Modems Made Easy

Karl Jeacle

Computer communications can be one of the most baffling areas of computing to the beginner. Jargon everywhere. Virtually impossible to figure out what's going on. This article is going to be as simple as possible - only explaining what you need to know. We'll just stick with the basics.

Flashing Lights

First, what is a modem, and what does it do? A modem is a little box about the size of an external 5.25" disk drive. It will usually have a panel of status lights on the front, and at the back, sockets where you can plug your computer and telephone line into. The status lights on the front tend to flash on and off quite a lot, and really do a great job of impressing your friends.

The modem plugs into your computer's serial port using an `RS-232' cable. Your computer sends digital `bits' of information to the modem which in turn, converts these into analogue `tones' which can be sent down the telephone line.

At the other end of the telephone connection will be another modem which just does the reverse of the above. It listens to the line, and converts tones back into `bits' for the computer at the far end. The computer at the other end of a connection is sometimes known as the `remote' computer.

Bulletin Boards & UseNet

What can you do with a modem? The main use would probably be to ring up local Bulletin Board Systems (BBS) where you can join discussions on all kinds of topics, send and receive messages to/from other BBS users, and `download' public domain and shareware software for your computer.

If you're a bit more adventurous, you could try connecting to UseNet, and get access to international electronic mail and news for about IR#50 a year. There's an extensive article on UseNet in the April 1991 issue of the CUGI newsletter.

The `V-Numbers'

The speed at which a modem can send or receive data is measured in `bits per second' (bps). These speeds are what many of the `v-numbers' are for:

v.21    = 300   bps
v.22    = 1200  bps
v.22bis = 2400  bps
v.32    = 9600  bps
v.32bis = 14400 bps

A normal character (numbers/letters of the alphabet) takes about 10 bits of data to transmit, so if you had a 2400bps modem, you would be able to receive about 240 characters per second. This isn't bad - 3 full lines of text per second.

2400bps modems are the most common modems around these days. 300 and 1200bps are really a little slow, and 9600 and 14400 while super fast, are also super expensive! 2400bps modems are currently sold new in the UK for about STG#120.

Correction & Compression

Sending data down a noisy phone line is bad news. Information can be lost completely or garbled beyond recognition. To combat this, there are two forms of error correction widely used - MNP.4 and v.42.

Not all modems have error correction, so if an error free connection is to be made, both modems must support the same correction scheme. When they first connect, they tell each other which schemes they support, and then decide which (if any) to use.

This is pretty much the same situation as with Data Compression. Two main types - MNP.5 and v.42bis, both modems must support a common scheme, and many modems support neither. MNP.5 is the older of the two, and has a compression ratio of 2:1, while v.42bis can achieve rates of up to 4:1.

In practical terms, this means that if both your modem, and the modem you're calling supports v.42bis, then depending on what you're sending up and down the line, transfer speeds of up to 4 times the normal rate can be achieved.

It depends on what you're sending since some types of data will compress better than others. Generally, you'll be transfering either plain text or pre-compressed files like .zip, .lzh or .zoo files. Since plain text is uncompressed, the modem can compress it and shoot it down the line at up to 4 times the normal rate. Pre-compressed files, however, can't be cut down anymore, so will just be transfered at the normal rate.

Beware of companies that market their modems as 9600bps, when they're really only 2400bps with data compression built-in. This is just a marketing scam. Sure, if you're transferring just text, you can achieve speeds approaching 9600bps, but a lot of the time you will be transferring precompressed data. Current UK price for a 2400bps with v.42/v.42bis would be about STG#175.

I lied above somewhat. I'll confuse you now: v.42 is actually two protocols in one - it is its own scheme and the MNP.4 scheme. Not only that, but a modem with MNP.5 automatically has MNP.4, and a modem with v.42bis automatically has v.42.

This means that if you have a v.42bis modem, you can talk to another v.42bis with compression rates of up to 4:1, but if you talk to an MNP.5 modem, you won't get any compression, just MNP.4 error correction.

HST, PEP, CSP & friends

Above speeds of 2400bps with v.42bis, things start to get a little tricky. From the consumer's point of view, anyhow.

Up to now, we've been talking standards. All those `v-numbers' are the work of the CCITT. The people in the CCITT tell the world what a modem needs to do to make it a v.22bis (or whatever), and all the manufacturers go out and make modems which comply with these instructions. This means that all modems talk the same language.

The problem with this is that the CCITT sometimes can be a little slow in getting these specifications out the door. This was the case after v.22bis. People were waiting for v.32 and then v.32bis to be finalised, so they could have 9600bps and 14400bps transfer speeds. They started to get impatient.

Companies like US Robotics produced the HST protocol, Telebit produced PEP, and now Compucom have CSP. All these protocols run at 9600bps or faster. Not only that, but the Compucoms are very cheap. The problem is, if you have a HST, it'll only run in high speed mode when talking to another HST. Same for Telebit and same for Compucom. You have to talk to a modem identical to yours. Otherwise the modem will just drop back and operate at v.22bis 2400bps.

Some of the companies, like US Robotics, provide `Dual-Standard' modems. These are fine. They feature both the proprietary high speed protocol (HST in this case) and the CCITT high speed protocol - v.32 or v.32bis.

It all depends on who you're calling. Many BBS use HST since they got good Sysop deals on these modems from US Robotics. Telebit modems seem to be popular for a lot of Unix installations. CCITT v.32/v.32bis is the norm for universities and industry in general.


If you just want a 2400bps modem, no problem, go out and buy one. If you can afford it, pay the extra and try to get MNP.5 or better still, v.42bis.

If you're looking for high speed, as far as I'm concerned, CCITT standards are the way to go. Get a v.32bis if you can. These are here to stay, and will certainly outlast the others.

Before you buy though, try to find out what kind of modem the sites you'll be calling use. Only then will you know what's best for you. If you're still not sure, track down the comms gurus at CUGI sometime...