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AMPS Mobility Management

Mobility management in AMPS networks appears to be based on the engineering assumption that most calls are originated by the mobile and seems optimized for mobiles that are usually in their home area.

Intelligent paging algorithms are employed by AMPS to reduce the collective forward bandwidth required for a page. The paging algorithm starts by paging in only a small area, based on where the mobile usually receives service (the home area) or perhaps where the mobile was last registered (i.e., the location area). When a mobile receives a page, it responds and proceeds to the assigned traffic channel pair.

AMPS mobility management has been greatly enhanced by IS-41, which defines the standard for interoperation between networks. Mobility management is handled by databases known as the home location register or HLR and the visiting location register or VLR.

The purpose of the HLR is to track the location of (the VLR serving) a mobile. The HLR also contains information about each of the mobiles associated with a home area (e.g., is the mobile allowed to originate an international call, etc.). This database logically unites data describing both the subscriber and the subscriber's equipment into a single service profile. Because this "permanent" information is critical to serving customers, the HLR function is typically supported by multiple distributed fault-tolerant computers.

The purpose of the VLR is to track all mobile stations currently roaming in the local MSC coverage area. The VLR contains the service profile of each roaming mobile as well as other information necessary for calls terminating at the mobile station. Because the VLR function is so closely aligned with an operating MSC, it is typically collocated with or part of that MSC. Since the information it contains is of a temporary nature, fault-tolerance is less critical than for the HLR.

Because base stations periodically broadcast the SID and location area identifiers on the forward control channel, the mobile station knows immediately when it has roamed into another system or location area. An option in IS-41, known as autonomous registration, allows the mobile to register to the host MSC. This registration is forwarded by the MSC to the VLR. Another IS-41 message is then used by the VLR to notify the HLR that it is currently hosting the mobile.

The HLR passes necessary service profile information to the VLR in another IS-41 message, enabling the host system to provide service to the mobile station. Information such as whether or not the mobile is allowed to originate international calls is contained in this service profile. Since the HLR now knows the location of the mobile, more efficient paging can be used for mobile-terminated calls. The HLR also notifies any other VLR which had been previously hosting the mobile to deregister the mobile.

A mobile-terminated call attempt is always first directed to the mobile station's HLR by the gateway MSC first contacted from the PSTN. The HLR is responsible for contacting the current serving system, obtaining a temporary local directory number (TLDN) from the serving system, and transferring the call to the TLDN. The serving system is responsible for the connection between the TLDN and the roaming mobile station.

IS-41 supports uninterrupted voice services while the mobile station moves between MSCs. This is equivalent to maintaining a session between a mobile data device and another host while the mobile host is in motion between areas. Because trunk connections are used to carry the voice traffic, the concatenated trunk length could grow as the mobile moves about while the conversation is active (i.e., the mobile "grows a tail"). This is usually not a significant problem because most conversations are only a few minutes in duration and even fast-moving vehicles covers only so much ground in a typical call period.

HLRs and VLR is are not affected by cell handoffs, only by wider-scale mobility (between MSCs). Mobiles reregister every time they cross boundaries separating location areas. These location areas are clusters of cells large enough to minimize the number of re-registration messages (on the contended reverse control channel) while also minimizing the number of paging channels involved in a page.