MSC-new MSC-old Fig. 5.7 Some protocol networks in the GSM-PCS To the left in Fig. 5.7, the authentication and ciphering (AC) protocol network. This network is a support function to the service handling (SH) protocol net- work. Every time an MS accesses the network, this function must be executed before the SH accepts the MS's request for service. The AC protocol network comprises protocols and protocol elements in the logical nodes, as shown in the model. None of the protocols indicated in the model are defined as separate pro- tocols in the GSM standard. At best, sequence diagrams are used to give partial information on how a particular network function works across the logical net- work. To the upper right, Fig. 5.7 shows BRO, the protocol network for broadcasting information to all MSs that exist and are active in the network. The information to be broadcast comes from the OSS and is sent to the BSC, which formats and encodes it for sending to MSs. BRO is a support function to three other protocol networks: the roaming protocol network (ROA), the service handling (SH) pro- tocol network and the locating (LOC) protocol network. To the lower right in Fig. 5.7, the protocol network for registration of the where- abouts of MSs, called REG. This network is also a support function to the SH network. The SH protocol element in MS uses the information provided by BRO to tell the network of its whereabouts. Forced registration is performed whenever MS moves from one location area to another, or at regular intervals. Registration is also performed each time an MS accesses the network. The SH in MSC orders the REG network to check location information in the VLR and update that information, if necessary. The HLR may also be informed if the MS has moved to a cell that is handled by a different MSC than the one that handled 5.2 An Example: The GSM Process Control System the last access from the MS. In that case, the REG protocol element in HLR also informs the other MSC that the MS is no longer in its service area. A numb