intra-plmn is private IP network, which is used to connect the GSN of the same PLMN. The use of private address space means that network nodes cannot be reached from external network, which ensures security and performance of the GPRS system. Two intra-plmn can be connected via Border Gateways and an inter-plmn network. Inter-PLMN can be either private IP network based on leased lines or tunnelled public Internet. The inter-plmn network type is selected by roaming agreement. The Figure 2-2 depicts backbone network structure in the GPRS system: [GSM0360] Packet Data Network Inter-PLMN Backbone Gi Gp Gi GGSN BG BG GGSN Intra-PLMN Backbone Intra-PLMN Backbone SGSN SGSN SGSN PLMN A PLMN B Figure 2-2 Intra- and Inter-PLMN backbone networks [GSM0360]

10 5 2.2 GPRS Mobiles The GPRS mobiles can be divided into three classes: A, B and C. Class A mobile is able to use simultaneously circuit switched and packet switched services e.g. having voice call and packet connection at the same time. Class B mobile can be simultaneously attached to both circuit switched and packet switched systems but only one service can be in use at a time. Class C mobile can only be attached to either circuit switched or packet switched service at a time. Table 2-1 Classification of the GPRS mobile terminals Class A Class B Class C Fully simultaneous use of packet and circuit mode No simultaneous traffic but automatic sequential Alternate use or a GPRS only MS. connections service. [GSM0760] 2.3 GPRS Interfaces and Reference Points The GPRS system introduces new G-interfaces to GSM system. Figure 2-3 shows overall logical structure of GRPS system: SM S-GM SC SM S-IW M SC SM -SC E Gd C MSC/VLR HLR D Gs A Gc Gr Gb Gi TE MT BSS SGSN GGSN PDN TE Gn R Um Gn Gf G p EIR SGSN GGSN Other PLMN Sig nalling Interface Sig nalling and Data Transfer Interface Figure 2-3 Logical structure of the GPRS system [GSM0360]

11 6 The Gs,Gr,Gd,Gc and Gf interfaces are subsets of the Mobile Application Part (MAP) and they are carried over the SS7 network. These interfaces connect the GPRS system to Network Subsystem (NSS) part of the GSM. The Gn interface is implemented trough the intra-plmn backbone network, Gb through the inter-plmn backbone network and Gi through external network. Gb interface carries both payload and signalling between the BSS and the SGSN. Flow control is based on Frame Relay. Gs is an optional interface between the SGSN and the MSC which is used to send location information to the MSC/VLR or to get paging requests from the MSC/VLR. This interface utilises BSSAP+ protocol. Gd resides between the SMS-GMSC and the SGSN. The Gc interface allows more efficient use of the SMS services. Gr is between the SGSN and the HLR. The SGSN exploits Gr interface to interrogate with HLR during the PDP context activation phase. Gc is an optional interface between the GGSN and the HLR. The GGSN may request location information during the Network Request Context Activation-phase via Gc interface. Gf interface locates between the SGSN and the EIR. The Gf gives an access to equipment information. Gn is between two GSNs from the same PLMN. The Gn provides a data and signalling interface in the intra-plmn backbone. Gp is between two GSNs from the various PLMNs. The Gp interface provides the same functionality as the Gn interface, but in addition to that it provides, by using the BG and Firewall, all the functions that are needed in the inter-plmn networking i.e. security, routing, etc. Gi resides between the GGSN and an external network. The external data network is connected to GPRS network via Gi interface. R resides between terminal equipment and mobile terminal. This reference point connects terminal equipment to mobile terminal. Um is located between MS and the GPRS fixed network part. The Um interface gives MS access to the GPRS network. [G_SYS00]

12 7 3 GPRS Protocol Architecture The GPRS protocol architecture separates transmission plane from the signalling plane. The transmission plane consist of a layered protocol structure providing user information transfer, along with associated information transfer control procedures (e.g. flow control, error detection, error correction and error recovery). The Figure 3-1 shows protocol stacks for the transmission plane. [WCDMA98] Application Um Gb Gn Gi IP / X.25 IP / X.25 Relay SNDCP SNDCP GTP GTP LLC Relay LLC UDP / TCP UDP / TCP RLC RLC BSSGP BSSGP IP IP MAC GSM RF MAC GSM RF Network Services L1bis Network Services L1bis L2 L1 L2 L1 MS BSS SGSN GGSN Figure 3-1 GPRS transmission plane architecture [GSM0360] 3.1 Transmission Plane The GPRS Tunnelling Protocol (GTP) encapsulates higher layer Protocol Data Units (PDUs) and tunnels user data and signalling information between the GPRS support nodes. It also provides flow control between the GSNs. The TCP and UDP protocols are situated below the GTP protocol. The TCP provides reliable transmission of the GTP PDU whereas the UDP is used for unreliable GTP PDU transmission.

13 8 The IP is used in the GPRS backbone network for routing user data and signalling information. The IPv4 will be supported in the first phase but in the future IPv6 might be adopted. The Subnetwork Dependent Convergence Protocol (SNDCP) handles the PDU transmission between the SGSN and the MS. It maps network layer characteristics into characteristics of the underlaying logical link. The SNDCP performs also multiplexing of multiple layer 3 messages into a single logical link connection. In addition, ciphering, segmentation and compression are performed by SNDCP. The Logical Link Layer (LLC) provides a highly reliable and secured logical link between the MS and the SGSN. The reliability of packet transmission is provided by retransmission of the packet in acknowledged mode. In unacknowledged mode retransmission is not used. The LLC shall be independent of the underlying radio interface protocols in order to allow introduction of alternative GPRS radio solutions with minimum changes to the NSS. The Base Station System GPRS Protocol (BSSGP) conveys routing and QoS-related information between the BSS and the SGSN. The Relay function in the BSS conveys LLC PDUs between the Um and Gb interface. In the SGSN, this function relays the PDP PDUs between the Gb and Gn interfaces. The Network Service (NS) performs the BSSGP PDU transmission and it is based on Frame Relay between the BSS and the SGSN. In the radio interface Radio Link Control (RLC) function provides reliable radio link to the upper layers. The another protocol, Medium Access Control (MAC), handles the use of physical layer functions e.g. channel allocation and the multiplexing. It also takes care of mapping LLC frames into the GSM physical channel. [GSM0360]

