Mobile phones and Cellular Networks
Wednesday, August 4th, 2010In America they call mobile phones cell phones. Perhaps this is because in the UK we are more interested in their portability, whilst in the USA they are more interested in how they work. Whatever we call them, they both work in the same manner; that is over a cellular network.
A cellular Network is a network of radio transmitters and receivers that is composed of cells, and each cell has a single fixed location which is known as its base station. The cells are interconnected so that they cover a large geographical area. The mobile phones connect with the base stations and the base stations communicate with each other so that they can relay the calls between phones. Phones can move through adjacent cells during a call without losing connection.
Cells are generally defined as hexagons, though alternative shapes are possible. Each cell will have a number of assigned radio frequencies that allow them to communicate with adjacent cells. With hexagonal cells there will be six frequencies, and with square cells there would be four. Specific frequencies can be re-used in different parts of the network as long as steps are taken (such as limiting signal strength) to ensure that the cells do not interfere with each other. The ability to re-use frequencies is one of the factors that make cellular networks so efficient. This network of cells is called the Base Station Subsystem.
Voice calls and text messages are handled by the core switched network; mobile data transmission is handled by a packet switched network; and a public switched network is used for making connections to land lines. A mobile phone connects to the network through the corresponding cell of a base station subsystem. This in turn connects to the mobile switching centre (core circuit or packet switched) which itself connects to the public switched network.
When a phone moves between cells, the mobile station that it is leaving looks for an alternative channel on an adjacent cell and when it has located one sends a signal to the mobile instructing it to switch to the new channel and simultaneously switching the call to that channel.
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