Handover – handover in mobile computing – handover in GSM
Handover – A handover is a process that occurs in a telecommunications system and mobile communications system in which a connected call or a data session is or transferred or handover from one cell to another cell without any disconnection.
Handover in mobile computing – handover in GSM
Handover is a necessary function of GSM to maintain the call connectivity. Mobile call connectivity based on the cellular system.
It needs the Handover procedure because a single cell cannot cover the whole service area of the GSM.
Causes of Handover in Mobile Computing and in GSM
There are two reasons for handover-
- Handover for Quality Improvement
Mobile station moves out of the range of a base transceiver system (BTS) or an antenna of a BTS.
The received signal intensity lows continuously until the signal falls below the minimum requirements for the communication.
The error rate may increase due to interference, the distance to the BTS may be too high also.
All these reasons may diminish the strength and quality of the radio signal and make the radio transmission impossible further.
- Handover for Load Balancing
To balance the load on MSC, and BSC the traffic in one cell may need to transfer on another cell is also another cause of handover
Handover Scenarios in Mobile Computing in GSM
In GSM following possible handover scenarios –
- Intracell handover
- Intercell, Intra BSC handover
- Inter BSC, intra-MSC handover
- Inter MSC handover
Intracell handover
Within a cell, narrow-band interference sometimes can make a transmission impossible. Therefore the BSC can decide to change the carrier frequency and therefore handover is required.
Intercell, Intra BSC handover
The mobile station moves from one cell to another cell in the same BSC. The BSC then do a handover. BSC assigns a new radio channel in the new cell and leaves the old one.
In the following figure inter-cell handover and intracell handover, both are shown-
Inter BSC, intra-MSC handover
BSC controls a limited number of cells. GSM do handovers between cells that are controlled by different BSCs. This kind of handover is controlled by the MSC.
Inter MSC handover
A handover could be possible between two cells controlled by two different MSCs. This handover is referred to as inter MSC handover.
To provide information for handover, mobile station and BTS both perform measurement of link strength and link quality of downlink and uplink.
Measurement reports are sent by the mobile station about every ½ second and this information contains the quality of the current link used for transmission and also the quality for some specific channels in neighboring cells.
The decision about the handover does not base on the actual value of the received signal level but based on the average value.