Just What Do Modems Do? from the August 2000 Actrix Newsletter

by Rob Zorn

Modems are devices that allow computers to "talk" to each other over a standard telephone line in much the same way humans use a telephone. One computer uses its modem to dial a phone number. At the other end of the line, another modem answers the call. The two modems then negotiate or shake hands, that is, they agree on the format of the data transmission and the transmission speed . After negotiation, the two computers transmit data back and forth through their respective modems.

During the negotiation phase of the connection, the two modems test the quality of the telephone connection. Based on line quality the two modems agree on how fast they will transmit data. Transmission speed is expressed in bits per second (bps). Typical modem connection speeds range from 2400 bps to 56,000 bps.

This month I thought it might be a good idea to try and reduce the complexities of how modems work into layperson's terms. The better we understand technology, the more at home we tend to be with it.

Firstly, the origin on the word "modem" is pretty telling. It is short for modulator demodulator. You see, computers tend to work with information in digital format, but telephone lines were designed to carry sound waves in analog format. It is the modem's job to turn your computer's digital information into analog "sound waves" (which is why you hear squealing when you pick up your telephone while online) at your end. This is called "modulation."  The modem in the computer at the other end of your internet call has the job of turning those sound waves back into a digital format to be used by the computer there. This is called "demodulation."

Wow! 28,800bps!
As long as your modem is active (ie, while you are online) it constantly transmits a signal called a "carrier" because that signal is the means by which information is "carried" across the phoneline. Modems transmit information by modulating or varying this signal. The carrier signal is characterised by the number of "pulses," that are transmitted per second. Each pulse is called a baud. The maximum number of pulses per second that can be transmitted across phonelines is 1200, so the faster the modem, the more information (bits) it is able to squeeze into each pulse.

To illustrate: one of the first modems used to access the Net was described as 1200bps. This meant that it was able to transmit 1 bit of information per pulse (therefore 1200 bits per second). 2400bps modems were able to transmit two bits of information per pulse or baud. They were still transmitting at a 1200 baud rate, but doubled the amount of information carried at that speed. 9,600bps modems quadrupled this, and so on as modem technology advanced eventually to 33,600 bps.

At that point the modem manufacturers agreed that the limit of technology had been reached, that modems could never go any faster. Now, however, all of the major manufacturers of modems produce 56,000 bps models, or 56K modems (K being short for kilobit).

In fact, It would actually be more accurate to say that modem makers discovered two ways to realise this increase in speed.

Shortly after U.S. Robotics announced their 56K technology, which they dubbed "X2," competitors Rockwell Semiconductor and Lucent Technologies unveiled another method of reaching this speed. This approach, which they named "K56 flex", was not compatible with X2 modems at 56K, and this is where things became crucial. If 56K was the way to go, which system should the Internet universally adopt?

In 1998 an international standards committee called the International Telecommunications Union decided on a hybrid of the two systems, and thankfully, major modem manufacturers agreed to go along. Hence today we have what is known as V.90 56K modems. What makes V.90 different and possible is probably too complicated to go into here, but it assumes that the only aspect of your entire internet connection that needs conversion from digital to analog is between your computer and your local telephone exchange. Everything else it treats as digital.

The V.90 system works by digitally encoding downstream data instead of modulating it as analog modems do. The data transfer is asymmetrical meaning that upstream transmissions (mostly keystroke and mouse commands from your computer, which require less bandwidth) continue to flow at the old conventional rates of up to 33,600bps. Only the down stream data transfer takes advantage of the high speed V.90 rates.

If My Modem is 56K, Why Do I Never Achieve Download Speeds of more that 5 or 6K?

If you're getting a 6 kilobyte download through a 56K modem, you are doing very well indeed.

The reason for the discrepancy here is that in each of these cases, the K stands for something different. Your modem is 56Kilobits. There are 8 bits in a byte, and 1024 bytes in a kilobyte. What this means is that your modem's maximum download speed (if all conditions are perfect) is 56,000 bits, or 7,000 bytes, or 7 kilobytes. Given the nature of 56K modems (see the article above) a 7K download is technically impossible.

Make sense?

Why a 56K Modem Does Not Transmit Data at 56K

Even though you own a V.90 56K modem and Actrix, your ISP, supports V.90 56K modem connections, you will still connect at a slower speed. In fact, you may connect as slow as 24K. There are many reasons for this.

Transmission speed is greatly affected by the quality of the telephone line and how far you are away from your local telephone exchange. In order for a 56K modem to connect at 53K, the telephone signal can only go through one analog to digital (A/D) conversion. If you live too far from the exchange, your service likely goes through more than one A/D conversion and your  56K modem may only connect at 24K or 28K. Funnily enough, I remember spending considerable help desk time with a person who was experiencing terribly slow speeds, but who said he was so close that he could "spit on his local exchange from his study window." You guessed it; he turned out to be too close to the exchange, but that is another story.

Line noise is also a definite speed killer. Faulty, wet or cracked phone lines will produce noise (often inaudible to the human ear) and 56K modems are particularly susceptible. When your modem and ours talk together during initial handshaking, they may agree to communicate at a slower speed so that your connection doesn't drop or stop as a result of noise.

Ironically, the very process of converting data from analog to digital format creates a form of line noise as a by-product that will slow your modem ever so slightly. Therefore it is safe to say that a 56K connection  is impossible to achieve with a mere 56K modem.