This overall process is shown in Figure 47.11, national calls. Speech Frame 260 Bits Signalling Frame 184 Bits Fire code 1/2 Rate ADD CRC convolutional code Parity bits Interleave Interleave Degree 8 Degree 4 8 time slots 4 time slots Figure 47.11 GSM channel coding and interleaving Detailed description of GSM 47/13 HLR or VLR BSSAP BSSAP LAPDm LAPDm Layer 1 Layer 1 Figure 47.12 GSM signalling model 47.5.4 GSM signalling scriber data to the new VLR and cancels the subscriber entry in the old VLR. Finally a confirmation message is set back to the mobile. Figure 47.12 shows the overall signalling model. The Air Interface There are, of course, many other signalling exchanges, dealing uses LAPDm Layer 2 signalling protocol and this is also used for with mobile terminating calls, supplementary services, and short the A-bis, BTS to BSC interface. message service. There is not space in this chapter to deal with the The layer 3 protocol consists of three sublayers, dealing with radio detailed signalling for these cases; the examples above describe the resource management (RR), mobility management (MM), and con- general principle and illustrate the roles of the MS, BSS, MSC, VLR nection management (CM). Radio resource management is con- and HLR. cerned with managing the logical channels, including paging, channel assignments, handover, measurement reporting, and other 47.5.5 Security features functions. The mobility management layer contains functions necessary to support the mobility of the user which include authentication, loca- The information on the air interface needs to be protected, to tion updating, attach and detach of IMSI (International Mobile provide user data (including speech) confidentiality and to prevent Subscriber Identity), and registration. The connection management fraudulent use of subscriber and mobile identities. The basic mech- layer is concerned with call control, establishing and clearing cir- anisms employed are user authentication and user data encryption. cuits, management of supplementary s