14 9 3.2 Signalling Plane Internal signalling in the GPRS system is handled by protocols, which can transmit both data and signalling information (LLC, GTP and BSSGP). The signalling plane supports and controls data transport plane functions such as attaching to and detaching from the GPRS network, activation of the PDP context, mobility management, radio resource management and providing supplementary services. The Figure 3-2 shows the GPRS signalling plane protocol stack. [WCDMA98] GMM/SM LLC RLC RLC Relay BSSGP GMM/SM LLC BSSGP MAC MAC Network Service Network Service GSM RF GSM RF L1bis L1bis MS Um Gb BSS SGSN Figure 3-2 GPRS signalling plane architecture [GSM0360]

15 10 4 PDP Context Functions and Addresses The PDP Context functions are considered only at the NSS level and does not directly involve the BSS. The GPRS subscription contains the subscription of one or more PDP addresses (i.e. IP or X.25 addresses) and each of these addresses are described individually in the PDP context database in the MS, SGSN and GGSN. Each PDP context can be individually in the one of two states. In INACTIVE stage subscriber has not activated certain PDP address, which means that no data can be sent. In INACTIVE stage the PDP context doesn t contain any routing or mapping information related to that PDP address. In STANDBY or READY stage mobile can initiate the PDP context activation procedure. The GGSN can also request PDP context activation from GPRS- attached subscriber. During the PDP context activation procedure, the PDP address is allocated to the specific PDP context. At the same time the PDP context database in filled to contain information related to routing, mapping and QoS. If subscriber wants to use several applications which each requires own QoS, a subsriber has to activate several PDP contexts at the same time. The PDP address allocation can be either static or dynamic. A static PDP address is assigned to a MS permanently by the HPLMN. A dynamic PDP address is assigned to a MS by the HPLMN when MS performs a PDP context activation. After finnishing the session, the PDP addresses have to released. [GSM0360]

16 11 MS SGSN GGSN 1. Activate PDP Context Request 2. Security Functions 3. Create PDP Context Request 4. Activate PDP Context Accept 3. Create PDP Context Response Figure 4-1 PDP Context Activation procedure [GSM0960]

17 12 5 Quality of Service Provision in the GPRS In GPRS A QoS profile is associated with each PDP context. The PLMN may support only limited subset of the possible QoS profiles and network attempts always to provide enough resources to support requested service. The QoS profile is a single parameter with multiple data transfer attributes that defines traffic in terms of: Precedence class Delay class Reliability class Peak Throughput class Mean Throughput class The Precedence class attribute indicates relative importance of the traffic. Data packets with higher precedence class are served before data packets with lower precedence class. The GPRS provides four delay classes. Delay class indicates scheduling order of the data packets from different subscribers and PDP contexts. The Reliability class indicates transmission characteristics that are required by an application. Data reliability defines probability of data loss, miss-sequencing, data duplication and corruption of data. These characteristics are mapped to the requirements of the network layer. The Peak Throughput class indicates maximum data rate at which data is expected to be transmitted for particular PDP context. There is no quarantee that this peak rate can be achieved or sustained for any period of time.

18 13 The Mean Throughput class specifies the average rate at which data is expected to be transferred across the GPRS network during the remaining lifetime of an activated PDP context. [GSM0260]

19 14 6 Summary The IP based data services are the most rapidly growing market in the telecommunication industry at the moment. In order to provide access to those services efficiently, a new mechanism that consumes resources only when data is sent and received is required. In the first phase this is done by utilising the General Packet Radio Service (GPRS) in GSM networks. In the network evolution path, the GPRS is seen as a stepping stone towards the third generation UMTS networks. The third generation applications like speed high-speed data, imaging and video conferencing are seen especially lucrative services from operator s point of view. However, while standards of the UMTS networks are still under construction, the GPRS network can seen as a platform to develop many of those services already now with existing GSM networks. This work introduced the main components that are needed to build GPRS network. In backbone side, the most important elements were Serving GPRS Support Node (SGSN) and Gateway GPRS Support Node (GGSN). In the radio interface side a new protocol stack that supported dynamic capacity allocation was required. The service provision platform of the GPRS was described in chapters 4 and 5.

20 15 References [GSM0360] ETSI, General Packet Radio Service (GPRS); Service description; Stage 2; version [GSM0260] ETSI, General Packet Radio Service (GPRS); Service description; Stage 1; version [GSM0960] ETSI, General Packet Radio Service (GPRS); GPRS Tunnelling Protocol (GTP); version [GSM0760] ETSI, General Packet Radio Service (GPRS); Mobile Station (MS) supporting GPRS; version [G_SYS00] GPRS System Desription;Ahti Muhonen; version 2.0; [WCDMA98] Tero Ojanperä, Ramjee Prasad, Wideband CDMA for Third Generation Mobile Communications, Artech House Publisher, 1998 [W_PAC98] Jaffa, WCDMA Packet Data System Concept, version [CONSE97] IEEE, Concept, Services, and Protocols of the new GSM Phase 2+ General Packet Radio Service, August 1997 [GENPA97] IEEE, General Packet Radio Service in GSM, October 1997