The structure of a GSM network relies on several functional entities, which have been specified in terms of functions and interfaces. It involves three main subsystems, each containing functional units and interconnected with the others through a series of standard interfaces.

• The MS, the handheld mobile terminal;
• The base station subsystem (BSS), which controls the radio link with the MS;
• The network and switching subsystem (NSS), which manages the function of connection switching to other fixed public network or mobile network subscribers, and handles the databases required for mobility management and for the subscriber data.

As can be seen from the figure, in a PLMN, the radio access network part (BSS) is logically separated from the core network (NSS), in order to ease the standardization of the different functions.

MS

The MS is made up of the mobile equipment (ME), and a SIM. It performs the following functions:

• Radio transmission and reception;
• Source and channel coding and decoding, modulation and demodulation;
• Audio functions (amplifiers, microphone, earphone);
• Protocols to handle radio functions: power control, frequency hopping, rules for access to the radio medium;
• Protocol to handle call control and mobility; Security algorithms (encryption techniques).

As mentioned, the SIM enables the user to have access to subscribed services irrespective of a specific terminal. The insertion of the SIM card into any GSM terminal allows the user to receive calls on that terminal, to make calls from that terminal, and to use the other subscribed services. The ME is identified with an international mobile equipment identity (IMEI).

The SIM card contains, among other information, the international mobile subscriber identity (IMSI) used to identify the subscriber to the system, and a secret key for authentication. The IMEI and the IMSI are independent, thereby allowing personal mobility.

BSS

The BSS is composed of several base station controllers (BSCs) and BTS. These two elements communicate across the Abis interface. The BTS contains the radio transceivers, responsible for the radio transmissions with the MS. This includes the following functions:

• Modulation and demodulation;
• Channel coding and decoding;
• Encryption process;
• RF transmit and receive circuits (power control, frequency hopping, management of antenna diversity, discontinuous transmission).

Several types of BTS exist: the normal BTS, the micro BTS, and the pico BTS. The micro BTS is different from a normal BTS in two ways. First, the range requirements are reduced, and the close proximity requirements are more stringent. Second, the micro BTS is required to be small and affordable in order to allow external street deployment in large numbers. The pico BTS is an extension of the micro BTS concept to the indoor environments.

The RF performances of these different BTSs are slightly different. The BSC manages the radio resources for one or more BTSs. It handles the management of the radio resource, and as such it controls the following functions: allocation and release of radio channels, frequency hopping, power control algorithms, handover management, choice of the encryption algorithm, and monitoring of the radio link.

Network Subsystem

The mobile services switching center (MSC) is the central part of the network subsystem (NSS). It is responsible for the switching of calls between the mobile users (between different BSCs or toward another MSC) and between mobile and fixed network users.

It manages outgoing and incoming calls from various types of networks, such as PSTN, ISDN, and PDN. It also handles the functionality required for the registration and authentication of a user, and the mobility operations. This includes location updating, inter-MSC handovers, and call routing. The BSS communicates with the MSC across the A interface.

Associated with the MSC, two databases, the home location register (HLR) and the visitor location register (VLR), provide the call-routing and roaming capabilities. The HLR contains all the administrative information related to the registered subscribers within the GSM network. This includes the IMSI, which unequivocally identifies the subscriber within any GSM network, the MS ISDN number (MSISDN), and the list of services subscribed by the user (such as voice, data service).

The HLR also stores the current location of the MS, by means of the address of the VLR in which it is registered. The VLR temporarily keeps the administrative data of the subscribers that are currently located in a given geographical area under its control. Each functional entity may be implemented as an independent unit, but most of the time, the VLR is colocated with the MSC, so that the geographical area controlled by the MSC corresponds to that controlled by the VLR.

The MSC contains no information about particular MSs, but rather, the information is stored in the location registers. Two other registers are used for authentication and security purposes:

• The equipment identity register (EIR) is a database that contains a list of all valid ME on the network, where each MS is identified by its IMEI. An IMEI is marked as invalid if it has been reported stolen.
• The authentication center (AuC) is a protected database that contains a copy of the secret key stored in each subscriber's SIM card, for authentication and encryption over the radio channel. The AuC verifies if a legitimate subscriber has requested a service. It provides the codes for both authentication and encryption to avoid undesired violations of the system by third parties.

Two other important entities of the NSS are the operations and maintenance center (OMC) and the network management center (NMC). These entities perform the functions relative to the network management (NM), such as the configuration of the system (locally or remotely), maintenance and tests of the pieces of equipment, billing, statistics on the performance, and gathering of all information related to subscriber traffic necessary for invoicing and administration of subscribers.