Classic Computer Magazine Archive COMPUTE! ISSUE 128 / APRIL 1991 / PAGE 40

Modems Made Easy. Master the Mysteries of Modem Speeds
by Michael A. Banks

Modern transmission speed is the source of a lot of confusion, even among otherwise informed computer and modern users. The problem stems from the interchangeable use of baud and bits per second. I suspect this is because it's easier to say baud than bits per second, though misinformation has a hand in it, too.

If you've ever found yourself confused by the relationship between bits and baud rate, or if you think that a modem's baud rate is the same as the number of bits or characters it transmits per second, read on.

Background Bits

Bits per second is a measure of the number of data bits (digital Os and Is) transmitted each second in a communications channel. This is often called the bit rate. Individual characters (letters, numbers, and so on), also referred to as bytes, are made up of a group of data bits.

Depending on its type, a character may be represented within a computer by a string of seven or eight bits. Seven-bit characters usually represent the 128 standard ASCII characters found on your computer keyboard.

In addition to the standard ASCII codes, 8-bit characters can represent control characters and special token or graphics characters peculiar to a specific machine. The line-drawing characters in the PC's extended character set are a good example.

Any character with an ASCII number higher than 128 is an 8-bit character. The number of bits sent for each character can change during transmission, as we'll see. (Again, the transmitted character is often referred to as a byte, or on older systems, a data word.)

While a modem's bit rate is related to its baud rate, the two numbers are not always the same.

Get Baudy

Baud rate is a measure of the number of times per second a signal in communications channel varies, or changes states. States can be many things, including frequencies, voltage levels, or frequency phase angles.

One baud is one such change. So, a 300-baud modem's signal changes in state 300 times each second.

However, contrary to what you might expect, a 1200-baud modem's signal doesn't change state 1200 times per second. And that fact has an important bearing on why it is inaccurate to say baud when you mean bps.

Raw Bits

Depending on the modulation technique it uses, a modern can send one or more (or less) bits with each baud or change in state. To put it another way, one change of state can send one bit - or more or less than one bit. Thus, baud and bps are very different.

As mentioned earlier, the number of bits modern transmits per second is based on the number of bauds (changes in signal) that take place each second.

The numbers for bps and baud are not always the same because one baud can represent one bit, more than one bit, or a fraction of a bit.

To illustrate this, first consider a modern with a baud rate of 300 using a transmission technique called FSK (Frequency Shift Keying, in which a signal changes to any of four different frequencies to represent digital 0 and 1 signals from both modems).

With FSK, each baud (which is, again, a change in the state of the signal) sends one bit. Only one change in state is required to send a bit. Thus, the modem's bps rate is also 300: 300 bauds per second X 1 bit per baud = 300 bps.

Similarly, if a modern operating at 1200 baud were to use one change in state to send each bit, that modern's bps rate would be 1200. (There are no 1200-baud modems, by the way. This is only a hypothetical example.)

Now, consider a hypothetical 300- baud modem using a modulation technique that requires two changes in state to send one bit, or half a bit per baud. This modem's bps rate would not be 300 bps, but 150 bps: 300 bauds per second x 1/2 baud per bit = 150 bps.

To look at it another way, bits per second can also be obtained by dividing the modem's baud rate by the number of changes in state, or bauds, required to send one bit.

Real Speed

Now let's move away from the hypothetical and into reality, as it exists in the world of modern modulation. First, lest you be misled into thinking any 1200-baud modern can operate at 2400 bps with a two-bits-per-baud modulation technique, remember that I said there are no 1200-baud modems.

Medium- and high-speed modems use baud rates lower than their bps rates. Along with this, however, they use something called multiple-state state modulation to send more than one bit per baud.

For example, 1200-bps modems that conform to the Bell 212A standard (which includes most 1200-bps) modems used in the U.S.) operate at 300 baud and use a modulation technique called phase modulation that transmits four bits per baud. These modems can operate at 1200 but not 2400 bps because they are not 1200-baud modems; they use a baud rate of 300.So, 300 baud X 4 bits per baud = 1200 bps, or 300 baud = 1200 bps X 1/4 baud per bit.

Similarly, 2400-bps modems that conform to the CCITT V.22 recommendation (which includes virtually all 2400-bps modems) actually use a baud rate of 600 when they operate at 2400 bps. To achieve a bps rate of 2400, they use a modulation technique that transmits four bits per baud: 600 baud X 4 bits per baud = 2400 bps, or 600 baud = 2400 bps X 1/4 baud per bit.

Thus, a 1200-bps modem is not a 1200-baud modem, nor is a 2400-bps modem a 2400-baud modem.

Now let's take a look at 9600-bps modems. Most of these operate at 2400 baud, but (again) use a modulation technique that yields four bits per baud. Thus: 2400 baud X 4 bits per baud = 9600 bps, or 2400 baud = 9600 bps X 1/4 baud per bit.

Characters per Second

Usually abbreviated cps, characters per second is the number of characters (letters, numbers, spaces, symbols, or control or other 8-bit characters) transmitted by a modern in one second.

Cps is often the bottom line in rating data transmission speed. It is certainly a more effective way of thinking about text data transfer than baud-or bit-rate.

Determining the cps rate is easy: Simply divide the bps rate by the number of bits sent for each character. You must of course consider more than just the seven or eight bits that make up each character binary digit. In fact, as many as ten bits may be transmitted for each character during ASCII transfer, whether seven or eight data bits are used.

This is because the sending computer adds bits that mark the beginning and end of each character (called start-and stop-bits). In addition, the system usually adds a parity bit during 7-bit ASCII transmission. (The computer's serial port takes care of adding the extra bits as well as stripping them out at the receiving end.)

So, in asynchronous data communication, the number of bits per character is usually 10 (either seven data bits plus a parity bit, a start bit, and a stop bit, or eight data bits plus a start bit and a stop bit). Thus, using 10 bits per word:

300 bps = 30 characters per second 1200 bps = 120 characters per second 2400 bps = 240 characters per second

Common Speeds

The most commonly used communications rates for dial-up computer communications (including BBSs and online services like CompuServe, DELPHI, and GEnie) are 300, 1200, and 2400 bps.

Some older systems - especially Telex - communicate at 110 bps, but these are gradually going the way of the dinosaur. Modems with speeds of 4800 and 9600 bps are available, but at present few online services or BBS's accommodate them. This will be changing soon, however, as the cost of high-speed modem technology drops and the demand rises.

Super Fast

Modems with even higher bps rates (19,200 and up) exist, but these are not used with CompuServe, GEnie, America OnLine, and most other dial-up systems; 9600 bps is generally accepted as the upper limit on asynchronous data transmission via voice-grade telephone lines.

At present, using higher transmission rates requires special dedicated lines that are conditioned (that is, lines are shielded from outside interference) as well as expensive modulation and transmission equipment.