GSM History

Global System for Mobile Communications

Before the Global System for Mobile Communications (GSM) was developed, the countries of Europe used a number of incompatible first-generation cellular phone technologies. GSM was developed to provide a common second-generation technology for Europe so that the same subscriber units could be used throughout the continent. The technology has been extremely successful and is probably the most popular standard, worldwide, for new implementations. GSM first appeared in 1990 in Europe. Similar systems have now been implemented in North and South America, Asia, North Africa, the Middle East, and Australia. The GSM Association claimed over a billion subscribers worldwide by early 2004, the bulk of these in Europe and Asia Pacific, but with growing market share in North and South America.

GSM Network Architecture

Figure 10.13 shows the key functional elements in the GSM system. The boundaries at Um, Abia, and A refer to interfaces between functional elements that are standardized in the GSM documents. Thus, it is possible to buy equipment from different vendors with the expectation that they will successfully interoperate. Additional interfaces are also defined in the GSM standards, but need not concern us here.

Mobile Station

 A mobile station communicates across the Um interface, also known as the air interface, with a base station transceiver in the same cell in which the mobile unit is located. The mobile equipment (ME) refers to the physical terminal, such as a telephone or PCS (personal communications service) device, which includes the radio transceiver, digital signal processors, and the subscriber identity module (SIM). The SIM is a portable device in the form of a smart card or plug-in module that stores the subscriber's identification number, the networks the subscriber is authorized to use, encryption keys, and other information specific to the subscriber. The GSM subscriber units are totally generic until an SIM is inserted. Therefore, a subscriber need only carry his or her SIM to use a wide variety of subscriber devices in many countries simply by inserting the SIM in the device to be used. In fact, except for certain emergency communications, the subscriber units will not work without a SIM inserted. Thus, the SIMs roam, not necessarily the subscriber devices.

Base Station Sub system:

A base station subsystem (BSS) consists of a base station controller and one or more base transceiver stations. Each base transceiver station (BTS) defines a single cell; it includes a radio antenna, a radio transceiver, and a link to a base station controller. A GSM cell can have a radius of between 100 m and 35 km, depending on the environment. A base station controller (BSC) may be collocated with a BTS or may control multiple BTS units and hence multiple cells. The BSC reserves radio frequencies, manages the handoff of a mobile unit from one cell to another within the BSS, and controls paging.

Network Subsystem:

The network subsystem (NS) provides the link between the cellular network and the public switched telecommunications networks. The NS controls handoffs between cells in different BSSs, authenticates users and validates their accounts, and includes functions for enabling worldwide roaming of mobile users. The central element of the NS is the mobile switching center (MSC). It is supported by four databases that it controls:

·         Home location register (HLR) database: 

            The HLR stores information, both permanent and temporary, about each of the subscribers that "belongs" to it (i.e., for which the subscriber has its telephone number associated with the switching center).

·         Visitor location register (VLR) database: 

           One important, temporary piece of information is the location of the subscriber. The location is determined by the VLR into which the subscriber is entered. The visitor location register maintains information about subscribers that are currently physically in the region covered by the switching center. It records whether or not the subscriber is active and other parameters associated with the subscriber. For a call coming to the subscriber, the system uses the telephone number associated with the subscriber to identify the home switching center of the subscriber. This switching center can find in its HLR the switching center in which the subscriber is currently physically located. For a call coming from the subscriber, the VLR is used to initiate the call. Even if the subscriber is in the area covered by its home switching center, it is also represented in the switching center's VLR, for consistency.

·         Authentication center database (AuC): 

            This database is used for authentication activities of the system; for example, it holds the authentication and encryption keys for all the subscribers in both the home and visitor location registers. The center controls access to user data as well as being used for authentication when a subscriber joins a network. GSM transmission is encrypted, so it is private. A stream cipher, A5, is used to encrypt the transmission from subscriber to base transceiver. However, the conversation is in the clear in the landline network. Another cipher, A3, is used for authentication.

·         Equipment identity register database (EIR):

           The EIR keeps track of the type of equipment that exists at the mobile station. It also plays a role in security (e.g., blocking calls from stolen mobile stations and preventing use of the network by stations that have not been approved).