Whenever a mobile user moves out of one cell to another it must be possible to retain the connection. The process by which this occurs is known as handover or handoff. Cellular systems require handover procedures, as single cells do not cover the whole service area. The term handover is more widely used within Europe, whereas handoff tends to be use more in North America. Either way, handover and handoff are the same process.
Reasons for a Handover:
- If a traffic in one cell is too high; then Handover may be is required to balance load.
- If the mobile station moves out of the range of a BTS, received signal level decreases.to overcome it handover is needed.
- In GSM system there are four types of handover:
- Intra-BTS handover: In this form of GSM handover, the mobile remains attached to the same base station transceiver, but change the channel or slot.
- This form of GSM handover occurs if it is required to change the frequency or slot being used by a mobile because of interference, or other reasons.
- Inter-BTS Intra BSC handover: It occurs when the mobile moves out of the coverage area of one BTS but into another controlled by the same BSC.
- In this instance the BSC is able to perform the handover and it assigns a new channel and slot to the mobile, before releasing the old BTS from communicating with the mobile.
- Inter-BSC handover: When the mobile moves out of the range of cells controlled by one BSC, a more involved form of handover has to be performed, handing over not only from one BTS to another but one BSC to another. For this the handover is controlled by the MSC.
- Inter-MSC handover: This form of handover occurs when changing between networks. The two MSCs involved negotiate to control the handover.
Figure 8 Different types of hand over in GSM