Mobile Fundamentals · Cellular Networks · Lesson 1 of 4

Talking to Towers

Cells, towers and handing a call off at 60 mph.

A map of cells

Your carrier can't cover a whole country with one giant antenna. Instead it splits the map into lots of small areas called cells. Each cell is served by an antenna on a tower or rooftop. One cell might cover miles of countryside, while in a busy city a cell may cover just a few blocks, because more, smaller cells can share the load.

Your phone is always listening to the cells around it and measuring how strong each signal is. When you're idle, it settles on the best one. During a call, the phone keeps reporting what it hears, and the network decides when to move you. Drive across town while talking and your call is passed from cell to cell, usually without you noticing a thing.

To deliver calls and texts, the network has to know where you are. When your phone is idle, the network tracks it by a group of nearby cells. During a call it knows the exact cell. Carriers keep records of this, which is why investigators often request cell site records to place a phone in an area at a certain time.

Name the move

Moving a live connection from one cell to the next is called a handover. In the US you'll often hear handoff instead. Both mean the same thing. The 3GPP standards, which define GSM, 3G, 4G and 5G, use handover.

Here's roughly how it works. While you're on a call or using data, your phone sends the network reports about the signal strength of its current cell and the cells next door. When a neighbor looks clearly better, the network prepares the new cell and then tells your phone to switch. Your call or download carries on.

When your phone is idle, there's no live connection to hand over. The phone just moves to a better cell by itself. That's called cell reselection.

Handovers matter in investigations too. Carrier records showing which cells a phone used during a call can show it moving along a road. Keep in mind that each cell only places a phone somewhere inside that cell's coverage area, which can be large.