Mobility Management

Mobility Management Mobility management is the one of the most important issues in PCS network. The performance of the PCS network is significantly affected by the way the network manages the movements of the mobile users. In the PCS system architecture, the mobile service area is covered by a set of base stations (BSs), which are responsible for relaying the calls to and from the mobile stations (MSs) located in their coverage area (or cells), the MSs are mounted on vehicles. The BSs are connected to mobile switching centers (MSCS) by land links. An MSC is a telephone exchanges configured specially for mobile applications. It interfaces the MSs (via BSs) with the PSTN. Two types of databases, the home location register (HLR) and the visitor location register (VLR) are used for roaming management. 1. Handoff. When a mobile user is engaged in conversation, the MS is connected to a BS via a radio link. If the mobile user moves to the coverage area of another BS, the radio link to the old BS is eventually disconnected, and a radio link to the new BS should be established to continue the conversation. This process is variously referred to as automatic link transfer, handover, Or handoff. 2. Roaming. When a mobile user moves from one PCS system to another the system should be informed of the current location of the user. Otherwise, it would be impossible to deliver the services to the mobile user. Handoff Three strategies have been proposed to detect the need for handoff. Mobile-Controlled Handoff (MCHO): In this strategy, the MS continuously monitors the radio signal strength and quality of the surrounding BSs. When predefined criteria are met, then the MS checks for the best candidate BS for an available traffic channel and requests the handoff to occur. MCHO is used in DECT and PACS. Network-Controlled Handoff (NCHO): In this strategy, the surrounding BSs, the MSC or both monitor the radio signal. When the signal's strength and quality deteriorate below a predefined threshold, the network arranges for a handoff to another channel. NCHO is used in CT-2 Plus and AMPS. Moble-Assisted Handoff (MAHO): It is a variant of NCHO strategy. In this strategy, the network directs the MS to measure the signal from the surrounding BSs and to report those measurements back to the network. The network then uses these measurements to determine where a handoff is required and to which channel. MAHO is used in GSM and IS-95 CDMA. Inter-BS Handoff or Intra-System Handoff: In inter-Bs handoff, the new and the old BSs are connected to the same MSC. Assume that the need for handoff is detected by the MS; the following actions are taken: 1. The MS momentarily suspends conversation and initiates the handoff procedure by signaling on an idle (currently free) channel in the new BS. Then it resumes the conversation on the old BS. 2. Upon receipt of the signal, the MSC transfers the encryption information to the selected idle channel of the new BS and sets up the new conversation path to the MS through that Channel. The switch bridges the new path with the old path and informs the MS to transfer from the old channel to the new channel. 3. After the MS has been transferred to the new BS, it signals the network and resumes conversation using the new channel. 4. Upon receipt of the handoff completion signal, the network removes the bridge from the path and releases resources associated with the old channel. This handoff procedure is used with the mobile controlled handoff strategy. For the networkcontrolled handoff strategy, all handoff signaling messages are exchanged between the MS and the old BS though the failing link. The whole process must be completed as quickly as possible, to ensure that the new link is established before the old link fails. If the new BS does not have an idle channel, the handoff call may be dropped. The forced termination probability is an important criterion in the performance evaluation of a PCS network. Forced termination of an ongoing call is considered less desirable than blocking a new call attempt. Most PCS networks handle a handoff in the same manner as anew call attempt. That is, if no channel in the old cell until the call is completed or when the failing link is no longer available. This is referred to as the non priotized scheme. Intersystem Handoff or Inter-MSC Handoff: In intersystem handoff, the new and old BSs are connected to two different MSCs. In the description that follows we trace the intersystem handoff procedure of IS-41, where networkcontrolled handoff is assumed. There will be a trunk connection before and after the intersystem handoff.