WITLICKI@WILLIAMS.BITNET (04/22/87)
In reference to the article Keith Petersen posted about Baud vs. Bits/sec. > ..... This is nothing new, texts on the subject generally avoid > the term "baud" Yes, but computer companies use the word continually. ...Rather than do a point by point response to what I feel is a very muddy, although perhaps technically correct article, let me give you a few of my explanations... (hopefully a little more clearer then the recursive definition of 'bit rate' in the glossary). We first have to separate our wires. The RS232 wire between your modem and your terminal or computer talks in binary (0 or 1) - the data rate here is _Bits Per Second_. The Phone line which is between your modems talks in _Baud_. Now comes in the common confusion. Computer and terminal makers typically give the speed of their RS232 interfaces in terms of 'Baud' - this is wrong - it should be Bits Per Second. At modem speeds of less than 2400 baud the modulation techniques used usually result in 1 baud = 1 bit per second, hence the casual and incorrect use of terms. Definition: A Sine Wave is a signal at one frequency and one frequency only. (no harmonics if you think in terms of music). On a telephone channel which is bandwidth limited (by the phone company) to 3000 Hertz - this is highest frequency (== fastest) which you can push through the wire. Because of signal degradation and the chaining of telephone company equipment, the maximum signal frequency used in practice is a few hundred Hertz lower. If you are still confused - think in terms of music - a higher frequency == a higher pitch. The Nyquist theorem says that for a sine wave of 2400 cycles per second (I never did like renaming cycles per second into Hertz, next thing you know, phonograph records will go at 33 1/3 Edisons, but I date myself and I digress...) ... for a a sine wave of 2400 hz you can have 1200 bits per second. Shannon, on the other hand, said hey fellows, we got us a Noiseless Channel. We don't have to push sine waves through it, we'll push voltage levels through it which you can sample at each interval. Heck, with no Noise at all, say we have at least 65,536 voltage levels in each sample of a 600 baud channel and we can do 9600 bits per second... (to use the example in the original piece). Life is not so simple. We do have noise. The Nyquist Theorem does apply (... but...). In the case of 9600 BPS over a voice grade channel, Phase Modulation uses each sampling interval (baud rate time interval) to push several bits of information over the line. I hope this helps some. Quite frankly, I think it's time to drop the term Baud from the data communications consumer's terminology (who's the consumer?, who's the engineer? who's the user? why do I have to go through this to set a few a dip switches and use the thing?). Computer systems people have serial interfaces which talk at Bits Per Second. I don't give a hoot what sideways modulation method and baud rate you use over the phone line. I want to talk in terms of 'I have a 9600 Bits Per Second Asynch. Serial Interface' and 'Does it talk to a Hayes or a Bell 212A' and 'Can I use a voice grade line or do I order a Type 3002 unconditioned line or what?'. - randy P.S. I am in the process of writing some introductory material on data communications and would appreciate any pointers to published stuff anyone has seen which explains bits per second, parity, etc. for the user/system manager who shouldn't be subjected to the Nyquist/Shannon intimidation.
steckel@alliant.UUCP (04/22/87)
BAUD = signalling elements per second BITS per SECOND = data bits per second Any ASYNCHRONOUS communications device on RS232 or RS422/423 has start and stop bits attached to each and every character transmitted; these are required for the devices on each end of the line to detect where the data characters live. Typical amounts are: ancient technology @ 110 baud: 1 start bit, 8 data/parity bits, 2 stop bits wrong-headed technology @ 134.5 baud: 1 start bit, 5 data bits, 1.5 stop bits newer technology @ 300 baud & up: 1 start bit, 8 data/parity bits, 1 stop bit These start & stop bits are transmitted (for instance) from terminal to host, over low speed asynchronous modems, etc. In these cases BAUD is the correct term to use to determine signalling speed. Bits per second will always be lower, but it is format and user dependent. SYNCHRONOUS communications does not use start and stop bits; it uses sync patterns at the beginning of a block to co-ordinate the sender and receiver. Again, the BAUD rate is the signalling rate over the channel, while the data rate depends on the size of sync pattern, block header and trailer, etc., etc., and is format and user dependent. I hope this clears up the confusion. geoff steckel (steckel@alliant.uucp, gwes@wjh12.uucp)
WITLICKI@WILLIAMS.BITNET (04/22/87)
Phil Ngia writes: > >You are wrong. Ever heard of Bell 212? Try looking it up. >Also, you surely mean less than 2400 bps, not baud. >-- ooops, sorry you're right. i guess that shows i can just as easily fall into the trap of equating bps and baud. (for the $ set terminal command on the VMS vax which I use every so often, the Help command says you are setting the speed in baud...) am i being uptight about people using 'baud' to describe bps, especially when it is RS232 (yes i know when you are doing binary signaling that baud does == bps, but i think bps is more proper). old habits die hard. - randy
pozar@hoptoad.uucp (Tim Pozar) (04/23/87)
If you want a very good definintion of how modems work (>300 b/s), get
the technical Manual for the Advanced Micro Devices Am7910 WORLD-CHIP FSK
modem. Section 2 of the manual has a tutorial for modems.
