Write the stepwise procedure for call delivery to a roaming user in GSM.

Subject: Mobile Computing

Difficulty: Medium

Marks: 4 Marks

1 Answer

The location of mobile station is maintained by HLR and the VLR. When an MS visit a new location, it must register in the VRL of the visited location.

Then the HLR must also be informed about this registration. To access MS, the HLR has to find the current VLR of the MS. The registration process of MS when it is moving from one VLR to another VLR is as follows.

Step 1: The MS periodically listens to the BCCH broadcast from the BSS. If MS detect that it has entered into a new area, then it sends registration message to VLR by usingSDCCH channel.

Step 2: Then new VLR communicates with old VLR to find the HLR of the MS. Then new VLR performed authentication process.

Step 3: After MS authentication, the new VLR sends registration message to the HLR. If registration request is accepted,then HLR provides the new VLR with all the subscriber information for call handling.

Step 4: The new VLR informs the MS about successful registration.

Step 5: After step 3, the HLR sends cancellation message to old VLR. The old VLR cancels the record for MS and sends on acknowledgement to HLR about the cancellation.

When MS is inactive due to switching off or SIM removal, it transmits a detach to deregister from network. MS has to periodically send registration message to network. The range of period may be from 6 minutes to slightly more than 24 hours.

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