What a modem (the word is derived from MOdulator DEModulator) does is to convert the digital signals produced by your computer into audio ones that can be sent down a telephone line. The modem at the other end converts the signal back from sound into an electronic form the remote computer can understand.
It isn't quite as simple as just buying a modem and plugging it in, however - you have to make sure you have the right features, the right standards and the right software. If you intend to connect your modem to BT's telephone network, it has to have British Approvals Board for Telecommunications (BABT) approval - it should have a little green circle on it somewhere. If you connect a modem that isn't BABT-approved to the BT network, then technically you are breaking the law.
BABT-approved modems are likely to have better design and components than non-approved modems, which have to display large red triangle stickers to show their unworthiness. (Of course, modems bought abroad will not have either set of markings; by definition, they're not BABT-approved.) In the past, approved modems were significantly more expensive, but increasing competition in the modem market has brought prices down significantly.
The prime measure of a modem is speed: how quickly it can squeeze the audio tones over the line. The faster the modem, the less time you'll spend racking up phone bills while sending and receiving data (although if you're reading text online, it won't make any difference - a point worth noting).
Speed is measured in bits per second (bps) and the numbers in a modem's specification tell you how fast it is. The minimum acceptable speed for cruising the Net is 14,400bps, while a 28,000bps modem is preferable in this age of graphics-laden World Wide Web pages. A 14,400bps model (often referred to as a "V32bis" modem, for reasons lost in the smoke-filled rooms of international standards organisations) will set you back around pounds 100, while a decent 28,800bps modem (V34) will cost another pounds 60 to pounds 80.
Modem prices have been falling, and manufacturers such as Motorola, Pace and US Robotics are trying to cater for Internet-hungry computer- users by producing bundles that include a modem and the software you need to get on the Net.
But before you plump for that super-fast 28,800 beast, check which speeds are offered by the services you intend to use. CompuServe only offers 28,800 access in London, for example, but it aims to upgrade all of its access nodes or "points of presence" (PoPs) by March 1996. Many Internet service providers are upgrading their PoPs to 28,800bps, making it worth splashing out because you'll be able to cut your phone bill by spending less time online.
When you have decided on the speed and sort of modem you want, you should then weigh up the ease of use and extra options provided. External modems that sit on your desktop are generally easy to set up; internal modems that fit in a slot inside the PC are neater and don't need a separate power supply, but are often harder to install.
If you're going to work from home, a fax/modem will probably be a good buy and nearly all modern modems now come with built-in fax capabilities. This means you can send faxes from your PC without having to print the document first.
The bad news, though, is that it is impossible to find a low-cost modem that is good and easy to use. When choosing one, first-time users should be careful to examine the software and manuals. Most modems are supplied with basic Windows and Macintosh communications and fax software, and other welcome extras are programs that give you limited trial access to CompuServe, Pipex or Demon. This way, you can see if cruising the Net is to your liking before you take out a monthly subscription to an online service.
The writer is the editor of '.net' magazine.
Sportster 14,400: pounds 139, US Robotic. Tel: 0800 225252.
MobiFax 144: pounds 99, Pace. Tel: 01274 532000.
Tornado 14,400 Fax Modem: pounds 104.57, Electronic Frontier. Tel: 01734 810600.
NetLink: pounds 139, Pace. Tel: 01274 532000.
Sportster 28800: pounds 233.83, US Robotics. Tel: 0800 225252.
Express 28.8E Fax Modem: pounds 217.38 (PC), pounds 240.68 (Mac), Express Technology. Tel: 01784 421123.
Frontier External V34: pounds 193.88, Electronic Frontier. Tel: 01734 810600.
Hayes Accura 288 V34 + Fax: pounds 292.58, Hayes. Tel: 01252 775577.
Internet Solution: pounds 233.83, Motorola. Tel: 01293 404343.Reuse content