lexw@idca.tds.PHILIPS.nl (Lex Wassenberg) (08/24/90)
Okay, in some recent articles there were these hints about the difference
between "baud" and "bits per second". Now I for one have always thought that
baud and bps is the same. So what's the difference? Has it something to do
with bits that don't carry actual data like start and stop bits? Or is it
more subtle? Thanks to anyone who can inform me (and others).
________________
/ / ___ _____/ Lex Wassenberg, Philips TDS
/ / /__ \/ ___/ Apeldoorn, The Netherlands
/ / ___/ /__ lexw@idca.tds.philips.nl
/ / /____/\___/
/ /____________/ It's said that only 10 people on the whole world understood
/_______________/ Einstein. I'm so brilliant that nobody understands me at all.
Disclaimer: Since nobody understands me, I speak only for myself.wsineel@wsooti06.info.win.tue.nl (Eelco Vriezekolk) (08/24/90)
In article <849@idcapd.idca.tds.philips.nl> lexw@idca.tds.PHILIPS.nl (Lex Wassenberg) writes: > >Okay, in some recent articles there were these hints about the difference >between "baud" and "bits per second". Now I for one have always thought that >baud and bps is the same. So what's the difference? Has it something to do As far a I know does 'baud' represent the number of *signal changes* per second. When you use phase modulation, you could easily trasmit two bits with one signal changes (when four phase-changes are possible). -- --------===========--------- Eelco Vriezekolk, wsineel@win.tue.nl Computer scientist looking for nice job in any civilized country. Low payment not necessarily a problem. Contact at above address.
sonny@charybdis.harris-atd.com (Bob Davis) (08/25/90)
In article <849@idcapd.idca.tds.philips.nl> lexw@idca.tds.PHILIPS.nl (Lex Wassenberg) writes: > >Okay, in some recent articles there were these hints about the difference >between "baud" and "bits per second". Now I for one have always thought that >baud and bps is the same. So what's the difference? Has it something to do >with bits that don't carry actual data like start and stop bits? Or is it >more subtle? Thanks to anyone who can inform me (and others). > ________________ Baud rate and Bit rate are two different things (although they are sometimes numerically equal). THE LIGHTS DIM AS THE LEGEND BEGINS... Initially, data rate requirements were low enough that folks were happy to send binary data ONE bit at a time. And the modems employed did SIMPLE things -- like: for a 0, send a tone; and for a 1, send a slightly higher tone. Or things like: for a 0, turn the tone off; and for a 1, turn the tone on. The RATE AT WHICH THE TONES ARE SENT IS THE BAUD RATE. In these simple modems, since ONE BIT WAS CARRIED PER TONE INTERVAL, the baud rate equaled the bit rate. The tone interval is called the baud time. Then, with increasing data rate requirements, the modems got more complicated. In order to double the bit rate, people decided to allow 4 different tone states per baud time, thus allowing 2 bits to be carried per baud time. Some folks chose to modulate the tone to one of 4 different phases depending on the 2 bits. Other folks chose to go to 4 different amplitudes. Thus, the 2 BITS PER BAUD modem arrived, and with it ETERNAL CONFUSION over just what the hay *is* the difference between bps and baud rate ANYWAY. For these more complicated modems, the BIT RATE WAS TWICE THE BAUD RATE. THE BIT RATE *ALWAYS* IS EQUAL TO THE BAUD RATE TIMES THE NUMBER OF INFORMATION BITS CARRIED PER BAUD TIME BY THE MODEM. WHY, you might ask, didn't they just double the rate at which tones were sent and still just use two tone states and get double the throughput that way? Good question. It turns out that real, bandwidth-limited channels - like the telephone channel - shut you down quicker as you try to increase the rate at which tones are sent THAN they do if you choose to send more tone states. The way they "shut you down" as you increase the baud rate is by interference between one tone and the next as you try to send them faster. Real channels "smear" the waveform of one tone into the following one. This is called intersymbol interference, or ISI. Once the designer of the modem has increased the baud rate to the point that he is hitting the ISI "stop", he