Global System for Mobile Communication (GSM) Network

Radio connections were first used for wireless communications in the late 19th century, we can consider this as first generation communication system.

A Further very significant development was second generation mobile system (e.g. GSM). GSM stands for Global system for Mobile Communication. The GSM standard is a consistent and open standard for Mobile communication system. GSM is the second generation mobile network system which provides very good voice quality with the coverage of wide frequency range from 900 MHz to 1800 MHz band. GSM is a digital Mobile communication system.

Some examples of the first and second generation mobile systems with GSM standards are:
1.Cellular mobile system
2.Satellite system
3.Paging system
4.Wireless Local Loop (WLL)

The GSM network Architecture



Mobile Station (MS) – carried by the user. Mobile station consists of Mobile equipment (ME) and Subscriber identity module (SIM).

Base Transceiver station (BTS) - it defines a cell* and responsible for radio link protocol with the MS.

Base Station Control (BSC) - controls BTS and manage radio channels setup.
Mobile Switching center (MSC) - is the main component of Network Switching Subsystem (NSS). It works as normal switching unit for ISDN and PSTN network and it’s provide the functionality as registration, authentication, location update, handover and call routing for the subscriber in roaming.

Home Location Register (HLR) – stores the current location of the subscriber, authentication details and the services which subscriber can access.

Visitor Location Register (VLR) – contains the some information of the subscriber from HLR. When subscriber move from one MSC to other MSC, VLR connected to the new MSC request the information from corresponding HLR.

Authentication Center (AC) - this unit is for security. AC provides the data for authentication and encryption function.

Equipment Identity Register (EIR) –is a database which contains the list of all valid mobiles equipments in the network. Every mobile equipment in the network has unique IMEI (international mobile equipment identity).

Operation and Maintenance control (OMC)

The GSM network structure is divided into three parts:
1.Network Switching Subsystem (NSS)
2.Base Station Subsystem (BSS)
3.Operation and Support Subsystem (OSS)


The NSS and BSS are the most responsible part in GSM network for call establishment. The NSS consists of MSC, HLR, VLR, AC and EIR. The NSS takes care of call control functions. Calls are always connected by and through the NSS.

The BSS consists of BTS and BSC. BSS is responsible for radio path control and every call is connected through the BSS.

The Operation and Support subsystem is connected to NSS and BSS. The OSS is the operation and maintenance related part of the network and it is needed for the control of the whole GSM network. The network operator observes and maintains network quality and service offered through the OSS.

All the three subsystems in GSM network are linked by air interface, A- interface and O&M interface.


*There are five different cell sizes in a GSM network—macro, micro, pico, femto and umbrella cells.