Mobile phone networks

cellphoneMany people have mobile phones now but how does a mobile phone network work? A mobile phone is simply a radio that can both send and receive voice sounds in the form of radio waves. It can do this at the same time because it uses one radio frequency to send voice signals and a different one to receive them. This is known as ‘duplex communication’. Consider a country such as the UK and a mobile phone company called Theteacher Telecom. Theteacher Telecom will split up the country into areas called ‘cells’. In the middle of each cell will be a ‘base station’. The base station is simply a radio transmitter/receiver. Base stations will typically be positioned 500 metres apart in cities but may be 10 km apart in the countryside.

When you switch on your mobile phone, your phone listens out for a special signal that is constantly being broadcast by the base stations. This signal is known as the System Identification Code, or SID. When your phone picks up a SID signal, it then knows that it is within range of a base station in a particular cell. If it cannot receive this signal then it displays a message on your phone that it can’t find your network. This means you can’t make a phone call! Assuming that your phone has received the SID, your phone then transmits a signal that identifies your phone and where you are (which cell you are in). This signal goes via a base station transmitter to the area Mobile Telephone Switching Office, or MTSO. The MTSO keeps track in a database of all the phones that are in that MTSO area at that time and which cell they are in.

When you want to make a phone call, the MTSO will allocate your phone a unique pair of frequencies to use from the hundreds it has available for the duration of the call. When you make a call, you will send your voice on one of these frequencies via the base station’s transmitter in the cell you are in to the MTSO. The MTSO will then relay your call to the destination using the normal Public Switched Telephone Network, the PSTN. The PSTN is the normal land-based telephone system that we use. When you listen to someone talking, the process is reversed using the other frequency you were allocated.

Whilst you are making a call, the strength of your radio signal is being monitored. The transmitter in the cell you are in as well as the transmitters of the adjacent cells do this. If you move towards the edge of a cell, your radio signal will reduce. As you cross into a new cell, your phone call will come under the control of a new transmitter and leave the control of the transmitter of the cell you were just in. This happens reasonably seamlessly as this change is coordinated by the MTSO. You may have noticed, though, that when you are in a train, for example, the signal does diminish sometimes. This is the effect of you moving to the extremes of a cell’s boundary and the limit of a particular transmitter.

Are mobile phone networks safe?
It has been known for a long time that radio waves can cause a heating effect on the body and can affect the efficiency of the nervous system. This had led to mounting speculation on the effects of mobile phones. Whilst people working with powerful radio waves have complained of problems in the past, mobile phones use very low power radio waves. To investigate the problems associated with mobile phones, the Government set up an independent enquiry called the Stuart Enquiry. The scientists reported in May 2000 and found: that there was little evidence that mobile phones caused health risks but that this may be due to gaps in their knowledge and the fact that the long-term effects of low power radio waves are not documented; they acknowledged that radio waves do affect cells in the body but this may not necessarily result in a health problem; children’s bodies are developing and they may therefore be more susceptible to health risks than adults; mobile phone companies should publish the SAR value of each phone. This is a measure of how much radio energy a type of phone transmits to the body; there was no evidence as yet of any damage to people living close to a base station; drivers are at risk of accidents if they use their phones whilst driving and should be encouraged to use hands-free sets. Various reports have recently indicated that pupils in schools may be at risk from both WIFI signals and from the radio signals from their phones. Reports can be found e.g. on the website.

Problems associated with mobile networks
There are problems associated with any mobile hardware, be it a mobile phone or a computer on a mobile link. These include:

    * poor bandwidth (limited information can be transferred to the mobile device)
    * poor reception (the signals received are subject to atmospheric conditions
    * the presence of mountains and valleys and the proximity of transmitters or base stations)
    * poor security (transmissions can be intercepted and captured with the right equipment)
    * hardware limitations (these include the power available from the battery, the limited display available, poor keyboard design and bandwidth limitations). 

Q1. What is meant by duplex transmission?
Q2. Another type of transmission is known as 'half-duplex'. Find out what this means.
Q3. What is a SID?
Q4. A mobile phone has a SIM card. Find out what SIM stands for?
Q5. Describe what happens when your mobile phone is switched on and you want to make a phone call.
Q6. How can you find out how much radiation a phone emits?
Q7. What did the Stuart Enquiry investigate?
Q8. What type of waves do mobile phones use?
Q9. What effect can these types of waves have on you?
Q10. What are the problems associated with using mobile phones?

Extension tasks
a) Are mobile phones safe? Investigate.
b) Is wifi safe? Investigate.

There are plenty of sites on the Internet that discuss these two questions. Draw up lists of the arguments for and against whether they are safe or not.