must increase the number of baud states; i.e., he must assign more bits per baud. Modern modems employ combinations of tone amplitude and phase to achieve 8 or more tone states to obtain even higher data throughputs over the fixed bandwidth telephone channel. Since 8 states will carry 3 bits, an 8-state modem has a bit rate equal to 3 times the baud rate. A 16 state modem has a bit rate = 4 times baud rate, and so on. BPS AND BAUD RATE ARE DIFFERENT. ______________________________________________________________________________ Bob Davis \\ INTERNET : sonny@trantor.harris-atd.com | _ _ | Harris Corporation, ESS \\ UUCP : ...!uunet!x102a!trantor!sonny |_| |_| | | Advanced Technology Dept.\\ AETHER : K4VNO |==============|_/\/\/\|_| PO Box 37, MS 3A/1912 \\ VOICE : (407) 727-5886 | I SPEAK ONLY | |_| |_| | Melbourne, FL 32902 \\ FAX : (407) 729-2537 | FOR MYSELF. |_________|
schweige@cs.nps.navy.mil (Jeffrey M. Schweiger) (08/25/90)
In article <849@idcapd.idca.tds.philips.nl> lexw@idca.tds.PHILIPS.nl (Lex Wassenberg) writes: > >Okay, in some recent articles there were these hints about the difference >between "baud" and "bits per second". Now I for one have always thought that >baud and bps is the same. So what's the difference? Has it something to do >with bits that don't carry actual data like start and stop bits? Or is it >more subtle? Thanks to anyone who can inform me (and others). The term 'baud' represents the number of times per second that a signal changes its value or state (for example, voltage level or frequency), the baud number itself is frequently referred to as the 'baud rate', and has units of measurement of inverse seconds. As Hertz (Hz) also has units of measurement of inverse seconds, an estimate fot the maximum baud rate of a channel is its bandwidth. Baud rate is not immediately useful as it actually represents how fast the signal modulation changes. Usually a data communication user is more interested in the actual speed of data transfer. Therefore, the measure of how many 'bits-per-second' (bps) is more useful in determining how hast data is transmitted. Baud and bps are related by how many bits are transmitted in one signal change. For low speed modems, such as the 300 bps modems following the Bell 103 or CCITT V.21, the baud rate and the bps are the same. With higher speed modems, the modem protocols used transmit more than 1 bit per signal change, and the bps will be greater than the baud rate. For example, 2400 bps modems following the CCITT V.22bis protocol are actually 600 baud modems as they transmit 4 bits per baud using a technique known as Quadrature Amplitude Modulation (QAM). Hope this helps. Jeff Schweiger -- ******************************************************************************* Jeff Schweiger Standard Disclaimer CompuServe: 74236,1645 Internet (Milnet): schweige@cs.nps.navy.mil *******************************************************************************
dmt@pegasus.ATT.COM (Dave Tutelman) (08/25/90)
>In article <849@idcapd.idca.tds.philips.nl> lexw@idca.tds.PHILIPS.nl (Lex Wassenberg) writes: >> >>Okay, in some recent articles there were these hints about the difference >>between "baud" and "bits per second". Now I for one have always thought that >>baud and bps is the same. So what's the difference? Has it something to do >>with bits that don't carry actual data like start and stop bits? Or is it >>more subtle? Thanks to anyone who can inform me (and others). >> ________________ In article <4188@trantor.harris-atd.com> sonny@trantor.harris-atd.com (Bob Davis) writes: > > Baud rate and Bit rate are two different things (although they >are sometimes numerically equal). > >THE LIGHTS DIM AS THE LEGEND BEGINS... ... [ excellent discussion of transmission symbols that carry ... more than one bit ] ... BUT ... > THE BIT RATE *ALWAYS* IS EQUAL TO THE BAUD RATE TIMES THE > NUMBER OF INFORMATION BITS CARRIED PER BAUD TIME BY THE MODEM. Close, but not quite. Lex's original speculation covers "the rest of the story". A "bit" is a unit of information. Start and stop pulses (frequently and inaccurately called start and stop bits) are really transmission related synchronizing signals, not information. The "old" modems Bob describes actually had a bit rate LESS than the baud rate. Dave
