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In today's market, the typical modem does a nominal 56kbps -- V.90 plus V.29 or V.17 fax transmission and reception. You don't see much in the way of slow/cheap to fast/expensive product ranges within a single brand, because competition is fierce and for many modem board designs (those featuring DSP (Digital Signal Processor) chips run by a program in ROM) adding a new protocol is basically a software change.
You probably want to buy a 56K modem. But if you live in a rural area, try a borrowed one out and watch for dropouts and glitches during a big file transfer (just watch zmodem's progress messages). If you see a lot of these, your copper may be too cruddy for 56K and you might as well settle for lower speed and lower cost (if you can find it; v90s have tended to drive out everything else).
Detailed discussion of the V-series standards can be found in the Glossary.
For much more information on high-speed modems (including breaking industry news), see the Navas 28800 Modem FAQ
Pay that premium --- being able to see the blinkenlights on the external ones will help you understand and recover from pathological situations. For example, if your UNIX system is prone to ``screaming-tty'' syndrome, you'll quickly learn to recognize the pattern of flickers that goes with it. Punch the hangup/reset button on an external modem and you're done --- whereas with an internal modem, you have to go root and flounder around killing processes and maybe cold-boot the machine just to reset the card.
The LineLink external modem is from Technology Concepts (actually manufactured by Prometheus). Their LineLink 144e gives you V.32bis, V.42bis and MNP 5, offers DCE speeds up to 57600. supports hardware handshaking, and will auto-answer. The price is $99. Yes, that's ninety nine dollars. You can buy it from MicroWarehouse at (800)-367-7080. Throw away the useless comm software bundled with the LineLink and you'll still have yourself a nice little dialout modem.
The LineLink has two significant drawbacks.
One is that the operation manual lacks a reference list of S-registers; thus, for example, you have to already know details of the Hayes-compatible interface, like the fact that setting S0 to a non-zero value will cause it to auto-answer on that many rings. However, you can download the S-register list from a BBS number MicroWarehouse will give you.
The second problem is functional and more serious. LineLinks have a tendency to go catatonic occasionally just after the DTR drop that terminates a connect (I found this out the hard way after our ISP bought six of them). This makes them chancy as dialin modems for ISPs and BBSes, though still quite OK for dial-out.
This problem may not be specific to the LineLink. Other ISPs using cheap modems based on the Rockwell modem-pump chip set (also used in the Boca and Zoom modems) have reported similar problems.
One V-series modem that seems to be performing well consistently on both dialout and dialin under UNIX is the U.S. Robotics Sportster 14.4. (I use these as 24-hours dial-in modems on that ISP I help run.) These have the minor disadvantage that their cases don't stack well. The 28.8 cousin is much less stable. Also I hear that Sportsters being used for dial-in have a strong tendency to flake out and get wedged on hangup if the tip and ring lines on the phoneline are reversed (which is not uncommon).
In the 28.8K domain, I've had limited but positive experience with the Cardinal 28.8K. I now use A U.S. Robotics Courier V.Everything 56K.