Introduction and history:-

                                                    GSM stands for Global System for Mobile Communication. It is a digital cellular technology used for transmitting mobile voice and data services. Important facts about the GSM are given below −
The concept of GSM emerged from a cell-based mobile radio system at Bell Laboratories in the early 1970s.
GSM is the name of a standardization group established in 1982 to create a common European mobile telephone standard.
GSM is the most widely accepted standard in telecommunications and it is implemented globally.
GSM owns a market share of more than 70 percent of the world's digital cellular subscribers.
GSM makes use of narrowband Time Division Multiple Access (TDMA) technique for transmitting signals.
GSM was developed using digital technology. It has an ability to carry 64 kbps to 120 Mbps of data rates.
Presently GSM supports more than one billion mobile subscribers in more than 210 countries throughout the world.
GSM provides basic to advanced voice and data services including roaming service. Roaming is the ability to use your GSM phone number in another GSM network.

GSM System Architecture

A GSM network comprises of many functional units. The GSM network can be broadly divided into −
The Mobile Station (MS)
The Base Station Subsystem (BSS)
The Network Switching Subsystem (NSS)
The Operation Support Subsystem (OSS)

The Mobile Station:

The MS consists of the physical equipment, such as the radio transceiver, display and digital signal processors, and the SIM card. It provides the air interface to the user in GSM networks. As such, other services are also provided, which include −
Voice tele-services
Data bearer services
The features' supplementary services

Base Station Subsystem (BSS):

The BSS is composed of two parts −
The Base Transceiver Station (BTS)
The Base Station Controller (BSC)
The BTS and the BSC communicate across the specified interface, enabling operations between components that are made by different suppliers. The radio components of a BSS may consist of four to seven or nine cells. A BSS may have one or more base stations. The BSS uses the interface between the BTS and the BSC. A separate high-speed line is then connected from the BSS to the Mobile MSC.

Network Switching Subsystem (NSS):

The Network switching system (NSS), the main part of which is the Mobile Switching Center (MSC), performs the switching of calls between the mobile and other fixed or mobile network users, as well as the management of mobile services such as authentication.

Home Location Register (HLR):

The HLR is a database used for storage and management of subscriptions. The HLR is considered the most important database, as it stores permanent data about subscribers, including a subscriber's service profile, location information, and activity status. When an individual buys a subscription in the form of SIM, then all the information about this subscription is registered in the HLR of that operator.

Mobile Services Switching Center (MSC):

The central component of the Network Subsystem is the MSC. The MSC performs the switching of calls between the mobile and other fixed or mobile network users, as well as the management of mobile services such as registration, authentication, location updating, handovers, and call routing to a roaming subscriber. It also performs such functions as toll ticketing, network interfacing, common channel signaling, and others. Every MSC is identified by a unique ID.

Equipment Identity Register (EIR):

The Equipment Identity Register (EIR) is a database that contains a list of all valid mobile equipment on the network, where its International Mobile Equipment Identity (IMEI) identifies each MS. An IMEI is marked as invalid if it has been reported stolen or is not type approved.

Operation Support Subsystem (OSS):

The operations and maintenance center (OMC) is connected to all equipment in the switching system and to the BSC. The implementation of OMC is called the operation and support system (OSS). Here are some of the OMC functions−
Administration and commercial operation (subscription, end terminals, charging, and statistics).
Security Management.
Network configuration, Operation, and Performance Management.
Maintenance Tasks.


Features:

Improved spectrum efficiency
International roaming
Low-cost mobile sets and base stations (BSs)
High-quality speech
Compatibility with Integrated Services Digital Network (ISDN) and other telephone company services
Support for new services