Handovers

Handover Types

There are four different types of handover in the
GSM system, which involve transferring a call
between:
• Channels (time slots) in the same cell
• Cells within the same BSS (same BSC)
• Cells in different BSSs (different BSCs) but
under the control of the same MSC
• Cells under the control of different MSCs
GSM handovers are ‘hard’ – i.e. mobile only
communicates with one cell at a time

In a cellular network, the radio and fixed links required are not permanently allocated for the
duration of a call. Handover, or handoff as it is called in North America, is the switching of
an on-going call to a different channel or cell. The execution and measurements required for
handover form one of basic functions of RR management. There are four different types of
handover in the GSM system, which involve transferring a call between:
• Channels (time slots) in the same cell
• Cells (Base Transceiver Stations) under the control of the same Base Station
Controller (BSC),
• Cells under the control of different BSCs, but belonging to the same Mobile services
Switching Centre (MSC), and
• Cells under the control of different MSCs.
Intra-BSS handovers are known as Internal handovers as they involve only one Base Station
Controller (BSC). To save signalling bandwidth, they are managed by the BSC without
involving the Mobile services Switching Centre (MSC), except to notify it at the completion of
the handover.
Inter-BSS (either intra- or inter-MSC) are known as external handovers and are handled by the
MSCs involved. An important aspect of GSM is that the original (or anchor) MSC, remains
responsible for most call-related functions, with the exception of subsequent inter-BSC
handovers under the control of the new (or relay) MSC.

Handover Causes

• Handover can be initiated by either MS or MSC
• Handover decision is based on the following parameters
(in priority order):
• Received signal quality
• Received signal strength
• Distance of MS from BTS
• Drops below power budget margin
• Each parameter has a operator-defined threshold and
 

handover decisions can be based on one or a
combination of the parameters

Handovers can be initiated by either the MS or the MSC (e.g. as a means of traffic load
balancing).
During its idle time slots of a multiframe, the MS scans the BCCH of up to 32 neighbouring
cells, and forms a list of the six best candidates for possible handover, based on the received
signal strength. This information is passed to the BSC and MSC, at least once per second, and
is used by the handover algorithm.

The algorithm for when a handover decision should be taken is not specified in the GSM
recommendations. There are two basic algorithms used, both closely tied in with power
control. This is because the BSC usually does not know whether the poor signal quality is due
to multipath fading or to the mobile having moved to another cell. This is especially true in
small urban cells.