GSM NETWORK COMPONENTS

The GSM network is divided into two systems. Each  of these systems are comprised of a number of functional units which are individual components of the mobile network. The two systems

are: 

• Switching System (SS)

• Base Station System (BSS)

In addition, as with all telecommunications networks, GSM networks are operated, maintained and managed from computerized centers.

SWITCHING SYSTEM (SS) COMPONENTS

Mobile services Switching Center (MSC)

The MSC performs the telephony switching functions for the mobile network. It controls calls to and from other telephony and data systems, such as the Public Switched Telephone Network (PSTN), Integrated Services Digital Network (ISDN), public data networks, private networks and other mobile networks.

Gateway Functionality

Gateway functionality enables an MSC to interrogate a network's HLR in order to route a call to a Mobile Station (MS). Such an MSC is called a Gateway MSC (GMSC). For example, if a person connected to the PSTN wants to make a call to a GSM mobile subscriber, then the PSTN exchange will access the GSM network by first connecting the call to a GMSC. The same is true of a call from an MS to another MS. Any MSC in the mobile network can function as a gateway by integration of the appropriate software.

Home Location Register (HLR)

The HLR is a centralized network database that stores and manages all mobile subscriptions belonging to a specific operator. It acts as a permanent store for a person's subscription information until that subscription is canceled. The information stored includes:

• Subscriber identity

• Subscriber supplementary services

• Subscriber location information

• Subscriber authentication information

The HLR can be implemented in the same network node as the MSC or as a stand-alone database. If the capacity of a HLR is exceeded by the number of subscribers, additional HLRs may be added.

Visitor Location Register (VLR)

The VLR database contains information about all the mobile subscribers currently located in an MSC service area. Thus, there is one VLR for each MSC in a network. The VLR temporarily stores subscription information so that the MSC can service all the subscribers currently visiting that MSC service area. The VLR can be regarded as a distributed HLR as it holds a copy of the HLR information stored about the subscriber. When a subscriber roams into a new MSC service area, the VLR connected to that MSC requests information about the subscriber from the subscriber's HLR. The HLR sends a copy of the information to the VLR and updates its own location information. When the subscriber makes a call, the VLR will already have the information required for call set-up.

AUthentication Center (AUC) 

The main function of the AUC is to authenticate the subscribers attempting to use a network. In this way, it is used to protect network operators against fraud. The AUC is a database connected to the HLR which provides it with the authentication parameters and ciphering keys used to ensure network security.

Equipment Identity Register (EIR)

The EIR is a database containing mobile equipment identity information which helps to block calls from stolen, unauthorized, or defective MSs. It should be noted that due to subscriber-equipment separation in GSM, the barring of MS

equipment does not result in automatic barring of a subscriber.

BASE STATION SYSTEM (BSS) COMPONENTS

Base Station Controller (BSC)

The BSC manages all the radio-related functions of a GSM network. It is a high capacity switch that provides functions such as MS handover, radio channel assignment and the collection of cell configuration data. A number of BSCs may be controlled by each MSC. 

Base Transceiver Station (BTS)

The BTS controls the radio interface to the MS. The BTS comprises the radio equipment such as transceivers and antennas which are needed to serve each cell in the network. A group of BTSs are controlled by a BSC.

NETWORK MONITORING CENTERS

Operation and Maintenance Center (OMC)

An OMC is a computerized monitoring center which is connected to other network components such as MSCs and BSCs via X.25 data network links. In the OMC, staff are presented with information about the status of the network and can monitor and control a variety of system parameters. There may be one or several OMCs within a network depending on the network size.

Network Management Center (NMC)

Centralized control of a network is done at a Network Management Center (NMC). Only one NMC is required for a network and this controls the subordinate OMCs. The advantage of this hierarchical approach is that staff at the NMC can concentrate on long term system-wide issues, whereas local

personnel at each OMC can concentrate on short term, regional issues. OMC and NMC functionality can be combined in the same physical network node or implemented at different locations.

MOBILE STATION (MS)

An MS is used by a mobile subscriber to communicate with the mobile network. Several types of MSs exist, each allowing the subscriber to make and receive calls. Manufacturers of MSs offer a variety of designs and features to meet the needs of different markets. The range or coverage area of an MS depends on the output power of the MS. Different types of MSs have different output

power capabilities and consequently different ranges. For example, hand-held MSs have a lower output power and shorter range than car-installed MSs with a roof mounted antenna.

GSM MSs consist of:

• A mobile terminal

• A Subscriber Identity Module (SIM)

Unlike other standards, in GSM the subscriber is separated from the mobile terminal. Each subscriber's information is stored as a "smart card" SIM. The SIM can be plugged into any GSM mobile terminal. This brings the advantages of security and portability for subscribers. For example, subscriber A's mobile

terminal may have been stolen. However, subscriber A's own SIM can be used in another person's mobile terminal and the calls will be charged to subscriber A.