a handoff
A handoff (called handover in many places) is the moment a moving device switches its connection from one base station or access point to another without dropping the conversation. Picture a relay race where the baton must pass cleanly from one runner to the next while everyone keeps moving — fumble it and the call drops or the download stalls. As you drive, walk through a building, or ride a train, your phone is constantly being handed off so it always talks to whichever cell or AP currently gives the best signal.
Here is roughly how it works in cellular. Your phone continuously measures the signal strength of its current base station and of neighbors it can hear, and reports these to the network. When a neighbor becomes clearly better — because you are moving toward it and away from the old one — the network arranges a handoff: it tells the new base station to expect you, coordinates the switch, and your traffic begins flowing through the new cell, ideally before the old link fails. A good system makes this seamless; a borderline area where two cells are equally weak can cause repeated back-and-forth handoffs, called ping-ponging. Wi-Fi does the analogous thing when you roam between access points sharing one SSID, though classic Wi-Fi handoff is often clumsier and may briefly break the connection.
Why it matters: handoff is what makes mobility usable — it is the difference between a call that follows you across town and one that dies the moment you leave a single tower's range. It also creates work elsewhere in the stack: because you may get a new IP address at the new location, higher layers can be confused about where you are (the problem Mobile IP tries to solve), and a TCP connection can misread the brief loss during a handoff as congestion and needlessly slow down.
Mid-sentence on a phone call in a moving car, you cross from one tower's coverage into the next. The network hands your call off so smoothly you never notice — but if you had stopped exactly on the boundary, you might hear it briefly stutter as the phone ping-pongs between two equally weak cells.
Pass the baton cleanly and the call follows you.
Handoff keeps the link alive, but it can change your IP address and cause brief loss — which is exactly why mobility confuses both addressing (the job of Mobile IP) and TCP, which can mistake handoff loss for congestion.