Mobility Management

Mobility mgmt function handles the function that arises due to mobility of the subscriber. Main objective of MM is location tracking & call set up. There are two aspects of mobility in a PCS n/w.

HANDOFF

When a mobile user is engaged in conversation, the MS is connected to BS via radio link. If the user moves to the coverage area of another BS, the radio link to old BS is disconnected and radio link to new BS is established to continue conversation. This process is called automatic link transfer or handoff. Depending on the mobility of MS, the handoff is divided into two categories:

Inter-BS Handoff/ Inter Cell Handoff:

Here MS usually moves from one BS to another BS under one MSC. Action taken for communication:

1. The MS momentarily suspends conversation & initiates the hand-off procedure by picking a channel in new BS. Then is resumes the conversation in old BS.
2. When MSC receives that signal, he transfers the information to the new BS & sets up new conversation path to MS through that channel.
3. After MS has been transferred to new BS, it starts the conversation channel with new BS & then MSC disconnect the link with old BS.

Inter-System Handoff/Inter-MSC Handoff

MS moves from one BS to another connected to two different MSCs.Action taken for communication:
1. MSC1 requests MSC2 to perform handoff measurement on the call in progress.
2. MSC2 then selects a BS by interrogating the signal quality and sends the information to MSC1.
3. Then MSC1 asks MSC2 to setup a voice channel.
4. Assuming that a voice channel is available in BS2.MSC2 instructs MSC1 to start radio link transfer.
5. MSC1 sends the MS a handoff order. Now MS can access BS2 of MSC2.MSC2 informs MSC1 that handoff is successful.MSC1 then connects call path to MSC2.
6. In the intersystem handoff process, anchor MSC is always in call path before & after handoff.

Path Minimization- When MS moves to MSC3, MSC2 may be removed from the call path. The link between MSC1 and MSC2 is disconnected and MS connects to MSC3 directly. This process is called path minimization.

Previous Next