How does a mobile network hand over between masts?
Continuously and invisibly — your phone is constantly measuring nearby cells and the network moves your connection between them, ideally without you noticing.
What a cell actually is. A mast site is usually divided into sectors, each with directional antennas covering roughly a third of the circle, and each sector runs cells on several frequency bands. A single mast therefore provides many cells, and "moving between masts" frequently means moving between sectors or bands on the same one.
Cell size varies enormously. Macrocells cover kilometres in rural areas; small cells cover a street or a building interior. Higher frequency bands carry more capacity over shorter distances, which is why dense urban areas use many small cells.
How handover works. Your phone continuously measures the signal strength and quality of its serving cell and of neighbouring cells, and reports those measurements to the network. When a neighbour becomes sufficiently better — by a configured margin, sustained for a short period — the network instructs the phone to switch.
The margin and timer matter. Without them, a phone sitting between two cells of similar strength would bounce between them repeatedly — ping-ponging — which consumes signalling capacity and degrades the connection. Hysteresis deliberately makes the phone reluctant to switch until the improvement is clear.
Hard versus soft handover. Some technologies connect to the new cell only after releasing the old one; others maintain both briefly so nothing is lost. Modern networks use techniques that keep the session continuous, which is why a call rarely drops at a boundary.
Why calls still drop. Handover fails if the phone moves too fast for the measurement cycle, if the target cell is full, if there is a coverage gap — inside lifts, tunnels and modern buildings with metallised glazing — or if the network is misconfigured.
Moving between technologies adds complexity, and falling back between generations for voice is a recognised source of setup delay and occasional failures.
Why your phone shows bars that do not match performance. Bars indicate signal strength, not quality or capacity. A strong signal from a congested cell performs worse than a weaker one from an empty cell — which is why full bars and no usable data is a normal, not a faulty, combination.