(This write up has been sourced from: http://media.wiley.com/product_data/excerpt/49/04700169/0470016949.pdf )
The GSM network consists mainly of the following functional parts:
• MSC – the mobile service switching centre (MSC) is the core switching entity in the network. The MSC is connected to the radio access network (RAN); the RAN is formed by the BSCs and BTSs within the Public Land Mobile Network (PLMN). Users of the GSM network are registered with an MSC; all calls to and from the user are controlled by the MSC. A GSM network has one or more MSCs, geographically distributed.
• VLR – the visitor location register (VLR) contains subscriber data for subscribers registered in an MSC. Every MSC contains a VLR. Although MSC and VLR are individually addressable, they are always contained in one integrated node.
• GMSC – the gateway MSC (GMSC) is the switching entity that controls mobile terminating calls. When a call is established towards a GSM subscriber, a GMSC contacts the HLR of that subscriber, to obtain the address of the MSC where that subscriber is currently registered. That MSC address is used to route the call to that subscriber.
• HLR – the home location register (HLR) is the database that contains a subscription record for each subscriber of the network. A GSM subscriber is normally associated with one particular HLR. The HLR is responsible for the sending of subscription data to the VLR (during registration) or GMSC (during mobile terminating call handling).
• CN – the core network (CN) consists of, amongst other things, MSC(s), GMSC(s) and HLR(s). These entities are the main components for call handling and subscriber management. Other main entities in the CN are the equipment identification register (EIR) and authentication centre (AUC).
• BSS – the base station system (BSS) is composed of one or more base station controllers (BSC) and one or more base transceiver stations (BTS). The BTS contains one or more transceivers (TRX). The TRX is responsible for radio signal transmission and reception. BTS and BSC are connected through the Abis interface. The BSS is connected to the MSC through the A interface.
• MS – the mobile station (MS is the GSM handset).
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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 kbit/s channel.
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