Subscriber Identity Modules, or SIMs to use the common acronym, are chips in the phone which hold a wide variety of information - anything from the network that it belongs to, to address books. SIMs come in two sizes - small and large. Large SIMs look like a credit card with a chip in the middle, and small SIMs look like a small flat computer chip; In fact, the only difference between the two is there is plastic around the chip for the large SIM - the chips work in precisely the same fashion. Each SIM contains an absolutely unique IMSI number which is used for billing customers and this information is never given out by the networks to avoid the possibility of 'cloning' the handset numbers.

When a GSM phone is switched on, the first thing that it will do is try to register with a 'Point of Presence' (POP) Site. POP sites are also sometimes called 'Base Stations', 'Ground Stations', 'Cell Sites' and a few variations like that. You normally don't notice these - they are designed to look like other things, like lamp-posts for example.

The mobile phone, when registering, will try to contact any POP sites in the area on a particular frequency, and will ask the nearest POP site for a frequency and a time-slot that it can use. Mobile phones are basically short-range radios, so as soon as it gets the frequency that it needs, it will retune itself to the new frequency, just like you might retune your radio to a different radio station.

While a phone is registering with a POP site, the POP site will contact the network that owns the POP site, to ask if that phone may be used on that network, and it will not allow the phone to register if not. If the mobile phone cannot register with one particular POP site, it will check all available POP sites in the area before giving up.

The reception of the signal depends on several factors, probably the most major being line of sight. If there can be a direct line drawn between the phone's transmitter and the POP site transmitter, then reception will be flawless. As the direct line slowly becomes blocked or has to curve round or go through objects then reception will slowly diminish. If the phone goes into a large tunnel, then chances are that the phone will lose communication with the POP site altogether, until the phone emerges at the other end of the tunnel.

Another factor to take into consideration is distance - the maximum distance between a mobile phone and a POP site is 35 kilometres, though this limit is unlikely to be reached in a densely populated area, or even a motorway. The network's tend to place POP sites where they are most used, but they do place them elsewhere to boost their claim that they have the best coverage of any network in their particular country. There may be a problem if you go to, say, the Orkney Islands off the coast of Scotland and try and use your phone there, or to anywhere with large amounts of granite such as the Lake District where phone signals are easily blocked by the absorbent stone.

One final factor, is quality of the ionosphere, which affects all radio signals. On normal radio signals (mainly LW/AM frequencies), the signal may 'fade' in and out, even though the radio and aerials are not moving; this is due to a shifting layer of the atmosphere about 15 kilometres up called the ionosphere which deflects radio waves, and cause this 'fading'.

The networks have fewer POP sites than you might think - at the time of writing, Cellnet currently has around 2,500 POP sites which serve the whole of the UK, and Cellnet is the second biggest network.

When a phone registers with it's own network, the network records the handset number and SIM, and places the information in what is known as a 'Home Location Register'. If a phone registers with another network apart from it's own (in another country, for example), the user's home network is contacted to get various details, and the results are placed in the 'Visitor Location Register'. That handset is then officially 'Roaming' on another network.

For example, if I have a Vodafone SIM and handset number, and I travel to Guernsey (one of the channel isles), the phone will try to register with a Guernsey Telecom POP site. Guernsey Telecom will contact Vodafone to get all details about the 'Visitor' handset, and the handset will become operational. Vodafone also knows now that the handset is roaming in Guernsey, and to route any calls made for that handset number to Guernsey Telecom.

If someone is roaming, and someone dials them, the user of the phone must pay some of the cost of the call. The reason behind this is, that whoever is dialling the mobile phone does not necessarily know where the user with the mobile phone is. The caller should pay the same amount for calling, wherever the owner of the phone is in the world, and the owner of the phone must pick up the difference. This is the part of the reason why 'Pre-pay' phones cannot normally roam outside of their own network. June 2001 Update: Some Vodafone and Cellnet pre-pay handsets can now roam abroad, but the number of countries is extremely limited.

Save yourself a fortune: Remember to remove all Diverts on your mobile phone, before you go abroad. I've seen bills of £8000 run up because people forgot. The reason is that if someone calls you when you're abroad, you pay for the leg of the journey out to you. If the handset then diverts back to the UK, you now have to pay for the second call which is diverting back. Quite easily, you can run a huge bill without ever making a call.

Networks and service providers are notorious for selling tariffs which are both difficult to compare and complex enough to discourage further investigation. Please note that the information about amounts is correct at the time of writing (21/07/00) but will change. Some notes follow about some of the offers that service providers use, and some the sneaky tricks that they can play.

