GSM Local Area Registration Initialization
Location Area Updates
MS location updating is performed to tell the system where to search for the MS during paging for an incoming call. If the location is known to a definite sub-region of a particular network, then this will reduce the number of cells where the MS has to be paged, thereby reducing the load on the system.
The MS location is determined from the cell identification of the strongest BCCH signal received by the MS. The MS regularly measures the received signal strengths of the BCCHs for all surrounding cells at least once every 6 sec. It stores at least the six strongest BCCH measurements and their identifications in the SIM, which can subsequently be used for hand-over decisions. The MS also transmits the Local Area (LA) of the strongest cell to the MSC during location updating.
The LA may consist of a few cells or a contiguous group of cells under the control of one BSC, however all cells must be the under control of a single network.
After channel acquisition, the MS decodes the Bearer Service (BS) information over the BCCH. The BS information includes Country Code, LAC and possibly the Cell Identity (CI). If any of these pieces of information do not match with the information stored in the SIM card, the MS thinks that it has been moved into a new area and reports its location to the system so that incoming calls can be routed correctly.
Also, the MS will periodically report its location if there has been no activity for a given amount of time. The time period of reporting is part of the information contained in the BCCH and may vary, due to network loading. The MS location is also updated in the system whenever the MS changes cells during conversation.
It is necessary for the Network to know exactly where the MS is located. When an MS needs to be reached to receive a call, GSM pages based on the previously reported location of the MS. All idle MS will receive this page and compare their ID numbers with the message to determine the intended recipient.
As soon as the MS is switched on, it retrieves the stored LAI and compares it with the one broadcast within its present cell. If they match, the MS does not have to do anything since the subscriber is already correctly located. However, it does not match, the MS identifies itself by transmitting its IMSI together with the identities of the previous and present LA. The BSS transmits this information to the associated VLR.
Each time an MS moves into a new LA, the corresponding VLR is informed. If both the present and previous areas are served by the same VLR, the MS is given a new TMSI and its location is updated in the VLR memory. On the other hand, if the MS enters a new VLR area, its HLR, the old VLR and the new VLR are informed. The old VLR erases the data for the MS and the new VLR records relevant parameters needed to process calls.
If the MS is switched on in an LA different from the previous one or it moves across boundaries of an LA in the idle sate, an Updating Request message, which is sent from MS to BSS, is relayed to MSC. The MSC in turn alerts “VLR by a MAP/B Update LA” message. The message contains the old LA that the mobile had in its store along with its TMSI.
If the equipment contains a cipher unit, it is at this point, that the authentication and ciphering will start. The process is initiated by a “Set Ciphering Mode from VLR to MSC” message as MAP/B protocol.
After completing the ciphering process, the message is sent from the VLR to MSC for reallocation of the TMSI if desired. A “TMSI reallocation complete” message is sent from the MS to BSS after reallocating a new TMSI. The HLR sends “ MAP/D Location Update Result” to the VLR, which in turn sends a “MAP/B Location Update Acknowledge” message to the MSC. This message is subsequently forwarded to the MS as an “RIL3-RR Location Update Accepted” message. In the event that the HLR rejects the request, a VLR “RIL3-MM Location Update Reject” message is sent from the MSC to MS (shown as dotted opposite). The MSC will initiate either accept or reject message. However, LA updating may not be accepted because of the following reasons:
· Unknown subscriber
· Unknown LA
· Roaming not allowed
· System failure
After receiving a location update accept or reject message, the MSC asks the BSS to release the allocated dedicated resource by sending a “BSSMAP Clear Command” to the BSS, which is then forwarded to the MSC as a “BSSMAP Clear Complete” message, which completes the location updating process.
0 Comments