Splat | WiseTome.com

In the last post on cellular network, you saw that the geographic area of a cell phone service provider is divided into cells - roughly hexagonal shaped areas that serve a particular region. The cell site provides coverage for that particular cell area. This results in the use of quite an interesting technique called handoff.

What happens when the caller (or the called) travels from one cell to the adjacent during a call? How does the network handle it? How does the network know which cellular network tower should be used to keep the connection between the caller and the called in order to continue the call?

The SIM card in a cell phone will be constantly identifying its location back to the tower. It will be keeping track of it and storing it in the LAI. A combination of the location and the usually the strength of the signals from the adjacent towers, helps the network make a handoff of the call. The moment a caller crosses the imaginary boundary of a cell and enters an adjacent one, handoff takes place, and the tower of the entered cell starts to handle the call.

There usually are two types of handoffs - hard handoff and soft handoff. Hard handoff is the break and make configuration. The call path is broken with the old cell and made to the new cell. In soft handoff, both the connections are kept for a duration, and after some time, the older cell connection is broken. The choice between the two types of handoff comes to the type of the network - GSM or CDMA. GSM being an analog technology, makes it costlier to opt for a soft hand off because the cell phone must be equipped with two transceivers. On the other hand, for digital networks like CDMA, since it is a logical division within the bandwidth, soft handoff can be used.

Handoffs are not usually a common occurance since a subscriber usually is within a particular cell. But numerous handoffs are made when you travel constantly over long distances. This is why, it is generally advised to switch off your cell when you travel on train or bus over long distances if you want to avoid discharging the cell battery. Handoffs increases the power consumption of the device since the signal strength near the borders of cells are weak. Amplification of signals involve use of more power from your cell phone battery.