Service Providers often offer 'bundles' of either free minutes or discounted calls for payment in advance. These almost always have exceptions - calls which cannot be included in bundles, and unsurprisingly, the calls which can't normally be included are the really expensive ones, like mobile to mobile. Additionally, if a call is made and the service provider puts the call in the bundle, the amount deducted from that bundle will be rated at Peak rate (and therefore be more) - this mainly affects money bundles, but can also affect time bundles, when a call is 'half in' and 'half out' of the bundle. In general, only take out a bundle of free minutes if most of your calls are to land-lines, or mobile phones on the same network as you.

Some unscrupulous service providers have been known to 'increase' the call duration slightly so that they can charge a little more, but this is uncommon nowadays as most phones have call monitors.

If you agree a line-rental deal with a service provider, this normally ties you in with them for 12 months where you have to use the tariff agreed when you bought the phone, and pay for any line-rental and call charges that are due. Most service providers will try to sell the actual handset very cheaply, and re-coup the cost in line rental and call charges over the full year, though some new players like Virgin are experimenting with it the other way around (pay the full price for the phone, and call charges and line rental have minimal margins).

Peak and off-peak times can vary between service providers, and often extend well beyond what BT offers for it's land lines. BT charges it's phone calls at peak rate between 8.00 and 6.00pm, whereas Vodafone uses 8.00am to 7.00pm as standard. If you make a 5 minute call that starts at 6:59 though, two rating mechanisms should be applied - one for the peak-rate portion of the call, and one for the off-peak portion of the call.

Calls to land-lines are normally fairly cheap nowadays, or given away in 'bundles' of free minutes of call time. However, the calls you need to look out for especially are mobile to other mobile network phone calls which in this country are extortionate, but don't pay any more than 50p per minute. Yes, believe it or not, it's cheaper to phone most countries than phone another mobile in this country.

Text messages are cheap to send, and should cost 10p no matter where they are sent. Some Internet companies also allow you to send text messages to mobiles for free. They can even be bundled sometimes. I was talking to a guy in a mobile phone shop the other day though, and he mentioned that in a couple of years, you would be able to send a text message just by voice-recognition - dictating the message. I pointed out to him that you could always phone the person instead. The march of technology goes ever onward. Orange and Cellnet currently have various services to provide you sport/lottery updates via text messages, and the other networks should be offering this service soon. These are not charged at the standard 10p rate, but at a charge set by the person providing you with the information (Pay no more than 30p).

International calls from mobiles are generally fairly reasonable - normally a little higher than the standard BT rate, but not by much. e.g. A 20 minute call on a mobile to the USA should cost about £3.10.

Roaming calls (when abroad) tend to be quite expensive. e.g. A 5 minute call from France back to the UK on Vodafone will cost about £3.70. Not everyone knows that if someone calls you, while you are roaming, that you will pay for part of the call's cost - in fact, the person phoning will pay the standard land-line to mobile rate (if he's calling from a land-line that is), you will have to pick up the charge for the international leg of the call's journey. Unsurprisingly this is quite expensive.

Other Services, such as Traffic update information, McDonalds Restaurant finder (1501 on Cellnet FindMe service) normally are quite expensive, either being comparable to mobile to other network rates or premium rate calls.

In general, find out what network your friends are on, and choose that network. The most expensive calls that you can make are mobile to other network's mobile, and cutting down this type of call out as much as possible will probably save you more money than the various competing deals that the networks can offer. The networks tend to price-match tariffs as much as possible, but Orange and One To One generally seem to be slightly cheaper than Vodafone or Cellnet.

The networks offer roughly the same quality of service as each other, but there are various 'definitions' of quality of service. The main ones are:

Coverage: Is the percentage of the total country's landmass that a call can be made from in theory. So, if Vodafone claims that it has 99% coverage, that means that in only 1% of the country can you not, in theory, make a call. This statistic is a little misleading however, because it assumes perfect conditions, so in practice the coverage is not as good as they say. But if you can't get a signal when you're in the local pub, it's probably because the pub has thick walls, rather than any fault of the network's. Vodafone have the best coverage of any network in the UK, though the difference between the networks is minimal nowadays.

Successful Calls: Is the percentage of calls that are a) made successfully, and b) completed successfully. Orange are easily the best network under this statistic in the UK.

There are numerous manufacturers of mobile phones, and it is sometimes difficult to choose which one is best. In general, phones made by Scandinavian companies (Nokia - Finland, Ericsson - Sweden, etc.) are of very high quality and have the most features. However, some US companies such as Motorola are almost as good. The reason why Scandinavian companies seem to dominate the market is because they were the first countries to truly adopt GSM, and so they have a head-start on all the manufacturers from other countries.

