Handover and handoff are different words for the same idea
On a WiFi system, where mobility is not the norm, a device will stay connected to a particular router. If the device moves then eventually the signal will be lost and the device will attempt to another router. There will be a break in transmission. This is a hard handover and in the mobile phone world this should be a last resort. It would be beneficial if a MS moved from one cell to another without losing the signal
There are a number of different reasons and types of handover :
A hard handover is where the communication with one cell is lost before the connection to the next cell is made. This will always be the case when roaming and a different mobile operator is chosen. Also if the MS needs to change the frequency used then a hard handover must be enacted; a MS cannot transmit / receive on two different frequencies at the same time.
A soft handover offers near continuous transmission when a MS moves from one cell to another. The MS continuously monitors the cell strengths of the different masts that it can see and reports back, through the Base Station, to the MSC. It is at the MSC the decision is made as to whether to switch. If the two cells use the same MSC then this is simpler than if there are two MSC involved. In either case the phone is effectively connected to the new cell and the current MSC will at some point make the switch, seamlessly.
A softer handover can be undertaken at the periphery of the network if the two cells involved are both connected to the same BSC. Then the BSC will do the switching, and inform the MSC of the change.
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