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GSM Networks: Protocols, Terminology, and Implementation

BSC takes care of all the central functions and the control of the subsystem, referred to as the base station subsystem (BSS). The BSS comprises the BSC itself and the connected BTSs.

1.2.2.5 Transcoding Ratę and Adaptation Unit

TRAU

One of the most important aspects of a mobile network is the effec-tiveness with which it uses the available frequency resources. Effective-ness addresses how many calls can be madę by using a certain bandwidth, which in tum translates into the necessity to compress data, at least over the Air-interface. In a GSM system, data compres-sion is performed in both the MS and the TRAU. From the architec-ture perspective, the TRAU is part of the BSS. An appropriate graphical representation of the TRAU is a black box or, morę symboli-cally, a clamp.

1.2.2.6 Mobile Services Switching Center

11 H I |j

A large number of BSCs are connected to the MSC via the A-interface. The MSC is very similar to a regular digital telephone exchange and is accessed by external networks exactly the same way. The major tasks of an MSC are the routing of incoming and outgo-ing calls and the assignment of user channels on the A-interface.

1.2.2.7 Home Location Register

The MSC is only one subcenter of a GSM network. Another subcen-ter is the HLR, a repository that Stores the data of a large number of subscribers. An HLR can be regarded as a large database that adminis-ters the data of literally hundreds of thousands of subscribers. Every PLMN requires at least one HLR.

1.2.2.8 Visitor Location Register

The VLR was devised so that the HLR would not be overloaded with inquiries on data about its subscribers. Like the HLR, a VLR contains subscriber data, but only part of the data in the HLR and only while the particular subscriber roams in the area for which the VLR is responsible. When the subscriber moves out of the VLR area, the HLR requests removal of the data related to a subscriber from the VLR. The geographic area of the VLR consists of the total area cov-ered by those BTSs that are related to the MSCs for which the VLR provides its services.