scjones@thor.UUCP (Larry Jones) (08/25/90)
In article <4188@trantor.harris-atd.com>, sonny@charybdis.harris-atd.com (Bob Davis) writes: > Baud rate and Bit rate are two different things (although they > are sometimes numerically equal). > > THE LIGHTS DIM AS THE LEGEND BEGINS... As the person who started all this (for which I really should appologize), I just wanted to say that Bob's explanation was exactly right. ---- Larry Jones UUCP: uunet!sdrc!thor!scjones SDRC scjones@thor.UUCP 2000 Eastman Dr. BIX: ltl Milford, OH 45150-2789 AT&T: (513) 576-2070 You should see me when I lose in real life! -- Calvin
sonny@charybdis.harris-atd.com (Bob Davis) (08/26/90)
In article <139@thor.UUCP> scjones@thor.UUCP (Larry Jones) writes: >In article <4188@trantor.harris-atd.com>, sonny@charybdis.harris-atd.com (Bob Davis) writes: >> Baud rate and Bit rate are two different things (although they >> are sometimes numerically equal). >> >> THE LIGHTS DIM AS THE LEGEND BEGINS... > >As the person who started all this (for which I really should >appologize), I just wanted to say that Bob's explanation was >exactly right. >---- Don't apologize, Larry...Sometimes when things get a mite slow and boring it's fun to mosy up and chunk a piece of meat in the water to see the piranhas churn. Heck, it's even fun to be one of the piranhas once in a while...;-) ______________________________________________________________________________ Bob Davis \\ INTERNET : sonny@trantor.harris-atd.com | _ _ | Harris Corporation, ESS \\ UUCP : ...!uunet!x102a!trantor!sonny |_| |_| | | Advanced Technology Dept.\\ AETHER : K4VNO |==============|_/\/\/\|_| PO Box 37, MS 3A/1912 \\ VOICE : (407) 727-5886 | I SPEAK ONLY | |_| |_| | Melbourne, FL 32902 \\ FAX : (407) 729-2537 | FOR MYSELF. |_________|
sonny@charybdis.harris-atd.com (Bob Davis) (08/27/90)
In article <5001@pegasus.ATT.COM> dmt@pegasus.ATT.COM (Dave Tutelman) writes: [Stuff deleted] >In article <4188@trantor.harris-atd.com> sonny@trantor.harris-atd.com (Bob Davis) writes: [Deletions] >> THE BIT RATE *ALWAYS* IS EQUAL TO THE BAUD RATE TIMES THE >> NUMBER OF INFORMATION BITS CARRIED PER BAUD TIME BY THE MODEM. ^^^^^^^^^^^^^^^^ > >Close, but not quite. >Lex's original speculation covers "the rest of the story". > >A "bit" is a unit of information. Start and stop pulses (frequently >and inaccurately called start and stop bits) are really transmission >related synchronizing signals, not information. The "old" modems >Bob describes actually had a bit rate LESS than the baud rate. > >Dave YOU ARE RIGHT, DAVE, but... Above, I stress "INFORMATION BITS" as being important. For the old modems which have the overhead of synchronization bits, as well as for newer techniques which have the overhead of redundant bits necessary to support error-correction coding strategies, I would say that the INFORMATION BITS carried per baud is reduced below the raw channel bits per baud. For example, in the old schemes using 1 start bit, 8 INFORMATION BITS, and 1 stop bit, I would say that the INFORMATION BITS per baud is equal to 0.8. This is because only 8 out of every block of 10 bits bears INFORMATION. Similarly, if a rate one-half error correction code is used on one of the newer schemes, I would say that the INFORMATION BITS per baud is one-half the raw channel bits per baud. Therefore, I stand by the statement: THE BIT RATE *ALWAYS* IS EQUAL TO THE BAUD RATE TIMES THE NUMBER OF INFORMATION BITS CARRIED PER BAUD TIME BY THE MODEM. ^^^^^^^^^^^^^^^^ Peace, ______________________________________________________________________________ Bob Davis \\ INTERNET : sonny@trantor.harris-atd.com | _ _ | Harris Corporation, ESS \\ UUCP : ...!uunet!x102a!trantor!sonny |_| |_| | | Advanced Technology Dept.\\ AETHER : K4VNO |==============|_/\/\/\|_| PO Box 37, MS 3A/1912 \\ VOICE : (407) 727-5886 | I SPEAK ONLY | |_| |_| | Melbourne, FL 32902 \\ FAX : (407) 729-2537 | FOR MYSELF. |_________|
scjones@thor.UUCP (Larry Jones) (08/27/90)
