Identification of GSM Entities

Since GSM systems are deployed in many countries, CCITT and ETSI have standardized the identification of GSM entities according to a numbering plan specified in CCITT Rec. E.212 [20]. This plan is different from the CCITT Rec. 163/164 numbering plan for fixed networks.

PLMN Identity. A PLMN is uniquely identified by its mobile country code (MCC) and mobile network code (MNC). MCC consists of three digits, of which the first one indicates a world zone:

2 Europe

3 North America

4 The Mid-East and Western Asia

5 Eastern Asia and Australia

6 Africa

7 Latin America

The second and third digits represent individual nations in these zones. For example, the MCCs of the U.K. and Malaysia are 234 and 502.

The MNC code identifies a PLMN in a country. It consists of two digits, and is allocated by individual national organizations for mobile telecommunication standards.

Location Area Identity (LAI). This uniquely identifies a GSM location area. The service area of a PLMN is divided into a number of MSC service areas, and each of these is subdivided into location areas that consist of a number of adjacent cells. A MSC keeps track of the location areas of mobiles currently registered in its service area. When a MS has to be paged, paging messages are sent out in all cells of the mobile's present location area. The LAI format is:


where the location area code (LAC) identifies a location area within a PLMN. The code consists of up to four hexadecimal digits. LACs are allocated by the operators of individual PLMNs.

Mobile Station Identity. We begin by pointing out a significant difference between U.S. mobiles and GSM mobiles. A GSM mobile consists of two parts: the mobile equipment (ME) and the subscriber identity module (SIM). The ME is an "almost complete" mobile station, containing the RF equipment, keypad, mouthpiece, earphone, etc. The SIM is a small package (smart card) with semiconductor chips that store permanent and temporary information about the ME user. A user can insert his SIM into—and extract it from—any ME. The ME is operable only when a SIM has been inserted.

Splitting a MS into two parts allows a ME to be used—at different times—by different people. For example, a business may have a number of employees, each of whom has a SIM. The business also owns a smaller number of MEs which are used by the employees on a "when needed" basis. Mobile station identities can be in two forms:

International Mobile Station Identity (IMSI). This uniquely identifies a mobile station in any GSM network. Its format is:


where MCC-MNC identifies the PLMN selected by the MS owner for mobile services, and MSIN (mobile station identity number) identifies a MS in that PLMN. The maximum length of MSIN is nine BCD digits.

An IMSI is allocated by the operator of the selected PLMN, and entered into permanent memory of the SIM.

The second MS identity format is a combination of LAI and TMSI.

Temporary Mobile Station Identity (TMSI). This is a 32-bit binary number which uniquely identifies the MS within one location area, or a group of adjacent location areas, of a PLMN. TMSI is a temporary identification, and is usually changed by the network when the MS enters a new location area. LAI and TMSI are stored in temporary SIM memory.

Most messages on the Um (radio) interface identify a MS by TMSI and LAI. IMSIs are used only in exceptional cases. TMSI gives protection against cloning (obtaining MS identifications for fraudulent use), because an intercepted TMSI no longer identifies the mobile after it has left the location area.

International Mobile Equipment Identity (IMEI). This is the counterpart of the mobile serial number (MSN) in AMPS. It uniquely identifies a ME. IMEI is a 15-digit number, entered into permanent ME memory by the manufacturer.

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