8.2 Network Architecture Description
The GPRS network has introduced a set of new functions, such as serving GPRS support node (SGSN), gateway GPRS support node (GGSN), border gateway (BG), and charging gateway function (CGF) for a public land mobile network (PLMN). It reuses some existing network elements (NEs) from the GSM CS network, such as home location register (HLR), authentication center (AuC), and equipment identification register (EIR). The details of the architecture are described in GSM TS 03.60 and are shown in Figure 8-1.
Figure 8-1. GPRS network architecture. Source: ETSI 2002. Further use, modification, and redistribution is
. ETSI standards are available from http://pda.etsi.org/pda/ and http://www.ets.org/eds/.
8.2.1 Gateway GPRS Support Node
The GGSN is the gateway to the external packet data networks for the GPRS. It tunnels the packet data units (PDUs) (e.g., an IP packet), to the MS's current point of attachment (i.e., the SGSN). The GGSN may query the HLR of a subscriber to get the current SGSN address. The GGSN also collects the charging information, but based on the usage of the external network resources. A firewall function is needed at the GGSN to filter out unauthorized and
packets from entering the GPRS network.
8.2.2 Serving GPRS Support Node
The SGSN provides mobility management,
authentication, and routes packet data. The SGSN collects charging information
to the usage of the network. The SGSN is also connected to the MSC/VLR via the Gs interface. The Gs interface helps in optimizing some CS network procedures by using the GPRS network. For example, the MSC/VLR may initiate paging or collect MS location information through Gs. SMS-GMSC and SMS-IWMSC are connected to the SGSN using the Gd interface. This interface allows GPRS-capable mobiles to send/receive SMS over GPRS radio channels.
8.2.3 Border Gateway
Two intra-PLMN backbone networks can be connected via the Gp interface using border gateways (BGs). The functions for a BG are not specified by ETSI. At minimum, it needs to have a security function for protecting the intra-PLMN network against external attacks. The security functionality is determined based on a roaming agreement between the two PLMNs.
8.2.4 Home Location Register
The HLR contains user subscription data and routing information. The routing information is the current SGSN address. The HLR has SS7-based interfaces with the SGSN and the GGSN. When the MS registers with a new SGSN, the SGSN informs the HLR using the Gr interface and the HLR sends back the
profile. The GGSN may
the HLR for a user's current location. The HLR from the GSM CS network is enhanced to support GPRS.
8.2.5 Charging Gateway Function
The CGF provides a mechanism to transfer charging information from the SGSN and GGSN to the billing systems (BSs). The CGF can be implemented as a centralized separate NE (CG) or as a distributed functionality resident in the SGSNs and GGSNs. Irrespective of the way in which the CGF is implemented, the functionality of the CGF stays the same.
8.2.6 SMS-GMSC and SMS-IWMSC
The SMS-GMSC and SMS-IWMSC are connected to the SGSN via the Gd interface to enable a GPRS MS to send and receive short messages (SMs) over GPRS radio channels. The SMS-GMSC is the gateway to the GPRS network for the SM delivery to a user
in the network. The SMS-IWMSC is the MSC connected to the SM-SC. It directs the SM originated by an MS to the SM-SC. Both of these functionalities could coexist in the same physical element.
8.2.7 Packet Control Unit
The radio functionalities introduced by GPRS are required to be an add-on function to the existing GSM BSS network. For this purpose, the packet control unit (PCU) is added to the GSM BSS. The PCU performs the radio
functions for the data packets. If the PCU is remote to a BTS, then the radio frames (PCU
) are carried in an extension of the Abis TRAU (
rate adaptation unit) frames, which are defined for the GSM voice services. In this way the BSS network doesn't need any modification to handle the PCU frames. The PCU
with the channel codec unit (CCU), which performs functions like interleaving and Forward Error Correction (FEC). Any control information between PCU and CCU is carried through inband signaling.
The PCU is not
in Figure 8-1 due to the flexibility in its location. The specifications have identified three different options for the location of PCU, without mandating any of them: BTS, BSC site, and GSN site (Figure 8-2).
Figure 8-2. PCU location options in the GPRS architecture. Source: ETSI 2002. Further use, modification, and redistribution is strictly prohibited. ETSI standards are available from http://pda.etsi.org/pda/ and http://www.ets.org/eds/.