One of the main features of GSM system is the automatic, worldwide localisation of it’s users. The GSM system always knows where a user is currently located, and the same phone number is valid worldwide. To have this ability the GSM system performs periodic location updates, even if the user does not use the MS, provided that the MS is still logged on to the GSM network and is not completely switched off. The HLR contains information about the current location, and the VLR that is currently responsible for the MS informs the HLR about the location of the MS when it changes. Changing VLRs with uninterrupted availability of all services is also called roaming. Roaming can take place within the context of one GSM service provider or between two providers in one country, however this does not normally happen but also between different service providers in different countries, known as international roaming.
To locate an MS and to address the MS, several numbers are needed:
MSISDN (Mobile Station International ISDN Number)16. The only important number for the user of GSM is the phone number, due to the fact that the phone number is only associated with the SIM, rather than a certain MS. The MSISDN follows the E.164, this standard is also used in fixed ISDN networks.
IMSI (International Mobile Subscriber Identity). GSM uses the IMSI for internal unique identification of a subscriber.
TMSI (Temporary Mobile Subscriber Identity). To disguise the IMSI that would give the exact identity of the user which is signaling over the radio air interface, GSM uses the 4 byte TMSI for local subscriber identification. The TMSI is selected by the VLR and only has temporary validity within the location area of the VLR. In addition to that the VLR will change the TMSI periodically.
MSRN (Mobile Station [Subscriber] Roaming Number)17. This is another temporary address that disguises the identity and location of the subscriber. The VLR generates this address upon request from the MSC and the address is also stored in the HLR. The MSRN is comprised of the current VCC (Visitor Country Code), the VNDC (Visitor National Destination Code) and the identification of the current MSC together with the subscriber number, hence the MSRN is essential to help the HLR to find a subscriber for an incoming call.
All the numbers described above are needed to find a user within the GSM system, and to maintain the connection with a mobile station. The following scenarios below shows a MTC (Mobile Terminate Call) and a MOC (Mobile Originated Call).