The following steps are used by the MSs outside their own subscription areas:
The process of transferring a mobile from one channel form one channel or base station to another is called handoff. When a mobile moves into a different cell while a conversation is in progress, the MSC automatically transfers the call is to a new channel belonging to the new base station.
Parameters influencing handoff
Handoff depends on cell size, boundary length, signal strength, fading, reflection and refraction of signals, and manmade noise.
If the number of channels per area increases if the number of channels allocated per cell is increased or if the area of each cell is decreased.
The base station monitors the signal level for a certain period of time before a handoff is initiated. The length of time needed to decide if a handoff is necessary depends on the speed of which the vehicle is moving.
Handoff can be initiated either by the BS or the MS, and it could be due to
Radio link type handoff is primarily due to the mobility of the MS and depends on the relative value of the radio link parameters. Radio link type handoff depends on
Network management may cause handoff if there is a drastic imbalance of traffic over adjacent cells, and optimal balance of channels and other resources are required.
Service related handoff is due to degradation of quality of services (QoS), and handoff could be invoked when such a situation is detected.
The factors that define the right time for handoff are:
The need for handoff is determined in two different ways:
Types of Handoffs
Handoff can be classified into two different types:
Hard handoff, also known as "break before make" is one, in which the channel in the source cell is released only and after that the channel in the target cell is engaged. Thus the connection to the source is broken before the connection to the target cell is engaged. Thus the connection to the source is broken before the connection to the target is made for these reason such handover are also known as break before make. Both FDMA and TDMA employ hard handoff.
Soft handoff "make before break" is one in which the channel in the source cell is oriented and used for a while in parallel with the channel in the target cell. In this case the connection to the target is established before the connection to the source is broken; hence this handover is called soft handoff.
It is also possible to move from a cell controlled by one MSC area to a cell connected to another MSC.