One analogy they draw is:
"Think of the information being transmitted as a stream of automobiles
with people in them moving down a highway (the phone line). Consider
each care a baud (symbol). Consider each person a bit.
"Bit Rate = Baud Rate * No. of Bits/Baud
or people/sec = cars/sec * people/car.
"Bit Rate >= Baud Rate.
A "baud" is the speed at which the carrier is Frequency Shifted. If
it is moved from, say 1000cps to 2000cps 100 times a second, it would
be delivering 100 "baud". If this shift shows is representive of a change
in state of data (1 to 0, or hight to low, or the other way around), then
it would be sending 100 bits per second or 100 b/s.
In order to put more that one person per car for FSK modems you can
phase shift the frequency. A phase shift of 90 degrees can have 4 people
in a car, or 2 bits per baud. Since 00b = 0, 01b = 1, 10b = 2, and 11b = 3.
This is how 2400b/s modems work. By spliting the phase "cycle" into
eigths, we can send 3 bits per baud, thus sending 4800b/s.
This is about the max we can send. What we are talking about is 45
degree phase angle shifts. We start getting into distortion with the
phone line that makes getting smaller incraments pretty rough. So,
there is one other way to get more bang for our buck. Quadrature Amplitude
Modulation or QAM. QAM phase shifts the frequency and amplitude shifts is
as well. Also the pattern of the bits are layed out differntly. Instead
of a circle, with each bit being a different point on the edge (the size
of the circle would represent the amplitude of the signal), the pattern
is broken up into squares:
0
1000 1100 | 0100 0000
|
1001 1101 | 0101 0001
270------------+------------90
1011 1111 | 0111 0011
|
1010 1110 | 0110 0010
180
Now we can send 4 bit/baud or 9600 b/s.
According to the tech manual:
"QAM is even more sensitive to line distortions than PSK (Phase Shift
Keying). Automatic equalizers are required to compensate for these
distortions. This type of modem is the most complex and expensive of
all the modems to implement.
--
Tim Pozar
UUCP pozar@hoptoad.UUCP
Fido 125/406
USNail KLOK-FM
77 Maiden Lane
San Francisco CA 94108howard@COS.COM (Howard C. Berkowitz) (04/23/87)
The previous article made some excellent points on the confusion between baud and bits per second. While I agree the "consumer" should exclusively use BPS/K[ilo]BPS/M[ega]BPS (and always spell out bytes per second), there is at least one place that knowing the true analog baud rate can be useful: generating "eye" patterns. An eye pattern is an oscilloscope display which gives an idea of total system transmission quality. Sales strikes again here, for most commercial "eye pattern generators" actually are constellation or phasor displays. For my purposes, eye patterns are NOT the neat geometric displays of dots (probably more useful than eye patterns) which the diagnostic sections of good high-speed modems can produce. Eye patterns look like they came out of a science fiction movie, and are good 1) for impressing management and visitors amd 2) getting a visual go-no/go check if you have no other equipment. Eye patterns are generated with an oscilloscope, using the signal on the line and the baud rate as the sweep rate. Baud, in this context, is defined as the number of times per second the line changes state. With this setup, you will see a number of overlapping more-or-less sine waves, which in their crossing produce a central empty "eye," something like the eye of a hurricane. The larger the eye opening, the better the transmission performane. This is basically for entertainment, although it is useful in some engineering applications. One point about the previous article -- I never thought before to use the rule that it's bps on the digital side and baud on the analog, and borrow it with gratitude as a teaching device. To be technical, though, baud=bps on the digital side, because there is one signal state change per bit on a digital channel. Randy's article was excellent for the consumer of modems. ...... "Consumers" of modems remind me of a note rumored to have generated the largest letter-writing campaign in the history of one IEEE magazine. We have, for a long time, tacitly understood that a Consumer of Single Socks lived in washing machines. The pubished note suggested this belief was wrong. It proposed that Socks are the immature but sexual form of the Wire Coat Hanger. Have you ever noticed how wire coat hangers multiply? This is due to one member each of two sock pairs (i.e., of different sock sexes) mate to form a wire coat hanger. The divorced partners of these socks are too shaken to mate (or afraid of incest), and thus become the odd socks in the drawer.
jdf@pbhyc.UUCP (Jack Fine) (04/28/87)
Ummm... I think I agree with most of the explanations I have seen here on what a bit versus a baud is. Though I am not sure if what I think I read is what I think I understand. Understand? Well anyway I have been advocate for sometime (teachine datacomm) of trying to get people to drop the baud term when referring to data communications. Yes a 300 bps modem's signial is modulated at 300 baud per second times one bit per second giving you 300 bps. However, 1200 bps modems divide each second into 600 parts which is done by a method called four-level phase shift keying. Each part then can then hold two bits of data. This is at 600 baud and if you multiply 600 times the 2 bits you get the 1200 bps transfer rate. With 2400 bps modems you use a 16-level phase shift keying (still at 600 baud) which allows 4 bits of data per baud. So I second the vote for dropping the term baud when used to refer to electronic data transmission. What may be better is the characters per second. With most transmissions it takes 10 bits for a character of data. So 1200 or 2400 divided by 10 would give 120 or 240 characters per second which has more meaning to me as a user. Now if we can just get the modem companies to advertise 240 character modems..... Jack