rformed by the MS, which reports to the PLMN as soon as it moves over boundaries of rout- ing areas or between cells. The PLMN might have to rearrange internal routes SO that the MS stays in contact with the PDN when it moves. A number of other problems must be handled by the process control system as well: an MS remains in the active state only when it actually communicates over the PDN. If it is passive too long, it is deactivated, which means that it is only attached. To become active again it must first make a new attach request. It must also be possible to call an MS from a PDN. The process control system will, in this case, have to page the MS. An MS can be paged only if it is in the attached state. Figure 4.10 also indicates that the PLMN may cooperate with other external PLMNs (i4.0), with or without GPRS functionality. This kind of cooperation may allow an MS to roam between different PLMNs and access PLMNs other than the one to which it is subscribed. The PLMN also cooperates with fixed networks (i5.0). This is, however, of no interest if the MS accesses the PLMN over i1.0. To the left in Fig. 4.11, the protocol stack for network interface 11.0 is shown. PDN applications gsm&gprs PDN applications Host, NSAPI SNDCP SAPI7 SAPI1 SAPI3,5,7,11 plmn.0 pdn.0 (GSM logical channels) pdn packet data network fxn. o GSM-RF fxn fixed networks GMM GPRS Mobility Management PDP Packet Data Protocol LLC Logical Link Control RF Radio transmission MAC Media Access Control RLC Radio Link Control NSAPI Network SAPI SMS Short Message Service PDCH Packet Data Channel SNDCP Subnetwork Dependent Convergence Protocol Fig. 4.11 The network interface for GPRS access The stack is characterized by discrimination functions in four steps: On the GSM-RF level, the data flow is discriminated into the PDCH (one or several timeslots) and the structures that are used for circuit-switched services. 4 Physical Network Modelling The MAC protocol provides access to the PDCH according to a special multiple access procedure. Since many MSs might