Subject: Mobile Computing

Difficulty: Medium

Marks: 6 Marks

HLR failure restoration:

• In GSM HLR, it is compulsory to save the updae into non-volatile storage.
• Changes of service information are backup immediately after every update and the location information is periodically transferred from HLR into backup.
• The service information is update infrequently because not all the subscriber chages their service profile after subscription.
• After HLR failure, the data in the backup are reloaded into the HLR. We also have 'uncovered period‘ as a time interval after last backup operation and before the restart of the HLR data that changes in the uncover period cannot be recoverd. The following HLR restoration procedure is executed.

Step 1: The HLR sends an signalling system 7 (SS7) TCAP (Transaction Capability Application Part) message. MAP_RESET to the all VLRs where its MSs are located (that is restoration signal).

Step 2: Each VLR that receives the restoration signal from HLR is queried to search the lost location information of user.

Step 3: All the VLRs derived all MSs of the HLR, and for each MS, they send an SS7 TCAP message, MAP_UPDATE LOCATION, to the HLR.