What is GSM – GSM Tutorial


What is GSM?

  • Global System for Mobile Communication
  • GSM is a digital cellular network system for voice and data communication
  • Mobile is a hand held module that will communicate with GSM network as a Voice and Data communication device.

Must Know

  • BTS : Base transceiver station
  • BSC : Base station controller
  • MSC : Mobile switching center
  • HLR : Home location register
  • VLR : Visitor location Register
  • STP : Signal Transfer Point
  • MGW : Media Gateway
  • EIR : Equipment identity Register
  • SMSC : Short Message Service Center
  • USSD : Unstructured Supplementary Service Data
  • SIM : Subscriber identity module
  • ICCID : Integrated Circuit Card Identifier
  • IMSI : International Mobile Subscriber Identity
  • MSISDN : Mobile Subscriber ISDN Number
  • IMEI : International Mobile (Station) Equipment Identity
  • GPRS : General Packet Radio Service
  • GGSN : Gateway GPRS support node
  • SGSN : Serving GPRS support node
  • EDGE : Enhanced Data Rates for GSM evolution
  • HSPA : High-Speed Packet Access
  • LTE : Long Term Evolution
  • 3GPP : 3rd Generation Partnership Project
  • ETSI : European Telecommunications Standards Institute
  • APN : Access point name
  • PDP : Packet Data Protocol
  • WAP : Wireless Application Protocol
  • MMS : Multimedia Messaging Service
  • MMSC  : Multimedia Messaging Service Center

How does GSM work

  • GSM needs base transceiver station module to communicate with mobile device.
  • Mobile device can change base transceiver station based on coverage and capacity.

GSM operates in different frequency bands, 900MHz and 1800MHz allocated for GSM communication. USA and Canada use 850MHz and 1900MHz for GSM. 1st generation GSM used 400MHz and 450MHz

GSM Network


Mobile Device

  • Mobile device is wireless transceiver module that will communicate with Base Transceiver Station.
  • Voice and data communication module where voice and data can be used simultaneously.
  • Several types of mobile devices available now. Categories based on capability of 2G and 3G


Base Transceiver Station

  • BTS is a wireless transmission and receiving terminal.
  • All mobile device connected with a nearest BTS.
  • A group of BTS connected with a BSC
  • Each BTS has a identification number in the network. Based on site and capacity it can be separated to 3-6 cell. There is a antenna for each cell.


Base Station Controller

  • BSC is a GSM node that controls one or more BTS in the network. BTS can be connected using microwave or optical fiber.
  • BSC connected with MSC for voice and signal communication. For data communication its connected with SGSN
  • Mobile device handover intelligence between BTS which is called BTS handover and call setup controlled by BSC.
  • Radio network management including radio frequency controlled by BSC

Mobile Switching Center

  • MSC is the sub center of large network or center of small network (GSM Core Network)
  • MSC is related with switching, call setup, release.
  • MSC control a group of BSC
  • In large GSM network MSC connected with STP for signal routing and MGW for Voice switching.

Home Location Register

  • HLR is a central database contains mobile subscribers details information which is used for core network. Every subscriber should be identified with IMSI/MSISDN pair and uniquely associated with one HLR

Visitor Location Register

  • VLR is a subscriber database having subscribers details information. VLR response will be faster than HLR, VLR store some additional information for which HLR need to communicate with voice network or radio network.

Signal Transfer Point

  • STP is SS7 message router, route messages between GSM signal end points and other STPs. STP connected with its signal end point using signaling links only.

Media Gateway

  • A MGW is a circuit switch or IP gateway that converts voice packet/data from format A to format B required by the destination or connected network/end points. Sometimes we called MGW a softswitch.

Equipment Identity Register

  • EIR is a central database, which contains subscribers handset IMEI. If network configured for check in EIR, then for any subscriber if the IMEI is enlisted in EIR, the mobile device will not be able to use in the network.
  • EIR can be used to prevent stolen or unauthorized mobile device use

Short Message Service Center

  • SMSC is a node that deliver SMS to subscriber. Any SMS either from subscriber to subscriber or application to subscriber must be sent via a SMSC.
  • Every subscriber SIM store a service center number where the subscriber initiated SMS send.
  • SMSC communicate with HLR for subscriber location and VLR for delivering the SMS

Unstructured Supplementary Service Data

  • USSD gateway is a node that provide USSD service. USSD can be mobile initiated or network initiated.
  • User can dial a USSD code and can access or avail any service easily.


