The various entities in the GSM network are connected to one another through signalling networks. Signalling is used for example, for subscriber mobility, subscriber registration, call establishment, etc. The connections to the various entities are known as ‘reference points’. Examples include:

• A interface – the connection between MSC and BSC;
• Abis interface – the connection between BSC and BTS;
• D interface – the connection between MSC and HLR;
• Um interface – the radio connection between MS and BTS.

Various signalling protocols are used over the reference points. Some of these protocols for GSM are the following:

• Mobile application part (MAP) – MAP is used for call control, subscriber registration, short message service, etc.; MAP is used over many of the GSM network interfaces;

• Base station system application part (BSSAP)– BSSAP is used over the A interface;

• Direct transfer application part (DTAP)– DTAP is used between MS and MSC; DTAP is carried over the Abis and the A interface. DTAP is specified in GSM TS 04.08 [49];

• ISDN user part (ISUP) – ISUP is the protocol for establishing and releasing circuit switched calls. ISUP is also used in landline Integrated Services Digital Network (ISDN). A circuit is the data channel that is established between two users in the network. Within ISDN, the data channel is generally a 64 Kbps channel.

The circuit is used for the transfer of the encoded speech or other data. ISUP is specified in ITU-T Q.763 [137]. When it comes to call establishment, GSM makes a distinction between signalling and payload. Signalling refers to the exchange of information for call set up; payload refers to the data that is transferred within a call, i.e. voice, video, fax etc.

For a mobile terminated GSM call, the signaling consists of exchange of MAP messages between GMSC, HLR and visited MSC (VMSC). The payload is transferred by the ISUP connection between GMSC and VMSC. It is a continue aim to optimize the payload transfer through the network, as payload transfer has a direct cost aspect associated with it. Some network services are designed to optimize the payload transfer. One example is optimal routing.