Lll System Architecture

In order to integrate GPRS into the existing GSM architecture (see e.g. Figure 3.9), a new class of network nodes, called GPRS Support Nodes (GSNs), has been introduced. GSNs are responsible for the delivery and routing of data packets between the mobile stations and external packet data networks (PDNs). Figure 11.1 illustrates the resulting system architecture.

Gprs Backbone Arsitektur
Figure 11.1: GPRS system architecture and interfaces

A Serving GPRS Support Node (SGSN) delivers data packets from and to the mobile stations within its service area. Its tasks include packet routing and transfer, functions

Parts of this chapter are based on the authors' publication: Ch. Bettstetter, H.-J. Vögel, J. Eberspächer. GSM Phase 2+ General Packet Radio Service GPRS: Architecture, Protocols, and Air Interface. IEEE Communications Surveys, Special Issue on Packet Radio Networks, vol. 2, no. 3, 1999, which can be obtained at http://www.comsoc.org/pubs/ surveys. © 1999 IEEE.

for attach/detach of mobile stations and their authentication, and logical link management. The location register of the SGSN stores location information (e.g. current cell, current VLR) and user profiles (e.g. IMSI, address used in the packet data network) of all GPRS users registered with this SGSN.

A Gateway GPRS Support Node (GGSN) acts as an interface to external packet data networks (e.g. to the Internet). It converts GPRS packets coming from the SGSN into the appropriate Packet Data Protocol (PDP) format (i.e. IP or X.25) and sends them out on the corresponding external network. In the other direction, the PDP address of incoming data packets (e.g. the IP destination address) is converted to the GSM address of the destination user. The readdressed packets are sent to the responsible SGSN. For this purpose, the GGSN stores the current SGSN addresses and profiles of registered users in its location register.

In general, there is a many-to-many relationship between the SGSNs and the GGSNs: A GGSN is the interface to an external network for several SGSNs; an SGSN may route its packets to different GGSNs.

Sgsn Interface

Figure 11.2: GPRS system architecture, interfaces, and routing example

Figure 11.2: GPRS system architecture, interfaces, and routing example

Figure 11.1 also shows the interfaces between the GPRS support nodes and the GSM network. The Gb interface connects the BSC with the SGSN. Via the Gn and the Gp interfaces, user and signaling data are transmitted between the GSNs. The Gn interface is used, if SGSN and GGSN are located in the same PLMN, whereas the Gp interface is used, if they are in different PLMNs.

All GSNs are connected via an IP-based GPRS backbone network. Within this backbone, the GSNs encapsulate the PDN packets and transmit (tunnel) them using the so-called GPRS Tunneling Protocol (GTP). In principle, we can distinguish between two kinds of GPRS backbones:

• Intra-PLMN backbones are IP-based networks owned by the GPRS network provider connecting the GSNs of the GPRS network.

• Inter-PLMN backbone networks connect GSNs of different GPRS networks. They are installed if there is a roaming agreement between two GPRS network providers.

Figure 11.2 shows, how two Intra-PLMN backbone networks of different PLMNs are connected with an Inter-PLMN backbone. The gateways between the PLMNs and the external Inter-PLMN backbone are called Border Gateways (BGs). Their main task is to perform security functions in order to protect the private Intra-PLMN backbones against unauthorized users and attacks. The illustrated routing example is explained later.

The Gn and Gp interfaces are also defined between two SGSNs. This allows the SGSNs to exchange user profiles when a mobile station moves from one SGSN area to another.

Across the Gf interface, the SGSN may query and check the IMEI of a mobile station trying to register with the network.

The Gi interface connects the PLMN with external PDNs. In the GPRS standard, interfaces to IP (IPv4 and IPv6) and X.25 networks are supported.

GPRS also adds some more entries to the GSM registers. For mobility management, the user's entry in the HLR is extended with a link to its current SGSN. Moreover, his or her GPRS-specific profile and current PDP address(es) are stored. The Gr interface is used to exchange this information between HLR and SGSN. For example: The SGSN informs the HLR about the current location of the MS. When an MS registers with a new SGSN, the HLR will send the user profile to the new SGSN. In a similar manner, the signaling path between GGSN and HLR (Gc interface) may be used by the GGSN to query the location and profile of a user who is unknown to the GGSN.

In addition, the MSC/VLR may be extended with functions and register entries which allow efficient coordination between packet switched (GPRS) and conventional circuit switched GSM services. Examples for this are combined GPRS and GSM location updates and combined attachment procedures. Moreover, paging requests of circuit switched GSM calls can be performed via the SGSN. For this purpose, the Gs interface connects the registers of SGSN and MSC/VLR.

Finally, it is worth mentioning that it is possible to exchange messages of the Short Message Service (SMS) via GPRS. The Gd interface interconnects the SMS Gateway MSC (SMS-GMSC) with the SGSN.

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  • petros
    How many SGSNs in a PLMN?
    7 years ago

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