Mobile phones – communications on the go
Key text
This topic is sponsored by the Australian Government's National Innovation Awareness Strategy.
Australia's mobile phone system is now into its third generation. But what is 3G, and how do mobile phones work anyway?
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You will get more from this topic if you have mastered the basics of electromagnetic radiation – these links will take you to an annotated list of sites with helpful background information. |
The main difference between a standard telephone and a mobile phone is that a standard telephone converts the sound waves of your voice into electrical signals which are sent along telephone lines and a mobile phone converts the sound waves of your voice into radio waves which are transmitted through the air.
Both types of phones require a transmission network. Standard phones use a network of wires; mobile phones use a network of antennas.
Base stations connect mobile phones to the communication system
Base stations consist of a number of antennas that both transmit and receive signals from mobile phones. These antennas are mounted on towers, or on the sides or roofs of tall buildings. After receiving the signal from a mobile phone, the base station then transmits the signals to a 'switching centre' - a telephone exchange for mobile phones. Here the call is connected - 'switched' – either to another mobile phone or to the telephone network. Although this sounds like a lengthy process, it is virtually instantaneous.
Base stations are in 'cells'
A mobile phone system requires a number of base stations. Each base station sits in the middle of a geographical area known as a 'cell'. (Mobile phones are also known as 'cellular' phones.) The number of calls (or 'traffic') that a base station can handle at any one time is limited by engineering design constraints. To operate the mobile phone network as efficiently as possible, base stations are located so as to maximise the number of calls that can be connected during peak use periods. Therefore, the geographic size of a cell depends on the traffic during these periods. Cells in populated areas, with many mobile phone users, will be smaller than cells in less populated areas.
As you cross the boundary of one cell, the cell next to it will automatically take over. This is called a 'hand-over' or 'hand-on', and it is controlled (usually without you noticing) by a computer in the switching centre. The switching centre knows which cell your mobile is in, and switches it to the next cell if you move across a boundary.
Your call will continue to get 'handed on' to each cell in turn until you reach the edge of the last boundary. Then you will be out of range of any cells and your call won't be transmitted.
Mobile phones 'check in' regularly to base stations
An important thing to remember is that mobile phones cannot talk directly to each other without going through a base station first.
- When you switch on a mobile phone it will send out a coded message
saying something like: 'I am here, is anybody listening?'
- If the
phone is inside a cell boundary there will be a base station receiver
in range, and it will send back a message saying: 'Yes I can hear
you, tell me what your number is and I will let you know if any
messages have come through for you.'
- When the base station at
the centre of the cell has got the number of the mobile handset,
it will then send out more messages. This time the base station
will communicate (by radio waves or telecommunication cables)
with a central computer in the switching centre and say: 'I have
a mobile phone number in my cell right now, are there any messages
for it?'
- If there is a message, the central computer will direct
it through to the cell's base station transmitter, which will
then broadcast it across its cell, where it can be picked up by
the mobile phone.
- If there are no messages, the computer will keep a record of which cell the phone is in, and if a message arrives later it will know where to send it. When it is switched on, a mobile phone talks to cell base stations at regular intervals. This means the central computer should always know where to send messages.
Australia's mobile phone systems
Mobile phones in Australia used to operate on both analogue and digital systems. The analogue Advanced Mobile Phone System (AMPS), was first established in 1987 and it formed the basis for the early expansion of mobile phone use across the nation. It used frequencies in the range 825-890 megahertz. Within this range of frequencies, there were 666 channels for receiving and 666 for transmitting. Analogue technology such as AMPS is referred to as first generation technology.
The digital Global System for Mobile Telecommunications (GSM), which has been in use in Australia since 1993, uses the frequencies 890-960 megahertz. Within this range of frequencies, there are 124 channels for receiving and 124 for transmitting. The digital system has the advantage that each channel can handle up to eight calls simultaneously.
In the early 1990s, the Australian government passed legislation ruling that GSM would take over from AMPS in the year 2000. However, because GSM has a much smaller range than AMPS, it is difficult for this system to provide adequate mobile phone coverage in country areas. Telstra upgraded its AMPS network to another digital technology, Code Division Multiple Access (CDMA). CDMA uses the same frequencies as AMPS and can provide the same coverage to country areas but it offers the benefits of digital technology. GSM and CDMA are both referred to as second generation – digital technology.
In 2003, the third generation (3G) network was launched in Australia. Unlike previous systems, 3G allows voice, data and video transmission and operates around the 2100 megahertz frequency band. Another change, important for those in remote areas, has been the launch of more telecommunications satellites allowing mobile phone coverage in areas beyond the reach of other networks. Base stations have been replaced by 'low Earth orbit' satellites (LEOs), which circle the Earth low enough to pick up your call. The LEO then re-transmits your message just like a base station.
The rapid change in mobile phone technology has come at a price, with an increasing number of users regularly discarding their old mobiles in favour of the latest technology. Mobile phone manufacturers have become aware of the need to consider the effect of this rapid turnover of mobiles on the environment (Box 1: Cradle to grave)
Mobile phones and health
Mobile phones are very useful and are growing in popularity. But some people are concerned that there is a possible health risk associated with mobile phone use. The mobile phone network uses radio waves (also called radiofrequency radiation), which is a part of the electromagnetic radiation spectrum. Some mobile phone users are concerned that having the phone's antenna close to their head exposes them to levels of radio waves that will increase the risk of brain cancer.
With the research that has been done so far, scientists generally agree that radio waves cannot cause or promote cancer, but there have been occasional conflicting studies. Research into these health issues continues around the world. The Australian government has supported research into possible links between mobile phones and cancers on a range of fronts. At the international level, the World Health Organization has initiated an Electromagnetic Fields Project to standardise international research efforts.
Precautionary options
Options to reduce exposure are available for mobile phone users who are concerned about the possible health risks. Minimising call times is obvious, but using hands-free or an earphone allows its use away from the user's head. Mobile phone covers that claim to shield the user's head are also available but there is little research to support this claim.
Page updated October 2008.