Following is the list of various identifiers used in GSM.
1. IMEI - International Mobile Equipment Identity
2. IMSI - International Mobile Subscriber Number
3. TMSI - Temporary Mobile Subscriber Identity
4. LMSI - Local Mobile Subscriber Identity
5. MSISDN - Mobile Subscriber International ISDN number
6. MSRN - Mobile Subscriber Roaming Number
7. LAI - Location Area Identity
8. CI - Cell Identifier
9. BSIC - Base Station Identity Code
IMEI - International Mobile Equipment Identity
Its an internationally-unique serial number allocated to Mobile Station (MS) hardware at the time of manufacture. It is registered by the network operator & stored in Authentication Center (AuC) for Validation purpose. This number consists of type approval code, final assembly code and serial number of the mobile station. The network stores the IMEI numbers in the Equipment Identity Register (EIR).
IMSI - International Mobile Subscriber Number
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.
TMSI - Temporary Mobile Subscriber Identity
Its used to protect the true identity (IMSI) of a subscriber. It is issued by & stored within a VLR (not in the HLR) when an IMSI attach takes place or a Location Area (LA) updates takes place. The issues TMSI only has validity within a specific LA. The TMSI is used for security purposes, so that the IMSI of a subscriber does not have to be transmitted over the air interface. Its a temporary identity, which regularly gets changed.
LMSI - Local Mobile Subscriber Identity
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.
MSISDN - Mobile Subscriber ISDN Number
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.
MSRN - Mobile Subscriber Roaming Number
It is temporary, location-dependent ISDN number issued by the parent VLR to all MSs within its area of responsibility. It is stored in VLR & associated HLR but not in the MS. The MSRN is used by VLR associated MSC for call routing within MSC/VLR service area.
MRSN = CC + NDC + SN
LAI - Location Area Identity
Each location area within PLMN (Public Landline Mobile Network) has an associated Intenationally unique identifier (LAI). The LAI is broadcasted regularly by the BTSs on the Broadcast Control Channel (BCCH), thus uniquely identify each cell within an associated location area (LA). It structure is as follows,
LAI = MCC + MNC + LAC
MCC= Mobile Country Code (of the visited country)
MNC= Mobile Network Code (of the serving PLMN)
LAC= Location Area Code
CI - Cell Identifier
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.
BSIC - Base Station Identity Code
Each BTS is issued with a unique identity, the BSIC & is used to distinguish neighbouring BTSs. It is needed to identify that the frequency strength being measured by the mobile station is coming from a particular base station.