GPRS Physical Network Level 2 If we apply the distinction between external and internal views of a network interface on i1.0, the physical subnetwork for the PLMN will look as shown in Fig. 4.12. The model shows how interface i1.0 is terminated in internal physical nodes (the network interface i4.0 is disregarded in this model). 4.4 General Packet Radio Service, a Case Study BSSGP plmn. 1.1 (GSM logical channels) GSM-RP SNDCP GGSN - j4.1 SAPI7 SAPI1 SAPI3,5,7,11 BSSGP GGSN - 4.1 (j1.1) pdn.0 GTP (j2.1) Fig. 4.12 GPRS network on physical network level 1 The model defines two new types of physical nodes for handling GPRS serv- ices: the serving GPRS support node (SGSN) and the gateway-GSN (GGSN). MSC is the circuit switching node in GSM, BSS is the access network (consisting of base station controllers, BSC, and base stations, BS) that is handled by one MCS and one SGSN. HLR is a database node where subscription data and the actual location of the MS are stored. SGSNs are nodes that contain the major part of the GPRS process control sys- tem, which deals with creating and managing connections between MSs and GGSNs and performing mobility control. GGSN connect to external PDNs. SGSNs and GGSNs are also routers. SGSN routes packages between MSs and GGSNs (path is i1.0-j1.1-j2.1). GGSNs route packages between PDNs and SGSNs (path is i3.0-j2.1). The GPRS part of a GSM network may contain many SGSNs and GGSNs. The GPRS part of an GSM network cooperates with the TDMA part over j3.1. For example, it is more effective to page and send short messages over GPRS than over MSC and BSS. GPRS therefore provides those services to the circuit-switched part of the GSM network. There is also some cooperation over j4.1 as regards mobility control and subscriber-data handling in the HLR. As can be seen from this model, the internal network interfaces (j1.1, etc.) are denoted as interfaces that do not include physical media. The reason is that GPRS does not specify its own transport mechanisms (except in i1.0). A GPRS network is based 