GSM Technology – Addresses and Identifiers



GSM distinguishes explicitly between user and equipment and deals with them separately. Besides phone numbers and subscriber and equipment identifiers, several other identifiers have been defined; they are needed for the management of subscriber mobility and for addressing of all the remaining network elements. The most important addresses and identifiers are presented in the following:

International Mobile Station Equipment Identity (IMEI):

The international mobile station equipment identity (IMEI) uniquely identifies a mobile station internationally. It is a kind of serial number. The IMEI is allocated by the equipment manufacturer and registered by the network operator and registered by the network operator who stores it in the EIR. By means of IMEI one recognizes obsolete, stolen or nonfunctional equipment.

There are following parts of an IMEI:

  • Type Approval Code (TAC): 6 decimal places, centrally assigned.
  • Final Assembly Code (FAC): 6 decimal places, assigned by the manufacturer.
  • Serial Number (SNR): 6 decimal places, assigned by the manufacturer.
  • Spare (SP): 1 decimal place.

Thus, IMEI = TAC + FAC + SNR + SP. It uniquely characterizes a mobile station and gives clues about the manufacturer and the date of manufacturing.

International Mobile Subscriber Identity ( IMSI):

Each registered user is uniquely identified by its international mobile subscriber identity (IMSI). It is stored in the subscriber identity module (SIM) A mobile station can only be operated if a SIM with a valid IMSI is inserted into equipment with a valid IMEI.

There are following parts of an IMSI:

  • Mobile Country Code (MCC): 3 decimal places, internationally standardized.
  • Mobile Network Code (MNC): 2 decimal places, for unique identification of mobile network within the country.
  • Mobile Subscriber Identification Number (MSIN): Maximum 10 decimal places, identification number of the subscriber in the home mobile network.

Mobile Subscriber ISDN Number ( MSISDN):

The real telephone number of a mobile station is the mobile subscriber ISDN number (MSISDN). It is assigned to the subscriber (his or her SIM, respectively), such that a mobile station set can have several MSISDNs depending on the SIM.

The MSISDN categories follow the international ISDN number plan and therefore have the following structure.

  • Country Code (CC) : Up to 3 decimal places.
  • National Destination Code (NDC): Typically 2-3 decimal places.
  • Subscriber Number (SN): Maximum 10 decimal places.

Mobile Station Roaming Number ( MSRN):

The Mobile Station Roaming Number ( MSRN) is a temporary location dependent ISDN number. It is assigned by the locally responsible VLR to each mobile station in its area. Calls are also routed to the MS by using the MSRN.

The MSRN has same structure as the MSISDN.

  • Country Code (CC) : of the visited network.
  • National Destination Code (NDC): of the visited network.
  • Subscriber Number (SN): in the current mobile network.

Location Area Identity (LAI):

Each LA of an PLMN has its own identifier. The Location Area Identifier (LAI) is also structured hierarchically and internationally unique as follows:

  • Country Code (CC) : 3 decimal places.
  • Mobile Network Code (MNC): 2 decimal places.
  • Location Area Code (LAC): maximum 5 decimal places or, maximum twice 8 bits coded in hexadecimal (LAC < FFFF).

Temporary Mobile Subscriber Identity (TMSI):

The VLR, which is responsible for the current location of a subscriber, can assign a temporary mobile subscriber identity (TMSI) which has only local significance in the area handled by the VLR. It is stored on the network side only in the VLR and is not passed to the HLR.

Together with the current location area, TMSI allows a subscriber to be identified uniquely and it can consist of upto 4×8 bits.

Local Mobile Subscriber Identity (LMSI):

The VLR can assign an additional searching key to each mobile station within its area to accelerate database access. This unique key is called the Local Mobile Subscriber Identity (LMSI). The LMSI is assigned when the mobile station registers with the VLR and is also sent to the HLR.

An LIMSI consists of four octets ( 4 x 8 bits).

Cell Identifier (CI):

Within an LA, the individual cells are uniquely identified with a cell identifier (CI), maximum 2 x 8 bits. Together with the global cell identity (LAI + CI) calls are thus also internationally defined in a unique way.

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