UNIX System Administration Handbook - Evi Nemeth [74]
External modems have an RJ-11 jack on the analog side and an RS-232 interface of some type on the digital side—usually a female DB-25. They usually have a series of lights on the front that display the modem’s current state and level of activity. These lights are incredibly useful for debugging, so modems should generally be located somewhere in plain sight.
Internal modems are usually seen only on PCs. They plug into an ISA, PCI, or PCMCIA slot and have an RJ-11 jack that sticks out the back of the computer’s case once the modem has been installed. They are cheaper than external modems but more troublesome to configure, and they generally lack indicator lights.
If you are considering an internal modem, you must check to be sure it’s specifically supported by your version of UNIX. Fast CPUs have made it possible to simplify modem hardware by performing some signal processing tasks on the host processor. Unfortunately, modems that work this way require sophisticated drivers and aren’t likely to be compatible with PC-based versions of UNIX.
Modems vary somewhat in general robustness, but this characteristic is hard to judge without direct experience. In the past, we have found some modems to be significantly more tolerant of line noise than others. These days, most designs use a standard chipset from one of several large manufacturers, so it’s likely that the variations among modems are not so great as they once were.
High-speed modems require complex firmware, and this firmware is occasionally buggy. Manufacturers share firmware among models when possible, so good or bad firmware tends to run in product lines. For this reason, we still recommend sticking with well-known brands. We’ve had good luck with 3Com products.
Modulation, error correction, and data compression protocols
Long ago, it was important to check the exact protocols supported by a modem because standards were continually changing and modem manufacturers did not always implement a complete suite of protocols. These days, modems of a given speed all support pretty much the same standards. The only real difference between them is the quality of the firmware, electronics, and support.
A protocol’s baud rate is the rate at which the carrier signal is modulated. If there are more than two signal levels, then more than one bit of information can be sent per transition and the speed in bits per second will be higher than the baud rate. Historically, the data speed and signaling speed of modems were the same, leading to a casual conflation of the terms “baud” and “bps” (bits per second).
The fastest modems available today use the “56K” V.90 standard, which doesn’t actually provide 56 Kb/s of throughput. At best, it allows 33.6 Kb/s from computer to ISP and 53 Kb/s in the other direction. Nevertheless, V.90 achieves speeds that are very close to the theoretical and legal limits of signaling over ordinary voice telephone lines, and it’s not expected to be superseded any time soon.
Two earlier “56K” systems, X2 and 56Kflex, were attempts by manufacturers (US Robotics—now part of 3Com—and Rockwell, respectively) to get V.90-class modems to market before a proper standard existed. Although they are still supported by some ISPs, these systems are expected to die out very soon. Many X2 and 56Kflex modems can be upgraded to V.90, so don’t throw them out.
A few modems with top speeds of 28.8 Kb/s and 14.4 Kb/s are still available, but they are becoming rare. You can pick up V.90 modems quite cheaply at sales and at on-line auctions, so there is no reason to compromise.
Line noise can introduce a significant number of errors into a modem connection. Various error correction protocols have been developed to packetize the transmitted data and provide checksum-based correction for errors, insulating the user or application from line faults. You used to have to know something about this to configure