Mobile Subscriber ISDN Number

  • MSISDN is a unique number for every mobile subscriber.
  • CC = Country code
  • NDC = National Destination Code
  • NPA = Numbering Plan Area
  • SN = Subscriber Number
  • Sample   0088-014 -12345678

Subscriber Identity Module

  • SIM is a physical module provided to subscriber. MSISDN is mapped with the SIM in reference with IMSI
  • SIM is electronic circuits that store some encrypted data written by service provider.
  • 3 type of SIM available




Integrated Circuit Card Identifier

  • ICCID is unique identification number for SIM.
  • Its 20 digit

International Mobile Subscriber Identity

  • IMSI is a SIM identification number registered in HLR. MSISDN mapped with IMSI. IMSI mapped with ICCID

International Mobile Equipment Identity

  • IMEI is a universal unique number for mobile device.
  • It’s a 15 or 17 digit number where last digit is the checksum. Using the IMEI GSM network can lock the stolen device
  • To check the IMEI user need to enter *#06#

General Packet Radio Service

  • GPRS is packet based data service for mobile users in GSM.
  • GPRS charged based on volume instead of minutes in circuit switched network.
  • Based on GPRS GSM provider offers MMS, Internet, push to talk service

Gateway GPRS  Support Node

  • GGSN is the main component of the GPRS network and this is the gateway between the GPRS network and external packet data networks like Internet.
  • From network point of view GGSN is a router to a sub network. GGSN convert the data packet coming from SGSN to PDP where incoming data packet address converted to GSM address of the destination user.

Serving GPRS Support Node

  • SGSN handles all packet switched data within the network.
  • Packet routing, packet transfer, mobility management .
  • SGSN stores user location and user profiles. All data users must be registered in SGSN to get GPRS service.

Enhanced Data Rates for GSM Evolution

  • EDGE is pre-3G technology which is backward compatible extension of GSM to allow improved data transmission rates.

High-Speed Packet Access

  • HSPA is a combination of two protocol HSDPA (High speed downlink packet access) and HSUPA (High speed uplink packet access)
  • HSPA extends and improves the performance of existing 3G mobile telecommunication networks utilizing the WCDMA protocol
  • Evolved HSPA (HSPA+) is a improved 3GPP standard

3rd Generation Partnership Project

  • 3GPP is a collaboration between groups of telecommunication association who prepare globally applicable 3G mobile phone system specification based on GSM specification

European Telecommunications Standards Institute

  • ETSI is an independent non-profit standardization organization in Europe telecommunications industry

Long Term Evolution

  • LTE is incompatible with GSM 2G/3G network and it need a separate radio spectrum. It reduces significant transfer latency for IP based system.
  • LTE together with WiMax and HSPA+ know as 4G

Access point name

  • APN is the gateway between a GSM, GPRS, 3G or 4G mobile network and another computer network like Internet
  • APN identifies the data network and the data service like WAP, MMS that a mobile data user wants to communicate.

Packet Data Protocol

  • PDP is a data packet structure that contains users session information. Before using data service user need attach PDP context and activate it.

Wireless Application Protocol

  • WAP is a technical standard or specification for a set of communication protocol to standardize the way to accessing information over a mobile wireless network by mobile devices. A WAP browser is a web browser for mobile devices such as mobile phones that uses the protocol.

Multimedia Messaging Service

  • MMS is a standard way to send multimedia content messages from mobile phones over a cellular network.
  • MMS messages send as MIME message. The message is then forwarded to the MMSC.
  • MMSC send the MMS containing the URL of the content as an SMS “control message” to trigger the receiver’s WAP browser to open and receive the content from the URL.

Multimedia Messaging Service Center

  • MMSC is carriers MMS store and forward server. MMSC can modify the content based on the receivers handset capability. MMSC act as a relay to send the MMS to the receiver on a different carrier from the sender. MMSC will forward the MMS to the receivers MMSC using internet.


Please go through the below video for brief description