Mobile phone networks - Answers
Q1. Duplex transmission is communication that can happen between two devices in both directions at the same time.
Q2. Half-duplex is communication that can happen between two devices in both directions but only one can communicate at a time e.g. walkie talkies.
Q3. A SID is a Sysytem ID code, broadcast by cell base stations.
Q4. SIM stands for Subsciber Indentity Module.
Q5. When your mobile phone is switched on and you want to make a phone call, 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.
Q6. You find out how much radiation a phone emits by looking up the SAR value for the phone. This is published by the manufacturers and is available online.
Q7. The Stuart Enquiry investigated whether mobile phones were safe.
Q8. Mobile phones use radio waves.
Q9. Radio waves can heat the body up and effect the nervous system - not good, especially when you are young and growing up.
Q10. 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).
Extension tasks
There are lots of sites on the Internet that discuss whether mobiles and wifi signals are safe. Students should be encouraged to use a variety of resources. They should take note of the sources and ask such questions as whether research done by mobile phone companies can be relied on, what might motivate different stakeholders to produce the evidence they have, whether phone companies or others might withold any research that isn't favourable to their own business.