Handoff
In
cellular telecommunications, 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
British English term for transferring a cellular call is
handover, which is the terminology standardised within such European originated technologies as
GSM and
UMTS.
In
telecommunications there are two reasons why a handoff (handover) might be conducted: if the phone has moved out of range from one
cell site (base station) and can get a better radio link 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 handoff is that used in
GSM and
analog cellular networks, where a
phone call in progress is redirected from one
cell site and its transmit/receive
frequency pair to another base station (or sector within the same cell) using a different frequency pair without interrupting the call. As the phone can be connected to only one base station at a time and therefore needs to drop the radio link for a brief period of time before being connected to a different, stronger transmitter, this is referred to as a
hard handoff. This type of handoff is described as "break before make" (referring to the radio link).
In
CDMA systems the phone can be connected to several cell sites simultaneously, combining the signalling from nearby transmitters into one signal using a
rake receiver. Each cell is made up of one to three (or more) sectors of coverage, produced by a cell site's independent transmitters outputting through antennas pointed in different directions. The set of sectors the phone is currently linked to is referred to as the "active set". A
soft handoff occurs when a CDMA phone adds a new sufficiently-strong sector to its active set. It is so called because the radio link with the previous sector(s) is not broken before a link is established with a new sector; this type of handoff is described as "make before break". In the case where two sectors in the active set are transmitted from the same cell site, they are said to be in
softer handoff with each other.
There are also
inter-technology handoffs where a call's connection is transferred from one access technology to another, e.g. a call being transferred from
GSM to
W-CDMA.
*
Cellular network*
Intra-MSC GSM Handover Call Flow*
Inter-MSC GSM Handover Call Flow*
Overview of cell phone handover or handoff
