1
GPRS (General Packet Radio Service)
GPRS operation is based on the principle of packet mode transfer using the Internet protocol (IP). GPRS technology also allows user allocation of more time slots within a frame. This exhibits traffic patterns such as frequent transmission of small volumes (e.g., typical web requests) or infrequent transmissions of medium volumes (e.g., typical web responses). Shortly, GPRS optimizes the use of network and radio resources. Strict separation between the radio subsystem and network system is maintained, allowing the network subsystem to be reused with other radio access technologies. For the new GPRS radio channels, the GSM system can allocate between one and eight time slots within a TDMA frame. Time slots are not allocated in a fixed, pre-determined manner but on demand. All time slots can be shared by the active users but up- and downlink are allocated separately. Allocation of the slots is based on current load and operator preferences. A mobile device connected to the GPRS is like a node on the LAN, i.e., it is always online. The mobile device is assigned an IP address whenever it is connected to the Internet. Depending on the coding, a transfer rate of up to 172 kbit/s is possible. Architecture In order to integrate GPRS into the existing GSM architecture, a new class of network nodes, called GPRS support nodes (GSN) has been introduced. GSNs are responsisble for the delivery and routing of data packets between the mobile stations and the external packet data networks (PDNs). The two network elements that are introduced with GPRS to the GSM architecture are: The Serving GPRS Support Node (SGSN). This node is responsible for the delivery of data packets from and to the mobile stations within its service area. The tasks include packet routing and transfer, keeping track of the mobile users, logical link management, and authentication and charging functions. At a high level, the SGSN provides similar functionality to the packet data network that the MSC/VLR in GSM provides to the circuit-switched network. The Gateway GPRS Support Node (GGSN). This node acts as an interface between the GPRS backbone network and the external packet data network. It converts the GPRS packet coming from the SGSN into the appropriate packet data protocol (PDP) format (e.g., IP or X.25) and the packet is forwarded (by tunneling) to the corresponding PDP. In the other direction, PDP addresses of incoming data packets are converted to the GSM address. Setting up the PDP context is more or less equivalent to obtaining an IP address. The readdressed packets are sent to the responsible SGSN. The system architecture is illustrated in Figure 1 (next page).
Figure 1. GPRS system architecture.
BSS (Base Station System) supports both circuit and packet switched traffic. GPRS uses circuit switched channels in a dynamic way. The channels are allocated only when data is sent or received. These features imply a more effective use of the bandwidth.
3 BTS (Base Transceiver Station) handles traffic between MC and BSC. Additional GPRS software is needed because of special coding and modulation. GPRS specific features are: o PDCH (Packet Data Channel) support o MAC (Medium Access Control and RLC (Radio Link Control) support o Administration of resources BSC (Base Station Controller) basically manages the BTSs. The number of BTSs per BSC are of the order of 25-150. The BSC handles radio related task like attach and detach of circuit and packet switched calls. Special software and hardware is needed. Fragmentation of packets is handled by the PCU(Packet Control Unit). GPRS specific parts in the BSC include: o GPRS mobility management o Communication with SGSN using Frame Relay and BSSGP (Base Station System GPRS Protocol) o PDCH allocation o SMS distribution PSS (Packet Switching System) is an IP-based network handling packet transfer between different SGSN and GGSN stations (nodes). Both nodes have IP routing functions and these nodes can be connected to external IP routers. The PSS network is divided into two sub networks : Management network. This part handles GPRS administration. Management network nodes are accessible only by GSN (GGSN, SGSN) stations. Service network. This network offers Internet service to the users. Typical examples are e-mail, public DNS and WWW services. These stations can be accessed from Internet and by mobile stations. SGSN and GGSN were already discussed in the previous text.
Figure 2. Two intra-PLMN backbone networks of different PLMNs connected with an inter-PLMN backbone.
CSS (Circuit Switching System) uses circuit switching technique. This part of the network handles traditional traffic over GSM. CSS is the ordinary GSM system including additional GPRS support. In the following lines we briefly discuss these additional features: MSC/VLR may be extended with functions and register entries that allow efficient coordination between packet switched (GPRS) and circuit switched (conventional GSM) services. Examples are combined (GPRS, non-GPRS) location updates and combined attachment procedures. Moreover, paging requests of circuit switched GSM calls can be performed via the SGSN. VLR keeps track of all mobile stations that for the moment are within the MSCs Local Area or within the SGNSs Routeing Area (see exercise). HLR stores the user profile, the current SGSN address, and the PDP address for each user in the PLMN. The SGSN informs the HLR about the current location of the MS. When the MS registers with a new SGSN, the HLR will send the user profile to the new SGSN. GGSN may use the HLR to query a users location and profile in order to update its location register. Therefore, the most important GPRS-HLR parameters are:
5 o PDP Type Defines the used protocol type. Example are IP and X.25. o PDP Adress This is the address of the MS. The address is static or dynamic. o GGSN Address This is the address of the GGSN currently in use. o Quality of Service This parameter gives the used bandwidth and it is normally dependent of the charge. AuC (Authentication Centre), EIR (Equipment Identity Register) and GMSC (Gateway Mobile services Switching Centre) are not modified. The task of the GMSC is to switch messages between GSM and PSTN. SMSC (Short Message Service Centre) handles sending/receiving of so called short messages via SMS Gateway MSC and SMS Interworking MSC. It is also possible to connect to SMSC via IP and X.25 from the Internet.
Miscellaneous Features Terminals. GPRS defines three classes of terminals: Class A. This class supports simultaneous circuit switched and packet switched traffic, i.e., the terminal can be connected to GPRS and GSM at the same time. Class B. A class B terminal can be attached to the network as both a circuit switched and packet switched client but can only support traffic from one service at a time. Class C. A class C terminal uses only packet switched services.
Protocols. The protocol stacks are illustrated in Figure 3. We can notice the following features: Applications use the IP (Internet protocol) protocol Packets between SGSN and GGSN use the tunneling protocol GTP (GPRS Tunneling Protocol). GTP. UDP/IP are transport protocols. SNDCP (Sub Network Dependent Convergence Protocol) and LLC (Logical Link Control) work between the SGSN and the MS. Compression (SNDCP) and encryption (LLC) are main tasks. LLC is used by the MS until a new SGSN + a new LLC are in use.
6 X.25 can be run on top of TCP/IP.
Figure 3. GPRS Protocol Stacks.
GPRS Security The GPRS system, like in GSM, provides subscriber identity authentication, user and signaling data confidentiality, and subscriber identity confidentiality. Subscriber Identity Authentication. The GPRS authentication procedure is handled in the same way as in GSM with the distinction that the procedures are executed in the SGSN. The procedure is shown below.
Figure 3. Subscriber Identity Authentication.
User and Signaling Data Confidentiality. A new ciphering algorithm GPRS-A5 is used. The scope of GPRS is from the SGSN to the MS. The ciphering is done in the Logical Link Control (LLC) layer where the GPRS Kc is handled by the SGSN independently from MSC. Subscriber Identity Confidentiality . GPRS uses the same procedure as in GSM. SGSN is used instead of MSC. The procedure is: 1. Identity Request is sent from SGSN to MS and Identity Response is received. 2. If SGSN decides to compare IMEI ( International Mobile Equipment Identity) to the value stored in EIR a Check IMEI is sent to EIR and the answer is Check IMEI Ack.
