How can a phone call follow you along a road?

During a modern cellular call, the microphone turns sound into an electrical signal. The phone samples and codes it, then sends voice data in packets over a radio link. A base station connects that radio link to the operator’s network, which routes the call toward the other person.

One base station cannot provide a useful connection everywhere. Networks divide service into cells, and a moving phone measures nearby radio signals as well as its current connection. The network can use these measurements to prepare a target cell and tell the phone to change its serving connection. This change is called handover.

The diagram follows one call from cell A to cell B. While B is being prepared, A still carries the illustrated packets. Once the change completes, B carries them instead. Coverage overlaps and real handovers involve much more signaling than shown. Brief interruptions and failed handovers are possible; a cell boundary is not a painted line on the road. The person’s voice continues through a routed communication system, rather than being shouted directly from one phone to the other.