Identifiers used in GSM System
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.
The most important addresses and identifiers are as follows:
IMSI (International Mobile Subscriber Identity)
SIM (Subscriber Identity Module)
MSISDN (Mobile System Integrated Service Digital Network)
LAI (Location Area Identity)
IMSEI (International MS Equipment Identity)
MSRN (MS Roaming Number)
TMSI (Temporary MS Identity)
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Identifiers:
1. International Mobile Subscriber Identity (IMSI):
• The IMSI is assigned to an MS at subscription time. It uniquely identifies a
given MS.
• When an MS attempts a call, it needs to contact a BS. The BS can offer its
service only if it identifies the MS as a valid subscriber.
• For this MS needs to store certain values uniquely defined for the MS, like
country of subscription, network type, subscriber ID and so on.
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The IMSI contains 15 digits and includes
Mobile Country Code (MCC)—3 digits (home country)
Mobile Network Code (MNC)—2 digits (Network provider Code)
Mobile Subscriber Identification code/Number (MSIC/MSIN)
National Mobile Subscriber Identity (NMSI)
Another use of IMSI is to find information about the subscriber’s
home PLMN
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Format of IMSI
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2. SIM (Subscriber Identity Module)
• Every time the MS has to communicate with a BS, it must correctly identify itself.
• An MS does this by storing the mobile phone number, personal information number
for mobile station, authentication parameters and so on, in the SIM card.
• Smart SIM cards have a flash memory –to store small messages to the unit.
• Advantage- it supports roaming with or without a cell phone, also called SIM roaming.
• Carry only the SIM card alone and insert in any GSM mobile phone to make a it work
as per customized MS.
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2. SIM (Subscriber Identity Module)
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3. Mobile System ISDN (MSISDN)
• It identifies a particular MS’s subscriber, with the format shown. MCC- 1 to 3
digits, NDC-variable, SN- variable
• The GSM actually does not identify a particular mobile phone, but a particular
HLR. It is the responsibility of HLR to contact the mobile phone.
• https://ind.areacodebase.com/ndc_list
3. Mobile System ISDN (MSISDN)
4. Location Area Identity (LAI)
The GSM service area is usually divided into a hierarchical structure that
facilitates the system to access any MS quickly.
•Each PLMN area is divided into many MSCs.
• Each MSC typically contains a VLR to inform the
system if a particular cell phone is roaming
•Each MSC is divided into many location areas (LAs).
•An LA is a group of cells and is useful when the MS is
roaming in a different cell but the same LA.
•LA identifier should contain the country code, the
mobile network code and LA code.
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5. International Mobile station
equipment identification (IMSEI)
• Each GSM mobile phone equipment is assigned a 15-bit long international MS
equipment identity number to contain manufacturing information.
• When the mobile phone equipment passes the interoperability tests, it is assigned
a type approval code.
• Since a single mobile unit may not be manufactured at the same place, a field in
IMSEI, called the final assembly code, identifies the final assembly place of the
mobile unit.
• To identify uniquely a unit manufactured, a Serial Number (SN) is assigned.
• A spare digit is available to allow further assignment depending on requirements.
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6. MS roaming Number (MSRN)
When an MS roams into another MSC, that unit has to be identified based on the
numbering scheme format used in that MSC.
Hence, the MS is given a temporary roaming number-called the MS Roaming
Number (MSRN), with the format shown in Fig.
This MSRN is stored by the HLR, and any calls coming to that MS are rerouted to
the cell where the MS is currently located.
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7. TMSI (Temporary Mobile Subscriber Identity)
As all transmission is sent through the air interface, there is a constant threat to the
security of information sent. A Temporary Mobile Subscriber Identity (TMSI)is
usually sent in place of IMSEI.
The TMSI is assigned to an MS by the VLR. The TMSI uniquely identifies an MS
within the area controlled by a given VLR. The maximum number of bits that can
be used for the TMSI is 32.
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