For months, full-page ads in computer magazines have proclaimed the wonders of a pocket-sized, 1,200-baud external modem from Migent Inc. The company calls it a "state-of-the-art information machine that looks like one." In its sleek red plastic case, it certainly does. And with its list price of $259, it may also look like an alternative to the $300-and-up internal modems made for most current lap-top computers.

A modem is indispensable for the lap-top user on the go because it allows transmission of computer data over phone lines. The internal modems offered with current lap tops are convenient because all you need do is run a phone cable from the back of your computer to a wall jack, and you're set. But external modems, such as the Migent, while they involve a bit more cabling, have the advantages of being cheaper and not permanently attached to a single computer.

While Migent has done a good marketing job, its product turns out to be the largest and most expensive of the three pocket modems now on the market. It also has the fewest features and performed poorly in some hands-on tests on noisy home telephone lines.

The Migent does not measure up to others I tested. It has neither a speaker nor LED indicators. It does sound an alarm through your computer's speaker when the battery is low, which is useful since low power -- like line noise -- can wreck a transmission and fill your screen with garbage. The Migent uses a nine-volt battery or AC adapter, but it continues to draw power when not in use, unlike the Worldport, which I'll get to in a minute. The Migent worked well enough on clean phone lines, but also failed once to hold a connection on a noisy line to a local bulletin board. It is not designed to work with acoustic cups or to work overseas. It appears that the modem's styling and promotion are where the company has put its money (Migent Inc., P.O. Box 6062, Incline Village, Nev., 89450, 702-832-3700).

The other two modems I tested are Novation's brand-new Parrot and Touchbase Designs' Worldport 1200. The Parrot wins the prize for technical innovation. It is the same size and weight as an ordinary audio cassette but, unlike the others, requires neither battery power nor a wall outlet. It gets all the juice it needs from your computer's serial port (external modems all hook up to your computer serial, or RS-232, port).

The Parrot has four red LED indicators to show when the modem is ready, off-hook, connected to a carrier and transmitting (or receiving) data. The Parrot also has a speaker, so you can hear your call going through. (It is a very faint speaker, but the only one in the group.)

Like the other two modems, the Parrot responds to the same software commands as the industry-standard Hayes modems. Hayes compatibility assures that most communications software will work.

I tried the Parrot (and the other two) with a desk-top IBM and a Toshiba T-1000 lap top, both using the popular Procomm communications program. It performed well online with a major commercial data base and with local computer bulletin boards, as long as there wasn't much line noise. But on noisy connections, the Parrot faltered, twice losing contact with one local bulletin board.

And the Parrot was the most difficult to hook up of the three. There is no serial connector on the modem itself. Instead, there is a connector similar to a phone jack that runs to a serial adapter that, in turn, must be attached to your computer. This leads to a lot of fussing around with cables. Still, at $119 without the cables, the Parrot is a bargain (from Novation Inc., 21345 Lassen St., Chatsworth, Calif., 91311, 818-998-5060).

The Worldport 1200 is an update of an earlier model from Touchbase Designs, and the company's experience shows. About the size of a pack of cigarettes, the modem has a serial connector on the back: You specify if you want a male or female connector. That means the modem can be attached directly to the back of your computer (unless, like the Toshiba T-1000, your computer has one of those pesky nine-pin connectors).

It has no speaker but does have a set of LED status lights, including one that warns when your battery is low. It takes a nine-volt battery that lasts about 10 hours. The modem turns itself on and off automatically when you start and stop using it. There is also an AC adapter for wall power and, unlike either of its competitors, the Worldport has an adapter to connect with the acoustic phone cups you may need in a hotel room with no phone jack.

In addition, the Worldport is compatible with the phone systems used in this country and overseas, which is not true of its competitors (hence the name "Worldport"). It performed the best of the three on noisy home-phone lines. Listing for $199 and seemingly the best overall value, it's available from Touchbase Designs, 16 Green Acre La., Northport, N.Y., 11768, (516) 261-0423.Brit Hume is a contributor to the Washington Post Writers Group. Hume is an ABC News Capitol Hill correspondent and the founding editor of a computer newsletter.