GSM Subsystems – Overview

A GSM network comprises several elements: the mobile station (MS), the subscriber identity module (SIM), the base transceiver station (BTS), the base station controller (BSC), the transcoding rate and adaptation unit (TRAU), the mobile services switching center (MSC), the home location register (HLR), the visitor location register (VLR), and the equipment identity register (EIR). Together, they form a public land mobile network (PLMN).

The figure provides an overview of the GSM subsystems.

GSM-PLMN contains as many MSs as possible, available in various styles and power classes. In particular, the handheld and portable stations need to be distinguished.

GSM distinguishes between the identity of the subscriber and that of the mobile equipment. The SIM determines the directory number and the calls billed to a subscriber. The SIM is a database on the user side. Physically, it consists of a chip, which the user must insert into the GSM telephone before it can be used. To make its handling easier, the SIM has the format of a credit card or is inserted as a plug-in SIM. The SIM communicates directly with the VLR and indirectly with the HLR.

A large number of BTSs take care of the radio-related tasks and provide the connectivity between the network and the mobile station via the Air-interface.

The BTSs of an area (e.g., the size of a medium-sized town) are connected to the BSC via an interface called the Abis-interface. The 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.

One of the most important aspects of a mobile network is the effectiveness with which it uses the available frequency resources. Effectiveness addresses how many calls can be made by using a certain bandwidth, which in turn translates into the necessity to compress data, at least over the Air-interface. In a GSM system, data compression is performed in both the MS and the TRAU. From the architecture perspective, the TRAU is part of the BSS. An appropriate graphical representation of the TRAU is a black box or, more symbolically, a clamp.

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 outgoing calls and the assignment of user channels on the A-interface.

The MSC is only one subcenter of a GSM network. Another subcenter 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 administers the data of literally hundreds of thousands of subscribers. Every PLMN requires at least one HLR.

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 covered by those BTSs that are related to the MSCs for which the VLR provides its services.

The theft of GSM mobile telephones seems attractive since the identities of subscribers and their mobile equipment are separate. Stolen equipment can be reused simply by using any valid SIM. Barring of a subscriber by the operator does not bar the mobile equipment. To prevent that kind of misuse, every GSM terminal equipment contains a unique identifier, the international mobile equipment identity (IMEI). It lies within the realm of responsibilities of a network operator to equip the PLNM with an additional database, the EIR, in which stolen equipment is registered and so can be used to bar fraudulent calls and even, theoretically, to track down a thief (by analyzing the related SIM data).