Mobile Phones: How They Work Part 3 — the Traffic Channels
Introduction
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Mobile Phones: How They Work Part 3 — the Traffic Channels
Introduction
In the second part of this series we learnt about control channels and how these control how a mobile phone connects to the network and is able to work when moving from cell to cell. We are now going to examine the second part of how a mobile telephone network functions by seeing how data is transmitted between the telephone and the network itself. This is done through a second set of data channels transmitted by both phone and base stations within a cell. These channels are called the traffic channels.
With the diagram below I am going to show how these data channels and the network as a whole actually works.
The Beginnings of Digital Networks
It could be argued that the European conference of Postal and Telecommunications Administrantion meeting in 1982 represents a pivotal point in mobile communications. At this conference it was decided to set up the GSM (Groupe Spécial Mobile) to generate an Europe-wide standard for second-generation mobile communications. It took five years for the group to finally decide to pursue digital technology and in 1987 (with a change of name to Global System for Mobile Communications) the GSM standard was proposed for worldwide adoption. By today GSM networks are by far the most common networks globally and account for over 70% of digital mobile phone communications. Indeed, GSM is the only network used in Europe, Australia, the Arab World, West and Sub-saharan Africa. It is also the dominant network in Asia. Even in North America and much of South America GSM networks cover a significant proportion of the population and are gaining in popularity because of global interoperability.
In Europe GSM networks send and receive data using radio waves that operate at either 900MHz or 1800MHz (dual band). In the USA the dominant frequency is 1900MHz (tri-band) but the 850MHz band is also used (especially in South America) necessitating a quad-band phone to cover all possibilities. It should also be noted that certain sparsely-populated areas and less developed countries use the 450MHz band due to its longer range but no mobile phone (which would be penta-band) currently covers this frequency as well as all the others.
Each of the bands described above is actually split into two frequency ranges of 25MHz each so that the 900MHz band is split into 900–925 MHz and 925–950 MHz ranges. One of these bands is used by the mobile phone to contact the network whilst the other is used by the network to contact the mobile phone. Both communications bands can be used simultaneously which is why mobile phones are actually full-duplex radio communicators. (Standard radio communicators such as CB radios and portable radio ransmitters are half-duplex as the same frequency is used by both receiver and transmitter and you have to physically change from receiver to transmitter mode). This is why mobile telephones, though radio-based deivces, give you simultaneous transmission and reception of voice data.
How the GSM System Works
If there was only one 25MHz band for transmission and one for reception then only a single user could use a mobile network. Which, obviously, makes no sense whatsoever. As a result the GSM standard stole a trick from the original analogue networks. Because the frequency ranges that each operator has are limited the area to be covered by various mobile networks are split into discreet cells (see image above). Each cell has its own base station and the power and configuration of this base station determines the size and shape of the cell. Most cells are circular and built-up areas use many low-powered cells (as this maximizes the number of calls that can be handled). Less populous areas have larger cells whilst rural areas can have very large cells indeed. Roads and other linkways have narrow beam base stations that are very long but narrow in shape. All the cells overlap so that mobile phones can move between cells without loss of communication (as described in the second part of this series). Each cell actually only uses a portion of the available frequencies. This allows the frequencies to be distributed between cells so that no ajoining cell uses the same frequency. This means that mobile phone users in ajoining cells can use their phones without causing each other interference.
To further increase the density of calls that can be made within a single cell each band is divided by Frequency Division Multiple Access (FDMA) into a number of carrier waves; each of which are 200kHz wide. Each carrier wave is then divided into eight equal time slots, called bursts which individually last more than a millisecond. In effect each burst is a new channel and this allows each carrier wave to support six separate calls. This system is called Time Division Multiple Access (TDMA) and all a mobile phone needs to work on this system is to know which frequency to tune into and which burst number in the frame represents the channel available to it (and this is where the Control Channed described in the previous section comes into the equation.