e and functionality of the GSM net- work, the reader should refer to [1, 2]. Mobile Communications Mobile Network stations, Base station subsystem, Network and switching subsystem, NSS management subsystem, NMS Operation and (GMSC) maintenance center, OMC transceiver stations station controller VLR = Visitors location register PSPDN HLR = Home location register AuC = Authentication center interface A-interface Radio EIR = Equipment identity register or air MSC = Mobile (services) switching center SMSC = Short message service center interface PSPDN = Packet switched public data network IWF = Interworking functions PSTN = Public switched telephone network TRAU = Transcoder (TC) and rate adaptor unit (RAU) EC = Echo canceller ISDN = Integrated services digital network Figure 5.9 Structure of the GSM network. 5.5.1.1 Radio Network MSs are connected to the mobile switching center (MSC), via a base station subsystem (BSS). The BSS consists of a base station controller (BSC) and many base transceiver stations (BTSs) that are controlled by one BSC. The roles of the network elements are introduced in the following sections. 5.5.1.2 MSC Like any local exchange, the MSC establishes calls by switching the incoming channels into outgoing channels. It also controls the communications, releases connections, and collects charging information. As a mobile switching system, the MSC together with the VLR per- forms additional functions such as location registration and paging. It also transfers encryption parameters, participates in the handover procedure when required, and supports short message service (SMS). The SMS is a serv- ice integrated into GSM that enables users to transmit and receive short text messages. In each cellular network there is at least one gateway MSC (GMSC) that provides connections to other networks. The MSC in Figure 5.9 per- forms gateway functions in addition to other MSC functions. The GMSC works as an interface between the cellular network and the fixed networks and it must handle the signaling protoco