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The effects of bit on baud demystified

Tue, 09/30/1997 - 8:00pm
Jim Farmer
i_farmer.GIF

The correct answer: "affect" is usually a verb, so use that word when you mean to do something. "Affect" can also be a noun, if you are referring to someone's "emotional affect," or the conscious subjective aspect of an emotion, separate and distinct from bodily changes. "Effect" is usually a noun, so use it when you are talking about something. However, if you want to talk about bringing something about, "effect" can be a verb, as in, effecting an improvement in your vocabulary (thanks to Lucent's Mark Rivecco for reminding me of that usage).

The best example of how to use the words came from my good friend Mike Wolcott at Scientific-Atlanta. I particularly liked his salutation, at least until I recalled that he is given to hyperbole.

Mike wrote:

"O wise and all-knowing one, Grand Poobah of knowledge and light, dispenser of great things, Greetings! As Brunswick should know, you put 'possum roadkill in the stew for effect, then step back to see how it affects the customers."

Mike is one of those guys who has a way with words. I have never been able to top him, and I won't try this time. But it will be a long time before I eat at his house.

Technical precision

Let's try a much misused technical term this month. Breeds there a man in the industry who knows the difference between a bit-per-second and a baud? If I have a cable modem, or even one of those old-fashioned things that are hooked to the phone line, I can get so many bits from my end to your end every second. That number of bits is the bit rate, or the speed at which the modem operates. Low-speed modems tend to use modulation techniques that transmit only one bit at a time. These modulation formats, which are also used extensively in cable work, are (usually) frequency shift keying (FSK) and biphase shift keying (BPSK). In FSK, you transmit 1s and 0s by shifting an oscillator between two frequencies. In BPSK, you change the phase of a carrier 180 degrees to indicate the two states.

The problem is that, if you transmit just one bit at a time, you don't make very good use of the spectrum. That is, your bandwidth efficiency (expressed as the number of bits-per-second for each hertz of bandwidth you use — bits-per-hertz) is not very good. To remedy this, you have to take baby steps back toward the analog domain, by transmitting more than one bit at a time. The more bits you transmit at a time, the more discrete states you have in your transmitted signal, and the more you start looking similar to an analog signal, which has an infinite number of states infinitely close together. It's not analog, and the spectrum doesn't look like analog, but with more states, you start approaching the continuous nature of analog.

When you are transmitting more than one state at a time, we can't talk just about the bits you are transmitting: we also need to talk about the collection of bits (two or more) you transmit at one time. We call this collection of bits a symbol. If you are using a modulation format such as QPSK, which transmits two bits at a time, the number of symbols transmitted per second is one-half the number of bits transmitted per second. Engineers, being the creative types we are, invented a new term for the number of symbols transmitted per second. Instead of the bit rate (number of bits transmitted per second), the number of symbols transmitted per second is called the baud rate. Where did the term "baud" come from? I haven't the foggiest notion, but I'm sure someone out there will tell me, and when he or she does, I'll tell you.

Thus, if we are transmitting QPSK, which puts two bits in a symbol, we have a baud rate of one-half the bit rate. A 19.2 kbps system is a 9.6 kbaud system if it uses QPSK. A 9.6 kbps system is also a 9.6 kbaud system if it uses BPSK, where only one bit is transmitted at a time.

In both cases, the same baud rate is used, but the number of bits transmitted per second varies by a factor of two. The bandwidth required is essentially the same, being a function of the baud rate, not the bit rate. In 64 QAM transmission, six bits are transmitted at a time, so a bit rate of, say, 30 Mbps works out to 30/6=5 Mbaud. If we are transmitting 256 QAM, we transmit eight bits in a symbol, so 5 Mbaud would be a bit rate of 40 Mbps. (Notice that you don't use a plural of baud, with an "s" at the end.)

You're right; they're wrong

Now that you know the difference between bits-per-second and baud, you are probably in the top one percent of telecommunications engineers, at least so far as that one fact goes. Now, how do you convince your co-workers that you are right, and they are wrong? The only way I know to do it is to cite an authority, so try mine: the IEEE Dictionary of Electrical and Electronic Terms.

Contact Jim Farmer jfarmer@ix.netcom.com

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