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LOCALIZATION AND CALLING
INTRODUCTION:
The fundamental feature of the GSM system is the automatic, worldwide localization of users for which, the system performs periodic location updates. The HLR always contains information about the current location The VLR currently responsible for the MS informs the HLR about the location changes. Changing VLRs with uninterrupted availability is called roaming. To locate and address an MS, several numbers are used:
1..MOBILE STATION INTERNATIONAL ISDN NUMBER
(MSIISDN)
The only important number for a user of GSM is the phone number. This number consists of the country code (CC), the national destination code (NDC) and the subscriber number (SN).
2.. INTERNATIONAL MOBILE SUBSCRIBER IDENTITY
(IMSI)
GSM uses the IMSI for internal unique identification of a subscriber. IMSI consists of amount of mobile country code (MCC), the mobile network code (MNC), and finally the mobile subscriber identification number (MSIN).
3.. TEMPORARY MOBILE SUBSCRIBER IDENTITY
(TMSI)
To hide the IMSI, which would give away the exact identity of the user signaling over the air interface, GSM uses the 4 byte TMSI for local subscriber identification.
4.. MOBILE STATION ROAMING NUMBER(MSRN)
Another temporary address that hides the identity and location of a subscriber is MSRN. The VLR generates this address on request from the MSC, and the address is also stored in the HLR. MSRN contains the current visitor country code (VCC), the visitor national destination code (VNDC), the identification of the current MSC together with the subscriber number. The MSRN helps the HLR to find a subscriber for an incoming call.
FOR A MOBILE TERMINATED CALL ,the following figure shows
the different steps that take place:
Upon receiving MSRN, the HLR determines the MSC responsible for MS and Forwards the information to the GMSC. STEP7: The GMSC can now forward the call setup request to the MSC indicated. STEP8: The MSC requests the VLR for the current status of the MS. STEP9: VLR sends the requested information STEP10: If MS is available, the MSC initiates paging in all cells it is responsible for. STEP11: The BTSs of all BSSs transmit the paging signal to the MS. STEP12: STEP13: If MS answers, VLR performs security checks. STEP15: Till STEP17: Then the VLR signals to the MSC to setup a connection to the MS.
STEP1:
The MS transmits a request for a new connection STEP2: The BSS forwards this request to the MSC STEP3: STEP4: The MSC then checks if this user is allowed to set up a call with the requested and checks the availability of resources through the GSM network and into the PSTN. If all resources are available, the MSC sets up a connection between the MS and the fixed network.
