Handoff
Skip contentAlso known as: Handover
Handover (or, in North America, handoff) occurs in both digital and analogue mobile networks when a mobile station (mobile phone or PDA) with an active data or voice call moves between cells. It is initiated by the network when signal strength drops so low that better signal quality or signal strength can be gained by switching to a new cell.
Various cell layout strategies can be used to help enhance network capacity and quality in different ways, including:
- Macrocells - Large cells each spanning several kilometres, which are perfect for rural areas
- Microcells - One-, two- or three-dimensional local area cells, spanning only 100 to 200 meters; these are often used to take advantage of cities laid out on a square grid, using half-square, full-square, or rectangular cell plans.
- Macrocell / Microcell Overlays - To catch spills when microcells become temporarily full, and to reduce the handover rate of moving mobile stations, one or more macrocells can span the area covered by a grid of many microcells.
- Integrated terrestrial system - The mobile network is integrated with an existing fixed line, wireless (e.g. Wi-Fi) or even satellite network; this has the most complex handover scheme but is also the most versatile.
If a cell is full and there are new calls queued, a handover will take priority over these - the network will keep an existing call open rather than cutting it off and replacing it with a new one. Successful GSM handovers can occur at maximum speeds of up to 250km/h.
In GSM terminology, the handover is managed by the Base Station Subsystem (BSS) or Mobile Switching Centre (MSC) depending on the network location of the Base Stations (BS) that the Mobile Station (MS) is moving between. The mobile station negotiates handoff using commands send over the Dedicated Control Channels (DCCH) based on the signal strength of the Slow Associated Control Channel (SACCH). It is necessary to renegotiate over the Command Control Channel (CCCH) after handover.
There are four different types of handover:
- Intra-BTS handover, switching between different channels of the same base station
- Inter-BTS handover, switching between different bases stations in the same BSS
- Inter-BSS handover, moving between cells under control of different base station subsystems but the same MSC
- Inter-MSC handover, moving between cells controlled by different mobile switching centres
