How Networks Operate

Mobile communications systems must establish a network of sites serving areas called cells, giving rise to the terms cellular communications and cellphone. Sometimes this is represented by a honeycomb pattern but in practice cell shapes are not so regular.

The size of each cell depends on current and projected number of calls, the physical terrain (signals are blocked by man-made and natural obstacles such as buildings, trees and hills) and the frequency of operation.

Each antenna site or base station consists of a mast or existing building to support the antennas and associated transmission and network equipment. The radio signals are transmitted by the antennas and not by the supporting structures. Base stations are connected to a mobile switching centre (similar to an exchange). A call goes from the mobile device via the base station to the mobile switching centre from there it is connected to its destination, another mobile or fixed phone or the Internet.

The number of frequencies available for use by mobile networks is small compared to the number of subscribers so the same frequencies have to be re-used. To avoid interference, base stations using the same frequency must transmit at low power and be separated by distance.

As the mobile phone user moves, the call is passed between neighbouring cells, a process called handover. If there are gaps in the available coverage the call could be dropped. As mobile phones are low power devices the antennas need to be located near to mobile phone users. This also ensures that the mobile phone transmits at the lowest power consistent with call quality. This also limits interference between cells and increases talk-time.

In order to avoid congestion as the number of users grows the network operator will reconfigure the network and divide large cells into smaller sizes. As the cell size decreases, so also does the transmit power. Larger cells are called macrocells and usually cover cell sizes of more than 1 km. A city centre may have microcells with a range of a few hundred metres. Inside a building such as a shopping centre even smaller cells my be present, these are picocells with ranges of a few tens of metres. A recent development is the femtocell that provides service to an individual home or office. Macrocells may transmit up to 100 watts but picocells or femtocells transmit at levels similar to other wireless home network equipment.