GSM ARCHITECTURE:

• The GSM system architecture consists of three major interconnected subsystems that interact between themselves and with the users through certain network interfaces.
• The subsystems are the Base Station Subsystem (BSS), Network and Switching Subsystem (NSS), and the Operation Suppon Subsystem (OSS).
• The Mobile Station (MS) is also a subsystem, but is usually considered to be part of the BSS for architecture purposes.
• Equipment and services are designed within GSM to support one or more of these specific subsystems.
• The BSS, also known as the radio subsystem, provides and manages radio transmission paths between the mobile stations and the Mobile Switching Center (MSC).
• The BSS also manages the radio interface between the mobile stations and all other subsystems of GSM.
• Each BSS consists ofmany Base Station Controllers (BSCs) which connect the MS to the NSS via the MSCs. The NSS manages the switrhing functions of the system and allows the MSCs to communicate with other networks such as the PSTN and ISDN.
• The OSS supports the operation and maintenance of GSM and allows system engineers to monitor, diagnose, and troubleshoot all aspects of the GSM system.
• This subsystem interacts with the other GSM subsystems, and is provided solely for the staff of the GSM operating company which provides service facilities for the network.

COMPONENTS OF THE GSM ARCHITECTURE:

• The Mobile Stations (MS) communicate with the Base Station Subsystem (BSS) over the radio air interface.
• The BSS consists of many BSCs which connect to a single MSC, and each BSC typically controls up to several hundred Base Transceiver Stations (BTSs).
• Some of the BTSs maybe co-located at the BSC, and others may be remotely distributed and physically connected to the BSC by microwave link or dedicated leased lines.
• Mobile handoffs (called handovers, or HO, in the GSM specification) between two BTSs under the control of the same BSC are handled by the BSC, and not the MSC.
• This greatly reduces the switching burden of the MSC.
• The interface which connects a BTS to a BSC is called the Abis interface.
• The Abis interface carries traffic and maintenance data, and is specified by GSM to be. standardized for all manufacturers.
• In practice, however, the Abis for each GSM base station manufacturer has subtle differences, thereby forcing service providers to use the same manufacturer for theBTS and BSC equipment.

THE BASE STATION COMMUNICATION SYSTEM:

• The BSCs are physically connected via dedicatedlleased lines or microwave link to the MSC.
• The interface between a BSC and a MSC is called theA interface, which is standardized within GSM. The A interface uses an SS7 protocol called the Signaling
• Correction Control Part (SCCP) which supports communication between the MSC and the ESS, as well as network messages between the individual subscribers and the MSC.
• The A interface allows a service provider to use base stations and switching equipment made by different manufacturers.

THE NETWORK SWITCHING CENTRE:

• The NSS handles the switching of GSM calls between external networks and the BSCs in the radio subsystem and is also responsible for managing and providing external access to several customer databases.
• The MSC is the central unit in the NSS and controls the traffic among all of the BSCs.
• In the NSS, there are three different databases called the Home Location Register (HLR), Visitor Location Register (VLR), and the Authentication Center (AUC).
• The HLR is a database which contains subscriber information and location information for each user who resides in the same city as the MSC.
• Each subscriber in a particular GSM market is assigned a unique International Mobile Subscrther Identity (IMSI), and this number is used to identify each home user.
• The VLR is a database which temporarily stores the IMSI and customer information for each roaming subscriber who is visiting the coverage area of a particular MSC.
• The VLR is linked between several adjoining MSCs in a particular market or geographic region and contains subscription information of every visiting user in the area.
• Once a roaming mobile is logged in the VLR, the MSC sends the necessary information to the visiting subscriber's FILE so that calls to the roaming mobile can be appropriately routed over the PSTN by the roaming user's HUt

THE AUTHENTICATION CENTRE:

• Authentication Center is a strongly protected database which handles the authentication and encryption keys for every single subscriber in the HLR and VLR.
• The Authentication Center contains a register called the Equipment Identity Register (EIR) which indentifies stolen or fraudulently altered phones that transmit identity data that does not match with information contained in either the HLR or VLR.
• The OSS supports one or several Operation Maintenance Centers (OMC) which are used to monitor and maintain the performance of each MS, BS, BSC, and MSC within a GSM system.
• The OSS has three main functions, which are to maintain all telecommunications hardware and network operations with a particular market, manage all charging and billing procedures, and manage all mobile equipment in the system.
• Within each GSM system, an OMC is dedicated to each of these tasks and has provisions for adjusting all base station parameters and billing procedures, as well as for providing system operators with the ability to determine the performance and integrity of each piece of subscriber equipment in the system.