Handovers equating to the scales of mobility.

Those between domains and those within a domain.

Handovers within a domain are expected to happen frequently and as much as is possible to be transparent to applications with active connections.

It is not entirely clear that in the type of in-building system we are discussing inter-domain handover is required. Inter-domain handover is achieved by migrating the FSP.

A new FSP is allocated in the new domain and a duplicate virtual path is created between the old FSP and the new FSP in the new domain. The mobile is allocated a new proxy ATM address which is routed to the new FSP. Connections which are created to the previous proxy ATM address for the mobile are routed via the old FSP. Connections created using the new ATM address will be routed directly through the new FSP.

Intra-domain handover - that between pico-cells in the same domain is by far the most important handover.

When a mobile is registered in a pico-cell it receives updates (using the handover signalling channel) of the adjacent available channels and base-stations from its currently selected base.

The mobile periodically polls these adjacent channels. At the physical layer it switches its currently selected channel and listens for base-station activity. If it detects base-station transmissions within a small number of cell-times it records the link quality to that base and switches back to the currently selected base.

As will be seen the MAC protocol is divided into frames and in some modes of operation, where base stations are synchronised with each other, there may be dedicated frames during which all bases will transmit and all mobiles will monitor adjacent channels. Alternatively if adjacent base stations are not synchronised then there may be dedicated times during which all mobiles within one cell will switch.

When a mobile detects that the link to the current base-station is worse than that measured to an adjacent cell it can initiate a handover. This form of controlled handover is called connected handover, and uses a backward handover mechanism in which the mobile uses the currently active channel to prepare the handover before committing to the new base.

In some cases, hopefuly rarely, the mobile will lose contact with the currently active base and will need to initiate handover via the new base-station. This is similar to the process required at the start of day.