Overviewman using phone

Global System for Mobile Communication (GSM) is a set of ETSI standards for a digital cellular service infrastructure that operates between frequencies 900 MHz and 1800 MHz. It is a circuit-switched technology dividing each 200 KHz channel into eight 25 KHz time-slots. GSM offers international roaming capability for accessing mobile phones globally and satellite roaming capability for accessing in areas where terrestrial coverage is not available.


specifications table

Frequency Bands

Frequency Bands

architectire diagram

A Mobile Station (MS) has Mobile Equipment (ME) and Subscriber Identity Module (SIM).

ME is a portable, handheld device. An internationally unique serial number or International Mobile Equipment Identity (IMEI) stored in the SIM card characterizes the mobile. The subscriber-specific call number and certain cryptographic algorithms for authentication and encryption of user data are incorporated into the SIM card. Just by changing the SIM card, the users can change operators while retaining the same handset.

A Base Station Subsystem (BSS) allows operation between components of different suppliers. It has Base Transceiving Station (BTS) and Base Station Controller (BSC).

BTS acts as an interface between the network provider and mobile phone by transmitting and receiving resources.

BSC manages Radio resources for BTS and communicates with Mobile Switching Centre (MSC) and BTS.

MSC controls the base station through switching nodes.

A Home Location Register (HLR) is a database of semi-permanent mobile subscriber information for a carriers’ entire subscriber base. The database contains information of International Mobile Subscriber Identity (IMSI), service subscription, location, service restrictions and supplementary services.

A Visitor Location Register (VLR) is a database that has temporary information related to the mobile subscribers currently located in a given MSC serving area.

An Authentication Centre (AUC) contains algorithms and subscriber-related keys to authenticate a mobile subscriber.

An Equipment Identity Register (EIR) contains the details of all the mobile transceivers permitted on the network, in the form of white, grey and black lists. The white list has all reliable functioning mobile phones, the grey list has all defective mobile phones, and the black list has all fault or stolen mobile phones.

EiR diagram

GSM uses Time Division Multiple Access (TDMA) technique that allows several different calls to share the same carrier with each call assigned with a particular time slot. When a user switches on his mobile phone, it gets registered with the network provider through the nearest base station.

GSM flow chart

Outgoing Call Process

MS sends called number to BSS

BSS sends called number to MSC

MSC checks VLR and asks BSS to allocate resources for the call

MSC routes the call to GMSC

GMSC routes the call to local exchange of the called user

Ring back tone is routed from called user to MS via GMSC, MSC, BSS

Incoming Call Process

Calling a GSM subscriber

Forwarding call to GSMC

Signal Setup to HLR

Request MSRN from VLR

Forward responsible MSC to GMSC

Forward Call to current MSC

Get current status of MS

Paging of MS

MS answers

Security checks

Set up connection


SIM card has four to eight digits Personal Identification Number to avoid unauthorized access. Authentication is ensured through AUC.

Data transmission can be encrypted on the radio link between the mobile phone and the base station. No other transmission paths use encryption.

Enhancements to GSM

High Speed Circuit Switched Data (HSCSD) is an enhanced version of the GSM and it uses multiple channels simultaneously for data transmission. The maximum number of timeslots used is 4. HSCSD is expensive and does not transmit data in packets.

General Packet Radio Service (GPRS) enhances GSM phones for rapid data transfer in video calling, video download, web browsing, email, etc. It is a packet-oriented service and uses IP address for every mobile data device. GPRS ensures permanent online connectivity.

Enhanced Data rates for GSM Evolution (EDGE) or EGPRS offers faster transfer rates than GPRS or HSCSD. It uses up to 8 timeslots.

GSM table

Enhancements to GSM

Dual Tone Multi Frequency (DTMF) is used for various control purposes.
Facsimile Group III is used to connect fax machines to a telephone using analog signals
Short Message Service allows sending and receiving messages of max160 alphanumeric characters.
Cell Broadcast allows broadcasting a message of a maximum of 93 characters.
Voice Mail works like an answering machine controlled by the subscriber.
Fax Mail allows the subscriber to receive fax messages at any fax machine.
Call Forwarding allows the subscriber to forward incoming calls to another number.
Barring of Outgoing Calls allows the subscriber to prevent all outgoing calls.
Barring of Incoming Calls allows the subscriber to prevent all incoming calls.
Advice of Charge (AoC) allows the subscriber to estimate the call charges.
Call Hold allows the subscriber to interrupt an ongoing call and then subsequently reestablish the call.
Call Waiting allows the subscriber to be notified of an incoming call during a conversation.
Multiparty Service allows the subscriber to establish a multiparty conversation.
Calling Line Identification supplies or restricts the called party with the integrated services digital network (ISDN) number of the calling party.
Closed User Groups (CUGs) similar to PBX allows a group of subscribers to only call themselves or certain numbers.


For Users

Superior sound quality International roaming capability Reduced congestion Longer battery life Data support (SMS, Web browsing, email) Flexibility of spectrum Better security against fraud Compatibility with ISDN Encryption capability for information security and privacy Instant transfer of SIM cards Wide variety of product selection

For Operators

Highly economical due to low cost infrastructure Increased capacity Highly scalable Spectral flexibility and efficiency Device selection Interoperability standard Flexible, cost-effective migration to 3G