GSM signalling

Figure 47.12 shows the overall signalling model. The Air Interface uses LAPDm Layer 2 signalling protocol and this is also used for the A-bis, BTS to BSC interface.

The layer 3 protocol consists of three sublayers, dealing with radio resource management (RR), mobility management (MM), and con­nection management (CM). Radio resource management is con­cerned with managing the logical channels, including paging, channel assignments, handover, measurement reporting, and other functions.

The mobility management layer contains functions necessary to support the mobility of the user which include authentication, loca­tion updating, attach and detach of IMSI (International Mobile Subscriber Identity), and registration. The connection management layer is concerned with call control, establishing and clearing cir­cuits, management of supplementary services and the short message service.

The BSC to MSC A-interface, and the various MSC to Register interfaces employ CCITT No.7 signalling using the Message Trans­fer Part (MTP), Signalling Connection Control Part (SCCP), Trans­action Capabilities Part (TCAP) and Mobile Application Part (MAP).

An example of the signalling messaging for establishing a mobile originated call is shown in Figure 47.13. The key events are:

1. Request and assignment of a channel, between MS and BSS.

2. A service request procedure which accesses the VLR.

3. An authentication and ciphering exchange which validates the mobile user and sets the encryption cipher.

4. Call set up which includes sending of dialled digits and estab­lishing the connection.

Location updating is shown in Figure 47.14 An update request is indicated by the mobile and passed to the VLR in the new location area. The new VLR requests the IMSI from the old VLR and then signals the new location to the HLR. The HLR provides the subscriber data to the new VLR and cancels the subscriber entry in the old VLR. Finally a confirmation message is set back to the mobile. There are, of course, many other signalling exchanges, dealing with mobile terminating calls, supplementary services, and short message service. There is not space in this chapter to deal with the detailed signalling for these cases; the examples above describe the general principle and illustrate the roles of the MS, BSS, MSC, VLR and HLR.

Figure 47.12 GSM signalling model

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