Mobility Management Mobility management is the one of the most important issues in PCS network. The performanceof the PCS network is significantly affected by the way the network manages the movements ofthe mobile users. In the PCS system architecture, the mobile service area is covered by a set ofbase 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 areconnected to mobile switching centers (MSCS) by land links. An MSC is a telephone exchangesconfigured specially for mobile applications. It interfaces the MSs (via BSs) with the PSTN. Twotypes of databases, the home location register (HLR) and the visitor location register (VLR) areused 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 linkto the old BS is eventually disconnected, and a radio link to the new BS should beestablished to continue the conversation. This process is variously referred to asautomatic 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 todeliver 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 thesurrounding BSs. When predefined criteria are met, then the MS checks for the best candidateBS for an available traffic channel and requests the handoff to occur. MCHO is used in DECTand PACS. Network-Controlled Handoff (NCHO): In this strategy, the surrounding BSs, the MSC or both monitor the radio signal. When thesignal's strength and quality deteriorate below a predefined threshold, the network arranges fora 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 thesignal from the surrounding BSs and to report those measurements back to the network. Thenetwork then uses these measurements to determine where a handoff is required and to whichchannel. 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 theneed 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 theconversation 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 thatChannel. The switch bridges the new path with the old path and informs the MS totransfer 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 network-controlled handoff strategy, all handoff signaling messages are exchanged between the MS andthe 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 anidle channel, the handoff call may be dropped. The forced termination probability is animportant criterion in the performance evaluation of a PCS network. Forced termination of anongoing call is considered less desirable than blocking a new call attempt. Most PCS networkshandle a handoff in the same manner as anew call attempt. That is, if no channel in the old celluntil the call is completed or when the failing link is no longer available. This is referred to asthe non priotized scheme. Intersystem Handoff or Inter-MSC Handoff: In intersystem handoff, the new and old BSs are connected to two different MSCs. In thedescription that follows we trace the intersystem handoff procedure of IS-41, where network-controlled handoff is assumed. There will be a trunk connection before and after the intersystemhandoff.