Classic Computer Magazine Archive COMPUTE! ISSUE 60 / MAY 1985 / PAGE 10


The Editors and Readers of COMPUTE!

If you have any questions, comments, or suggestions you would like to see addressed in this column, write to "Readers' Feedback," COMPUTE! P.O. Box 5406, Greensboro, NC 27403. Due to the volume of mail we receive, we regret that we cannot provide personal answers to technical questions.

The True Meaning Of Baud

What's the difference between 300 baud and 1200 baud?

Karl Stephens, Jr.

Most people use the term baud when referring to telecommunications transmission speed. In popular usage, the higher the baud rate, the faster the modem can transmit characters over the phone lines. You'll hear people referring to 300 baud, 1200 baud, and 2400 baud modems. However, this is technically incorrect. The more correct term is bits per second, abbreviated bps. A 1200 bps modem can transmit characters four times as fast as a 300 bps unit.

Baud is used to reference the division of each second into tiny, discrete pieces (a process known as signal modulation) by the modem's electronic circuitry. A 300 bps modem's signal is indeed modulated at 300 baud. Since each tiny division holds one bit of data, the effective transmission rate is 300 baud per second times 1 bit per baud, or 300 bps.

But 1200 bps modems are another story. A 1200 bps modem divides each second into 600 pieces. Using a technique called four-level phase shift keying, each piece can represent two bits. Multiply 600 baud per second by 2 bits per baud and you end up with 1200 bits of information per second (1200 bps).

High-speed 2400 bps modems also use a modulation rate of 600 baud. What's different is the method of phase shift keying. They use 16-level phase shift keying, so each piece or baud can represent four bits. Multiply 600 baud per second by 4 bits per baud and you have 2400 bits of information per second.

That's why it is technically inaccurate to say medium- and high-speed modems transmit at 1200 baud and 2400 baud. Both devices are actually 600 baud modems.

With most of today's transmission schemes, it actually takes ten bits to send one character of data. Therefore, the approximate character-transmission speeds of 300, 1200, and 2400 bps modems are 30, 120, and 240 characters per second, respectively. If you want to calculate how long it would take to transfer a certain file, divide the transmission speed in characters per second into the length of the file, remembering that 1K equals 1024 characters. For instance, at 300 bps, it would take about 546 seconds or 9 minutes to transfer a 16K file.

For a fuller discussion of modem transmission speeds and whether it's worth investing in a faster modem, see Arlan R. Levitan's "Telecomputing Today" column in the January 1985 issue of COMPUTE!.