Roaming Management Roaming is a facility, which allows a subscriber to enjoy uninterrupted communication fromanywhere in the entire coverage space. A mobile network coverage space may be managed by anumber of different service providers. They must cooperate with each other to provide roamingfacility. Roaming can be provided only if some administrative and technical constraints are met. Roaming Constraints Some roaming constraints are given below: 1. Administrative constraints: Billing. Subscription agreement. Call transfer charges. User profile and database sharing. Any other policy constraints. 2. Technical constraints Bandwidth mismatch. For example, European 900 MHz band may not be available inother parts of the world. This may preclude some mobile equipment for roaming. Service providers must be able to communicate with each other needs some standard. Mobile station constraints. Integration of a new service provider into the network. A roaming subscriber must beable to detect this new provider. Service providers must be able to communicate with each other. Quick MU (Mobile User) response to a service provider's availability. Limited battery life. Two basic operations in roaming management are 1. Registration (Location update). The process of informing the presence or arrival of a MU to acell. 2. Location tracking. The process of locating the desired MU. Location tracking is requiredwhen the network attempts to deliver a call to the mobile user. The roaming management strategies proposed in the IS-141 and GSM MAP standards are two-level strategies in that they use a two-tier system of home and visited databases. When a usersubscribes to the services of a PCS network, a record is created in the system's database, calledthe home location register(HLR). This is referred to as the home system of the mobile user. TheHLR is network database that stores and manages all mobile subscription of a specific operator.Specifically, the HLR is the location register to which an MS identity is assigned for recordpurposes, such as directory number, profile information, current location, and validationperiod. When the mobile user visits a PCS network other than the home system, temporaryrecord for the mobile user is created in the visitor location register of the visited system. TheVLR temporarily stores subscription information for the visiting subscribers so that the"other"location register used to retrieve information for handling calls to or from a visiting mobile user.
Registration Process Steps Step 1: Suppose that the home system of a mobile user is in A When the mobile user movesfrom one visited system to another, it must register in the VLR of the new visited system. Step2: The new VLR informs the mobile user's HLR of the person's current location -the addressof the new VLR. The HLR sends an acknowledgment, which includes the MS's profile to thenew VLR. Step 3: The new VLR informs the MS of the successful registration. Step 4: After step 2 the HLR also sends a deregistration message to cancel the obsolete locationrecord of the MS in the old VLR. The old VLR acknowledges the deregistration. The MS first contacts the MSC in the visited PCS network. The call request is forwarded to the VLR for approval. If the call is accepted the MSC setup the call to the called partyfollowing the standard PSTN call setup procedure. Call Delivery Steps: Step 1: If a wire line phone attempts to call a mobile subscriber, the call is forwarded to a switch,called the originating switch in the PSTN, which queries the HLR to find the current VLR of theMS. The HLR queries the VLR in which the MS resides to get a routable address. If theoriginating switch is not capable of querying the HLR (i. e, it is not equipped to supportmobility), the call is routed through the PSTN to the subscriber's gateway MSC, which queriesthe HLR to determine the current VLR serving the MS. Step 2: The VLR returns the routable address to the originating switch through the HLR. Step 3: Based on the routable address, a trunk (voice circuit) is setup. *Abbreviations: VLR: Visitor location register HLR: Home location register MSC: Mobile switching center BS: Base station. PSTN: Public switching telephone network.