Traffic cases when MS is in idle mode
ATTACHING TO THE NETWORK
IMSI attach
When an MS is switched on, the IMSI attach procedure is executed. This involves the following steps:
1. The MS sends an IMSI attach message to the network indicating that it has changed state to idle.
2. The VLR determines whether there is a record for the subscriber already present. If not, the VLR contacts the subscriber’s HLR for a copy of the subscription information.
3. The VLR updates the MS status to idle.
4. Acknowledgement is sent to the MS.
IMSI attach
Location Updating, type IMSI Attach
If the MS has changed LA while powered off, the IMSI attach procedure may lead to an update to the location of the MS. During IMSI attach, the VLR may determine that the current LAI of the MS is different from the LAI stored in the MS’s subscription information. If so, the VLR updates the LAI of the MS.
ROAMING IN THE NETWORK
Changing Cells within an LA
MS’s are constantly moving around in the cellular network. The MS location information stored in the VLR is the LA. If an MS changes cells within a LA, the network is not updated. The MS knows that the new cell belongs to the same LA by listening to the BCCH in the new cell. The BCCH broadcasts the cell’s LAI. The MS compares the last LAI received with the new LAI. If they are the same, it means that the MS has not changed LA's and does not need to inform the network.
Location Updating, Same MSC/VLR
If an MS detects a change in LAI on the BCCH, it informs the network. When the MS sends the Location Updating message, the MSC/VLR determines whether it is an MS, which is already registered, or if it is an MS visiting from another MSC/VLR.
Location updating, same MSC/VLR
1. The MS listens to BCCH in the new cell to determine the LAI. The received LAI information is compared to the old one. If they differ, a location update is necessary.
2. The MS establishes a connection with the network via SDCCH. Authentication is performed.
3. If authentication is successful, the MS sends a Location Updating Request to the system.
4. The system acknowledges Location Updating and requests RBS and MS to release the signaling channel.
Location Updating, New MSC/VLR
When an MS roams into a new LA, location updating is performed. However, unknown to the MS, the LA may belong to a new MSC/VLR. When the Location Update Request is received by the new VLR, it executes the procedure below.
Location updating, new MSC/VLR
1. Authentication is performed. If authentication is successful, the VLR checks its database to determine whether or not it has a record for this MS-subscription.
2. When the VLR finds no record for the MS, it sends a request to the subscriber’s HLR for a copy of the MS-subscription.
3. The HLR passes the information to the VLR and updates its location information for the subscriber. The HLR instructs the old VLR to delete the information it has about the MSsubscription.
4. The VLR stores its subscription information for the MS including the latest location and status (idle). The VLR sends acknowledgement to the MS.
Location Updating, type Periodic Registration
Periodic registration is a feature which forces MS’s to send a registration message to the network at predefined intervals. If an MS should miss such a registration, the network will mark the MS as detached. This may occur if an MS is out of the area of coverage and ensures that needless paging is not performed. If the network uses periodic registration, the MS will be informed, on the BCCH of how often periodic registration must be performed. Periodic registration has an acknowledgment message. The MS tries to register until it receives this message.
DETACHING FROM THE NETWORK
IMSI Detach
IMSI detach enables the MS to indicate to the network that it is switched off. At power off, the MS sends an IMSI detach message to the network. On reception, the VLR marks the corresponding IMSI as detached. The HLR is not informed. No acknowledgement is sent to the MS.
Implicit Detach
If the MS sends an IMSI detach message to the system and the radio link quality is poor, the system might not be able to decode the information. Because no acknowledgment is sent to the MS, no further attempt is made. In this case, the system still regards the MS as attached. If periodic registration is in use, the system will soon determine that the MS is detached. The VLR then performs an implicit detach, marking the MS as detached.
MS Purging
MS purging is used to inform the HLR that the VLR is about to remove a subscriber record from the VLR. The HLR then sets the MS purged flag and treats the subscriber as unreachable. This saves unnecessary network signaling and database lookup. For example, an UK MS travels to Australia and performs a location update in an MSC/VLR in Australia. Later, the subscriber travels back to the UK, which takes some time. During this period, the subscriber is not active. If MS purging is not used, when a caller makes a call to the MS, the HLR identifies the MS as registered in the Australian MSC/VLR and routes the call to it. The MSC/VLR then informs the HLR that the subscriber is unreachable. If MS purging is used; the UK subscriber’s record will have been purged from the Australian MSC/VLR. When a call is made to the subscriber, the HLR identifies the MS as unreachable and does not contact the Australian MSC/VLR.
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