There has been much hype in the press over whether mobile-phone radiation can cause brain tumours. Although some of the claims can be denied, several studies have proved that radiation from phones has in the past been a factor in causing tumours. Much of the problem stemmed from the immensely powerful transmitters in old analogue phones that are now obsolete. As transmitter power levels continue to drop, the risk should similarly drop. Output from a 3.5 volt battery is hardly likely to fry your brain nowadays!

There has never been any causal link proved between brain tumours and proximity of a POP site - the POP site only generates slightly more radiation than the mobile phones, and people generally don't put their heads next to the aerial itself, so as a result, there is no risk from aerials.

WAP provides a minimalistic access to the internet via a mobile phone. And it's as rubbish as people say it is. Roll on GPRS!!!

In Spring 2000 the UK government auctioned off licenses for the next generation of mobile phones. The next generation look like they will be a cross between a phone and a small hand-held computer. The next generation will be 'packet-switched', enabling a permanent connection between the phone and the network it is using. Additionally, the speed of information transfer will multiply immensely, offering up to 2Mbps worth of transmission speed. This will allow data fast enough to provide video-streaming to your mobile phone. So for the price of a quick call, you'll be able to watch TV Soaps such as Neighbours on the bus on the way home - the miracles of modern technology?!?

The next generation will be able to do everything that the existing mobile phones can do, but with the added bonuses that fast-transmission speeds can offer. The 3^rd generation phones are likely to be expensive to buy and use however, as an immense amount of money needs to be ploughed in to get these new phones off the ground. It has been estimated that because of the investment that the networks have put in to buy licenses, install infrastructure and implement control systems, they will have to make £70-100 per year off each user - and that's without allowing for any profit margins or running costs.

UK Networks
• Vodafone
• Cellnet
• Orange
• One To One

GSM information
• GSM World contains a wealth of information about GSM and networks across the world.
• Oftel is the government regulator for telecommunications in the UK, and should have some reliable data on QOS of each network.

Phone Manufacturers
• Nokia
• Ericsson
• Motorola

Service Providers
• Intercity are an independent (this is rare nowadays) service provider specialising in corporate accounts, with the focus on excellent quality service.
• Virgin mobile have good low-cost charges, even if they do run on the One 2 One Network (They won't tell you this, but they do!)
• Mobile Needs are a pure internet-based mobile phone service provider aimed at individuals, with low call costs and reliable service.
• Car Phone Warehouse are the UK's largest dealer who've recently moved into service provision. Watch out for the hidden charges though!

SIM - Subscriber Identity Module. This is the removable chip inside the phone (normally found underneath the battery) that contains information such as the handset number, your address book, etc.

GSM - Group Speciale Mobile (or Global System for Mobile telecommunications, depending on who you talk to).

ESN - Electronic Serial Number. Related to analogue phones, though these are obsolete now.

TACS/ETACS - Is the system that is used to make analogue mobile phone calls, though it has been replaced by the GSM system now.

MSISDN - (Deep Breath…) Mobile Subscriber International Subscriber Dialling Number. Your phone number.

GPRS - Global Packet Radio System. Will be used by Cellnet and other networks to provide reasonably fast access to the internet, as soon as someone invents equipment to use the new technology. Speeds of 60kbps are likely eventually, before becoming obsolete and being replaced by the next generation of mobile.

SMS - Short Message Service - the text messages that you can send on your phone.

IMSI - International Mobile Subscriber Identifier - a number, never given out by the networks that is used for billing purposes

IMEI - International Mobile Equipment Identifier - the unique number that identifies the hand-portable phone you are using (e.g. Nokia 3210 phone).

Cloning - Taking a handset and 'cloning' the phone number and SIM of another handset so that it can be used to illegally make calls. The old analogue phones used to be very prone to this, but so far (touch-wood) no one has managed to clone a GSM phone.

HLR - Home location register - contains a list of all the phones that can be reached that are on the home network

VLR - Visitor location register - contains a list of all the phones that can be reached that are not on the home network.

WAP - Wireless Application Protocol. Uses SMS (text) messaging to access the internet. Very slow and boring, not much there to look at just yet. See GPRS.

TETRA - Tetra operators, such as Dolphin are not GSM network operators, but the networks work on many of the same principles. The network equivalent of VHS and Betamax (GSM being the VHS). Tetra networks require different equipment anyway, so that you can't use your Nokia 3210 phone on the Dolphin network.