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Cover Story
POTS Gets Panned

-- by Cynthia Morgan and Eric Sherman

Plain Old Telephone Service (POTS) voice line is an analog line, meaning it transmits an electronic signal that rises and falls with the volume of what it hears and creates an electronic copy of the sound. Because it offers just a single communications channel, you can use it for only one application-voice, data or fax-at a time. But POTS lines are inexpensive and widely available, at least in the U.S.

Before you can send the PC's digital information through an analog phone line, you need to translate it into waves that mimic how phones transmit sound. That's why you need a modem. But travel through phone wires weakens, or attenuates, the signal during transmission, so that it must be periodically regenerated or amplified. Unfortunately, the signal also picks up noise (distortions caused by transmission error, problems with the physical line or even the amplification process itself) and amplifies it right along with the original signal. Too much noise, and the receiving modem will reject the transmission, forcing repeat transmissions until the data either arrives correctly or the modem gives up in that frustrating phenomenon known as "timing out."

The Switch Is On

Digital services, now gaining in popularity, don't use analog waves to carry data. Instead the data remains in digital form, which eliminates many of the problems typical of analog lines and increases maximum possible transmission speed. Telephone networks have been converting from analog to digital lines for some time. By now, about the only remaining analog portion in most areas of the U.S. is the so-called "last mile." This consists of the wires between the phone company's local Central Office (CO) switch and the customer.

If you're using a modem over an ordinary analog phone line, the modem converts the digital data from your computer to analog electrical waves, which are converted back to digital form at the CO switch. The conversion is reversed when it reaches its destination. Phone company networks are capable of incredible throughput without modification, but line noise and the inefficiency of this analog/digital conversion process limit transmission rates to about 35Kbps, a restriction known as Shannon's law.

The current standard speed in dial-up modems, 33.6Kbps, is approaching that limit, although modems reach that rate only under ideal conditions. Throughput rates are usually about 10 percent lower than the manufacturer's claims. In fact, one reason to buy a 33.6Kbps modem is to actually achieve the older 28.8Kbps rate. The difference between a 28.8Kbps and a 33.6Kbps is not the raw physical speed of the modem, but the encoding methods. High-speed modems actually work at far lower physical speeds, but use complex algorithms to encode data in a more compact format.

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Copyright © 1997 CMP Media Inc.

(From Windows Magazine, January 1997, page 200.)