Subject: Mobile Computing

Difficulty: Medium

Marks: 4 Marks

VLR Failure Restoration: Ater a VLR failure, the service information of a VLR record is recovered by the first contact between the VLR and the HLE of the corresponding MS. The location information is recovered by the first radio contact between the VLR and the MS. The mobile station information is recovered either by contact with the HLR or MS. VLR record restoration is intitiated by one of the three events.

1) MS registration 2) MS call originaton 3) MS call termination

1) MS Registraion: Since the VLR record was erased by the failure, the VLR considers the registration as a case of inter-VLR movement. Following the normal registration procedure defined in inter-VLR movement, the VLR record is recovered. In this case, TMSI sent from the MS to the VLR cannot be recognized, and the MS is asked to send IMSI over the air.

2) MS call originaton: When the VLR receives the call origination request MAP_SEND_INFO_FOR_OUTGOING_CALL from the MSC, the VLR record for the MS is not found. The VLR considers the situation as system error, with the cause unidentified subscribe. The request is rejected, and the MS is asked to initate the location registration procedure. After the registration procedure, the VLR record is recovered.

3) MS call termination:

Step 1 and 3: The IMSI and the MSC no. are provided in the MAP_PROVIDE_ROAMING_NUMBER message sent from the HLR to the VLR. The VLR searches the MS record by using the received IMSI. Since the record has been erased after the failure, the search fails. The VLR creates a VLR record for the MS. Neither the service nor the location information is available in this record.

Step 4 and 7: Since the VLR does not have the routing information, it uses the MSC no. provided by the MAP_PROVIDE_ROAMING_NUMBER message to create the MSRN. The no. is sent back to the gateway MSC to set up the call.

Step 5 and 6: The VLR recovers the receive information of the VLR record by sending a MAP_RESTORE_DATA message to the HLR.

Step 8: After the gateway MSC receives the MSRN. The SST ISUP message IAM is sent to the target MSC.

Step 9 and 11: The target MSC does not have the LA information of the MS. In order to procedd to set up the call, the MSC sends the message MAP_SEND_INFO_FOR_INCOMING_CALL to the VLR. The VLR does not have the LAI information either. Hence, the VLR asks the MSC to determine the LA of the MS by sending a MAP_SEARCH_FOR_MOBILE_SUBSCRIBER message.

Step 12 and 13: The MSC initiaties paging of the MS in all Las. IF the paging is successful, the current LA address of the MS is sent back to the VLR. At this point the location information of the VLR record is recovered.