Fast modems will make Infobahn driving easier

HOME COMPUTING

April 17, 1995|By MICHAEL J. HIMOWITZ

A lot of folks who bought computers a couple of years ago are finding that the 2,400-baud modems that came bundled inside aren't up to the demands of today's communications.

The major on-line services -- Prodigy, America Online and CompuServe -- all have shiny graphical interfaces that pass a lot of data back and forth.

At low speeds, they're maddeningly slow. And older modems are virtually useless for surfing the Internet with graphical World Wide Web browsers such as Mosaic and Netscape.

Luckily, replacing a modem is relatively easy and not all that expensive. You can move right into the mainstream with a 14,400 bps modem for $100 or so, and for $250 to $300, you can travel in the fast lane of the Infobahn with a 28,800 bps model.

One problem modem buyers face is deciphering the techno-gobbledygook on the package. Do you want V.34, V.32, V.42 (or the famous V.42bis), V.17, V.29, MNP-5, LAP-M, Bell 212A, or the laughably archaic Bell 103?

These codes all describe standard methods that modems use to communicate with one another. Modems that meet the standards described by this alphabet soup can talk the same language, although theory and practice occasionally diverge.

To decipher this technobabble, it helps to understand what modems do. Essentially, they get phone lines to do something they were never designed to do.

The phone system was designed to transmit the human voice, with its continuous range of frequencies. While the phone system is basically reliable, it has never had to be perfect, because your brain will compensate for small errors or distortions in the sound you hear.

Computers don't really speak to one another. They exchange the equivalent of binary ones and zeros. A modem turns those binary ones and zeros into tones of fixed frequencies which can travel over phone lines and be turned back into ones and zeros at the other end.

The first thing that distinguishes modems from one another is how fast they can send and receive those ones and zeros, known in the trade as bits. The speed of the modem, commonly but inaccurately referred to as the baud rate, is measured in bits per second (bps).

Here comes the arithmetic. It generally takes 10 bits to transmit a single byte of data, which is the equivalent of a single alphabetical character. A 2,400 bps modem can theoretically send and receive 240 characters per second, while a 28,800 bps modem will move 2,800 characters per second.

This column is about 6,000 characters long. Sending the column over the phone with a 2,400 bps modem takes about 25 seconds. With a 28,800 baud modem it takes just over two seconds. So you can understand the advantage of a high-speed modem when you're moving data and graphics files that occupy hundreds of thousands or even millions of bytes.

What happens when a slower modem tries to talk to a faster modem? Usually, they can communicate. Almost all modems on the market today can sense the speed of the modem at the other end and negotiate a mutually acceptable rate of communication.

The methods modems use to negotiate and maintain communications at different speeds are governed by either industry or international standards.

These account for most of the "V" numbers and other codes you see in modem advertisements. At the high end, a V.34 modem theoretically can talk to any other V.34 modem at speeds up to 28,800 bps. A V.32 modem can communicate at 14,400 bits per second. Standards for lower speeds include V.23, V.22, V.21 and Bell 212A. You'll also see the word "bis" after the designation of some standards. That means the standard has been modified over the years.

Because it's hard to maintain accuracy at high speeds, modem makers have added error-checking capability to their hardware. This means that your modem constantly checks with mine to make sure that what I sent is what you received. Most modems can handle the most common error correction standards, known as V.42, LAP-M and MNP 2 through MNP 4. Internationally, V.42 is the most important.

Finally, to squeeze the highest possible speed out of the phone system, manufacturers are now building data compression capability in their modems. In theory, data compression can quadruple the effective speed of a modem, but I've never seen it work that efficiently. In any case, the two most common compression standards are the international V.42bis and the older industry standard MNP-5. Of the two, V.42bis is more important.

There are other standards that deal with a modem's fax capabilities. A V.17 fax can send faxes at 14,400 bps, while a V.29 modem operates at 9,600 bps. Here, the higher speed is nice, but not necessary.

Now that you've passed Technobabble 101, you can go shopping, and when you do, you'll realize that most modems have most or all of this stuff.

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