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2.1.1 GSM (2G) Network Architecture

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Consider the legacy GSM CS network shown in Figure 2.1. It consists of the different network subsystems and elements: MS, BSS (Base Station Subsystem), and Network switching subsystems (NSS). A GSM network provides the CS voice call services to subscribers.

As illustrated in Figure 2.1, a GSM CS communications network is broadly divided into various subsystems, which are described below.

 BSS

It consists of the network elements: BTS and BSC. A BTS is a hardware component that is installed to provide communications services in a GSM cell. A BTS transmits and receives information with mobile devices through radio frequency communications. A BSC is responsible for the allocation of radio frequency resources in one or more cells and controls one or multiple BTSs. The GSM BSS, consisting of BTS and BSC, is the interface between a mobile device and the rest of the GSM network or public switched telephone network (PSTN). A BTS is connected to a BSC through a logical interface called A‐bis interface. An MS communicates with the BSC through the physical air interface, known as Um.

 Network switching subsystem (NSS)

It consists of the network elements: MSC, Home Location Register (HLR), Visitor Location Register (VLR), and so on. The MSC performs all the necessary functions to provide CS voice call services, both for the mobile originated (MO) and mobile terminated. The gateway MSC performs the routing functions on behalf of an MS that is being served by another MSC. A VLR contains information about all the MS currently being served by a particular MSC. An MSC contacts the VLR to find and retrieve the current location of an MS. The HLR is a central database that stores the permanent information of subscribers.

NSS is also known as the Core Network (CN) and facilitates seamless communication services to freely moving users within its coverage area or between the networks of different operators or between a mobile and fixed‐line network.

The vertical dotted lines in Figure 2.1 indicates the separation of one network element from another one or an entire network from another network through a particular interface with a set of protocol layers on it. Each such interface has its name, for example, air interface (Um), A‐bis interface, A‐interface, and so on, as shown in Figure 2.1. More about the mobile communications network interfaces, both physical and logical, are described in Chapter 3.


Figure 2.1 Network architecture and elements of a GSM network.


Figure 2.2 Network architecture and elements of a GPRS network.

Mobile Communications Systems Development

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