Handoff

In telecommunication, the term handoff refers to the process of transferring an ongoing call or data session from one channel connected to the core network to another. In satellite communications it is the process of transferring satellite control responsibility from one earth station to another without loss or interruption of service.

The actual process of transferring the call is referred to as a handover. In telecommunication there are two reasons why a handover might be conducted. If the user has moved out of range from one base station and can get a better connection from a stronger transmitter or if one base station is full the connection can be transferred to another nearby base station.

The most basic form of handover is that used in most 1G and 2G systems where a phone call in progress is redirected from one cell transmitter and receiver and frequency pair to another cell transmitter and receiver using a different frequency pair without interrupting the call. If the terminal can only be connected to one base station and therefore needs to drop the connection for a brief period of time before being connected to the other, stronger transmitter, this is referred to as a hard handover.

In CDMA systems the user can be connected to several base stations simultaneously, combining the data from all transmitters in range into one signal using a RAKE receiver. The set of base stations the terminal is currently connected to is referred to as the active set. A soft handover happens when there are several base stations in the active set and the terminal drops one of these to add a new one. In W-CDMA there is a special case called softer handover where several connections in the active set point to the same base station. The softer handover happens when one of these connections is dropped for another from the same base station.

There are also inter system handovers where a connection is transferred from one access technology to another, e.g. a call being transferred from GSM to W-CDMA.

External Links

• Intra MSC GSM Handover Call Flow (http://www.eventhelix.com/RealtimeMantra/Telecom/GSM_Handover_Call_Flow.pdf)

• Inter MSC GSM Handover Call Flow