ROAMING:
ROAMING:
When a mobile user moves from one PCS system to another, then the system should be informed of the current location of the user. Otherwise it is impossible to deliver services. Two basic operations are performed under roaming management.
1. Registration (location update): Where MS informs the system its current location.
2. Location tracking: Process during which a system locates MS. Location tracking is required when n/w attempts to deliver call to a mobile user.
The roaming management follows a two level strategy where two tier systems of home and visited databases are used. When a user subscribes to the services of a network, a record is created in the systems database called HLR. This is referred to as home system of the mobile user. HLR is a n/w database, where MS's identity, profile, current location & validation period is stored.
When the mobile user visits a new network other than home system, a temporary record for the mobile user is created in the VLR of visited system. VLR temporarily stores information for visiting subscribers so that corresponding MSC can provide service.
Registration Procedure includes following steps:
1. When mobile user enters into new PCS n/w, it must register in VLR of new system.
2. The new VLR informs mobile user's HLR regarding the current location & address of user. The HLR sends an acknowledgement which includes MS's profile, to the new VLR.
3. New VLR informs MS about successful registration.
4. HLR sends a deregistration message to cancel the location record of MS in old VLR. The old VLR acknowledges the deregistration.
To originate a call, MS first contacts with MSC in the new PCS n/w .The call request is forwarded to VLR for approval. If it is approved, MSC sets up the call to the user following the standard PSTN procedures.
1. If a wireline phone attempts to call a mobile subscriber, the call is forwarded to switch called the originating switch in PSTN. The switch masses a query to HLR to find current VLR of MS. The HLR queries the VLR in which MS resides to get a communicable address.
2. The VLR returns the address to switch through HLR.
3. Based on address, a communication link is established between MS through visited MSC.