The Network

Global System for Mobile Communications (GSM) does not provide requirements but only recommendations. The GSM specifications define the functions and interface requirements in detail, but do not address the hardware. The reason for this is to give minimal limitation to the designers and make it possible for users to buy equipment from different suppliers.

The GSM network is divided into three major systems: the Switching System (SS), the Base Station System (BSS), and the Operation and Support System (OSS).

The Switching System (SS) is responsible for performing call processing and subscriber-related functions. The switching system includes the following functional units:

· Home Location Register (HLR)-a database used for storage and management of subscriptions. The HLR stores permanent data about subscribers and is considered the most important database. The user is registered in the HLR at the time they buy a subscription from one of the Personal Communications Service (PCS) operators.

· Mobile Services Switching Center (MSC)-The MSC performs the telephony switching functions of the system. It controls calls to and from other telephone and data systems. It also performs such functions as toll ticketing, network interfacing, common channel signaling and others.

· Visitor Location Register (VLR)-The VLR is a database that contains temporary information about subscribers that is needed by the Mobile Services Switching Center (MSC) in order to service visiting subscribers. The VLR is always integrated with the MSC. When a mobile station roams into a new MSC area, the VLR connected to that MSC will request data about the mobile station from the HLR. Later, if the mobile station makes a call, the VLR will have the information needed for call setup without having to question the HLR each time.

· Authentication Center (AUC)-A unit called the AUC provides authentication and encryption parameters that verify the user's identity and ensure the confidentiality of each call. The AUC protects network operators from different types of fraud found in today's wireless world. · Equipment Identity Register (EIR)-The EIR is a database that contains information about the identity of mobile equipment that prevents calls from stolen, unauthorized, or defective mobile stations. The Authentication Center (AUC) and EIR are implemented as stand-alone nodes or as a combined AUC/EIR node. The Base Station System (BSS) is where all radio-related functions are performed. The BSS consists of Base Station Controllers (BSCs) and the Base Transceiver Stations (BTSs).

· Base Station Controller (BSC) provides all the control functions and physical links between the Mobile Services Switching Center (MSC) and Base Transceiver Station (BTS). It is a high-capacity switch that provides functions such as handover, cell configuration data and control of radio frequency (RF) power levels in base transceiver stations. A number of BSCs are served by an MSC.

· Base Transceiver Station (BTS) handles the radio interface to the mobile station. The BTS is the radio equipment (transceivers and antennas) needed to service each cell in the network. A group of BTSs are controlled by a BSC.

The Operation and Support System (OSS). The Operations and Maintenance Center (OMC) is connected to all equipment in the switching system and to the Base Station Controller (BSC). The implementation of OMC is called the operation and support system (OSS). The OSS is the functional entity from which the network operator monitors and controls the system. The purpose of OSS is to offer the customer cost-effective support for centralized, regional and local operational and maintenance activities that are required for a Global System Mobile Communication (GSM) network. An important function of OSS is to provide a network overview and support the maintenance activities of different operation and maintenance organizations.

Additional functional elements include: Message Center (MXE), Mobile Service Node (MSN), Gateway Mobile Services Switching Center (GSMC) and GSM Interworking Unit (GIWU).