2.2 GSM Identifiers

GSM IIddeenttiiffiierrs

• IMEI – International Mobile Equipment Identifier
• IMSI – International Mobile Subscriber Identifier
• TMSI –Temporary Mobile Subscriber Identity
• MSISDN – Mobile Subscriber ISDN number
• MSRN – Mobile Station Roaming Number
• LAI – Location Area Identity
• CI – Cell Identifier
• BSIC – Base Station Identity Code

2.2.1 IMEI – INTERNATIONAL MOBILE EQUIPMENT IDENTIFIER.

The IMEI is an internationally-unique serial number allocated to the MS hardware at the time
of manufacture. It is registered by the network operator and (optionally) stored in the AuC
for validation purposes.

2.2.2 IMSI – INTERNATIONAL MOBILE SUBSCRIBER IDENTIFIER

When a subscriber registers with a network operator, a unique subscriber IMSI identifier is
issued and stored in the SIM of the MS. An MS can only function fully if it is operated with a
valid SIM inserted into an <MS with a valid IMEI.

2.2.3 TMSI –TEMPORARY MOBILE SUBSCRIBER IDENTITY

A TMSI is used to protect the true identity (IMSI) of a subscriber. It is issued by and stored
within a VLR (not in the HLR) when an IMSI attach takes place or a Location Area (LA)
update takes place. At the MS it is stored in the MS’s SIM. The issued TMSI only has validity
within a specific LA.

2.2.4 MSISDN – MOBILE SUBSCRIBER ISDN NUMBER

The MSISDN represents the ‘true’ or ‘dialled’ number associated with the subscriber. It is
assigned to the subscriber by the network operator at registration and is stored in the SIM. It
is possible for an MS to hold multiple MSISDNs, each associated with a different service.
2.2 GSM Identifiers
2. Architecture Overview

2.2.5 MSRN – MOBILE STATION ROAMING NUMBER

The MSRN is a temporary, location-dependant ISDN number issued by the parent VLR to all
MSs within its area of responsibility. It is stored in the VLR and associated HLR but not in the
MS. The MSRN is used by the VLR-associated MSC for call routing within the MSC/VLR
service area.

2.2.6 LAI – LOCATION AREA IDENTITY

Each Location Area within the PLMN has an associated internationally-unique identifier
(LAI). The LAI is broadcast regularly by BTSs on the Broadcast Control Channel (BCCH),
thus uniquely identifying each cell with an associated LA. The purpose of LAs is covered
later in this course.

2.2.7 CI – CELL IDENTIFIER

The CI an identifier assigned to each cell within a network. However, the CI is only unique
within a specific Location Area. When combined with the internationally unique LAI for its
associated LA, the Global Cell Identity (GCI) is produced which is also internationally
unique.

2.2.8 BSIC – BASE STATION IDENTITY CODE

Each BTS is issued with a unique identity, the BSIC and is used to distinguish neighbouring
BTSs.