Cellular System Components

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  • Topic: GSM, Subscriber Identity Module, Universal Mobile Telecommunications System
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Introduction to GSM Networks
Figure 1.1 is a schematic overview of the main components in a GSM network. The various interface labels are the formal names given to these interfaces. More details about these interfaces are found in GSM TS 03.02 [26]. 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). CAMEL has no interaction with the EIR and AUC; hence EIR and AUC are not further discussed. • 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. The structure of the MS will be described in more detail in a next section. A GSM network is a public land mobile network (PLMN). Other types of PLMN are the time division multiple access (TDMA) network or code division multiple access (CDMA) network. GSM uses the following sub-division of the PLMN: CAMEL: Intelligent Networks for the GSM, GPRS and UMTS Network  2006 John Wiley & Sons, Ltd Rogier Noldus


CAMEL: Intelligent Networks for the GSM, GPRS and UMTS Network

To HLR from other PLMN D




Core network
To/from other network







BSC Abis BTS Um Um

Base station system

Air interface





Figure 1.1

GSM network architecture

• Home PLMN (HPLMN) – the HPLMN is the GSM network that a GSM user is a subscriber of. That implies that GSM user’s subscription data resides in the HLR in that PLMN. The HLR may transfer the subscription data to a VLR (during registration in a PLMN) or a GMSC (during mobile terminating call handling). The HPLMN may also contain various service nodes, such as a short message service centre (SMSC), service control point (SCP), etc. • Visited PLMN (VPLMN) – the VPLMN is the GSM network where a subscriber is currently registered. The subscriber may be registered in her HPLMN or in another PLMN. In the latter case, the subscriber is outbound roaming (from HPLMN’s perspective) and inbound roaming (from VPLMN’s perspective). When the subscriber is currently registered in her HPLMN,...
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