Introduction to GSM

What is GSM?

The Global System for Mobile Communications (GSM) is a standard developed by the European Telecommunications Standards Institute (ETSI) which is overcome of 1st(1G) mobile communication with the use of TDMA(Time Division Multiple Access) modulation technology. It was first deployed in Finland in December 1991.The GSM standard comprised of circuit-switched network optimized for full duplex voice telephony.


  • A mobile station may be referred to as a handset or mobile or a portable terminal or a mobile equipment ME). It includes a subscriber identity module (SIM) that is normally removable and comes in two sizes. Each SIM card has a unique identification number called IMSI (ITERNATIONAL MOBILE SUBSCRIBER IDENTITY). In addition, each MS is assigned a unique hardware identification called IMEI (ITERNATIONAL MOBILE EQUIPEMENT IDENTITY). Mobile transmits and receives voice at 13 kb/s over the air interface


  • MSISDN (Mobile Station ISDN Number) is the identity denotes the MS or customer own the MS.A MSISDN number is a format of CC+NDC+SN where CC stands for Country Code example 91 for India, NDC stands for Network Destination Code which is different and unique for each operator and SN denotes last 5 digit of the number printed in SIM


  • IMSI(ITERNATIONAL MOBILE SUBSCRIBER IDENTITY) is a 15 digit format of MCC+MNC+MSIN , where MCC stands for Mobile Country Code(2 digit), MNC stands for Mobile Network Code(3 digit) and MSIN stands for Mobile Subscriber Identity Number(last 10 digit the number printed in SIM)


  • IMEI (ITERNATIONAL MOBILE EQUIPEMENT IDENTITY) is a 15 digit format of TAC+FAC+SN+SP, where TAC stands for Tracking Area Code(6 digit) , FAC stands for Factory Assembly Code, SN stands for Serial number of SIM and SP stands for Spare Digit used to specify software version of SIM manufacturer.


  • A complete GSM architecture mainly contents three different structure.
  • RAN (Radio Access network)
  • CORE Network
  • Packet Switch(PS) network


  • RAN network includes BTS, BSC nodes
  • CORE network includes MSC/GMSC, VLR, HLR
  • Packet Switch network includes SGSN, GGSN



                GSM uses a series of radio transmitters called BTSs to connect the mobiles to a cellular network. Their tasks include channel coding/decoding and encryption/decryption. A BTS is comprised of radio transmitters and receivers, antennas, the interface to the PCM facility, etc. The BTS may contain one or more transceivers to provide the required call handling capacity. A cell site may be omni-directional or split into typically three directional cells.



A group of BTSs are connected to a particular BSC which manages the radio resources for them. The primary function of the BSC is call maintenance. BTS is connected to the BSC through Abis Interface which is 2Mbps and transmits and receives voice at 13kbps over air interface to the mobiles. The mobile stations normally send a report of their received signal strength to the BSC every 480 ms. With this information the BSC decides to initiate handovers to other cells, change the BTS transmitter power, etc.



  It is standard exchange like fixed network and additionally provides all the functionality needed to handle a mobile subscriber. The main functions are registration,authentication, location updating, handovers and call routing to a roaming subscriber. The signaling between functional entities(registers) in the network subsystem uses Signaling System 7 (SS7).If the MSC also has a gateway function for communicating with other network operator which is called POI(Point of Interconnect), it is called Gateway MSC (GMSC).



 A database used for management of mobile subscribers. It stores the international mobile subscriber identity(IMSI), mobile station ISDN number (MSISDN) and current visitor location register (VLR) address. The main information stored there concerns the location of each mobile station in order to be able to route calls to the mobile subscribers managed by each HLR. The HLR also maintains the services associated with each MS. One HLR can serve several MSCs.



Contains the current location of the MS and selected administrative information from the HLR, necessary for call control and provision of the subscribed services, for each mobile currently located in the geographical area controlled by the VLR. A VLR is connected to one MSC and is normally integrated into the MSC’s hardware.



A protected database that holds a copy of the secret key stored in each subscriber’s SIM card, which is used for authentication and encryption over the radio channel. The AuC provides additional security against fraud. It is normally located close to each HLR within a GSM network.



 The EIR is a database that contains a list of all valid mobile station equipment within the network,where each mobile station is identified by its international mobile equipment identity (IMEI). The EIR has three databases:

  •  White list: for all known, good IMEIs
  • Black list: for bad or stolen handsets
  •  Grey list: for handsets/IMEIs that are uncertain



 A Short Message Service Center is a network element in the mobile telephone network. Its purpose is to store, forward, convert and delivers Short Message Service messages. SMSC number is stored in MS or handset according to which SMS is routed or deliverer via that SMSC node.


SMS service can be directed in 3 ways:

  • From mobile to another mobile – referred to as MO-MT (Mobile Originated – Mobile Terminated)
  • From mobile to a content provider (also known as Large Account / ESME) – referred to as MO-AT (Mobile Originated – Application Terminated)
  • From application to a mobile – referred to as AO-MT (Application Originated – Mobile Terminated)




 The Serving GPRS Support Node (SGSN) is the node that used to serve the MS or UE which is responsible for provide DATA service in the form of GPRS.  SGSN supports GPRS and/or UMTS The SGSN keeps track of the location of an individual and performs security functions and access control.

The SGSN is connected to the RAN base station system through the Gb or Iu interface and/or to the UTRAN through the Iu interface. A SGSN is responsible for the delivery of data packets from and to the mobile stations within its geographical service area. Its tasks include packet routing and transfer, mobility management (attach/detach and location management), logical link management, and authentication and charging functions. The location register of the SGSN stores location information (e.g., current cell, current VLR) and user profiles (e.g., IMSI, address(es) used in the packet data network) of all GPRS users registered with it. SGSN supports GPRS tunneling protocol to communicate with MSC and GGSN



 (GGSN) is a main component of the GPRS network. The GGSN is responsible for the internetworking between the GPRS network and external packet switched networks. GGSN receives data from SGSN in the form of packet data request, called PDU(Protocol data units) which contains subscriber identity details (IMSI/IMEI) , depend on which GGSN check in own database about subscriber profile. PDP addresses of incoming data packets are converted to the GSM address of the destination user. The readdressed packets are sent to the responsible SGSN. For this purpose, the GGSN stores the current SGSN address of the user and his or her profile in its location register. The GGSN is responsible for IP address assignment and is the default router for the connected user equipment (UE). The GGSN also performs authentication and charging function. The GGSN also performs authentication and charging functions.

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