“Ex-CELL-ent…”

A Telstra mobile phone base station
Wireless Hill, Ardross,
Western Australia.

Mobile phones work through a system of radio waves and towers or antennas to transmit calls. Each area around a tower or antenna is known as a cell.

Radio waves travel through the air from the base station in each cell to mobile phones. Each base station typically comprises of a tower and a small building containing radio equipment.

The tower transmits radio signals at low power. Mobile phones have low-power transmitters inside them that ‘talk’ to the tower of the cell they are in. Both towers and mobile phones transmit at low power, as the transmission does not reach beyond the cell.

Because each transmission stays within a cell, it is possible to reuse the same radio frequencies in different cells. This means that a smaller range of frequencies is required for people to have separate phone conversations. This compares to walkie-talkies where anyone using the same frequency and within range can eavesdrop on a conversation.

Each cell has a certain number of duplex voice channels available. Duplex voice channels have two frequencies – one for listening and one for speaking. The number of separate phone conversations possible in that cell is limited to the number of available duplex channels.

Let us suppose there are a total of 392 voice channels available in a town. The town is made up of a grid that has 7 cells.

Each cell will therefore have 56 unique voice channels (392 divided by 7). This means that in any cell, 56 people can be using their mobile phones at the same time.

Let us suppose a big city is made up of several grids with seven cells each.

This means that all the same colour cells can use the same 56 voice channels without interfering with each other because each transmission between the base station and mobile handset is low-powered enough not to step outside the cell.

This means that as the number of mobile users in an area increases, the network provider must build more radio towers and make cells smaller to prevent network congestion where all the available channels are in use. This is particularly important at major events where thousands of people might be attempting to make phone calls, surf the internet or send and receive photos and SMS messages at the same time.