In article <5001@pegasus.ATT.COM>, dmt@pegasus.ATT.COM (Dave Tutelman) writes: > A "bit" is a unit of information. Start and stop pulses (frequently > and inaccurately called start and stop bits) are really transmission > related synchronizing signals, not information. The "old" modems > Bob describes actually had a bit rate LESS than the baud rate. Well, everything's relative, but I have to disagree with you here. A modem converts a serial bit stream into a signal suitable for transmission over a phone line and vice versa. The start and stop "pulses" ARE, in fact, BITS in the serial bit stream -- the modem doesn't have to do anything special with them, it just transmits them like any other bits. Thus, they should be counted when calculating the bps rate. (And as far as I know, they always are. I've never heard anyone refer to a modem as being 1200 bps synchronous, 960 bps async). Of course, they DO make a difference when calculating the characters per second rate, which is why cps isn't simply bps / 8 that you might expect. But that's a function of serial communication, whether modems are involved or not. ---- Larry Jones UUCP: uunet!sdrc!thor!scjones SDRC scjones@thor.UUCP 2000 Eastman Dr. BIX: ltl Milford, OH 45150-2789 AT&T: (513) 576-2070 Bad news, Mom. I promised my soul to the Devil this afternoon. -- Calvin
cgordon@vpnet.chi.il.us (Gordon Hlavenka) (08/28/90)
>THE LIGHTS DIM AS THE LEGEND BEGINS...
(... a good explanation deleted here, go back and read it)
One that you probably use every day, with four bits per symbol: Touch tones!
Each key on the keypad generates a pair of tones: One is determined by the
ROW, and one by the COLUMN. There is a "hidden" column (its tone is 1633
Hz) which is not present on a standard telephone keypad.
So, with 4 rows and 4 columns, there are 16 possible tone combinations.
This allows the encoding of 4 bits (called a "nibble" for you budding
lexicologists) per symbol.
The typical minimum usable burst length is 50 milliseconds, with a 50
millisecond pause between bursts. So this gives a baud rate of ten, and a
BPS rate of 40. (The math is left as an exercise etc.)
-----------------------------------------------------
Gordon S. Hlavenka cgordon@vpnet.chi.il.us
Disclaimer: I've fallen! And I can't get up!
So how could I have expressed an opinion?rcsmith@anagld.analytics.com (Ray Smith) (08/28/90)
lexw@idca.tds.PHILIPS.nl (Lex Wassenberg) writes: >Okay, in some recent articles there were these hints about the difference >between "baud" and "bits per second". Now I for one have always thought that >baud and bps is the same. So what's the difference? Has it something to do >with bits that don't carry actual data like start and stop bits? Or is it >more subtle? Thanks to anyone who can inform me (and others). Here is something I snarfed of the net some time ago. I thought it may help. -Ray > From: WSDZ@SIMTEL.ARPA (Keith Petersen) > Newsgroups: comp.dcom.modems > Subject: Bits, baud rate, and bps > Message-ID: <KPETERSEN.12407361536.BABYL@SIMTEL.ARPA> > Date: 18 Jun 88 07:50:00 GMT The following is presented "as-is" for its informational value. I am not the author. This file was uploaded to my BBS. --Keith Petersen ---cut-here--- BAUD-BPS.INF ---cut-here--- (NOTE: This article is excerpted from THE MODEM REFERENCE, a book written by Michael A. Banks, to be published in August, 1988, by Brady Books/Simon & Schuster. Permission is granted to post this article on bulletin board systems and online services provided this notice and the copyright notice are included. If you want to know more about how computer communications works, ask for the book (ISBN# 0-13-586646-4) at your local B. Dalton's or independent bookstore. THE MODEM REFERENCE is recommended by a number of reviewers, including Jerry Pournelle, who said in his May, 1988, BYTE column, "If you're thinking of getting into computer communications, get this book first." BITS, BAUD RATE, AND BPS Taking the Mystery Out of Modem Speeds by Michael A. Banks (Copyright, 1988, Michael A. Banks) Modem transmission speed is the source of a lot of confusion, even among otherwise informed computer and modem users. The root of the problem is the fact that the terms "baud" and "bits per second" are used interchangeably and indiscriminately. I strongly suspect this is a result of the fact that 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, please read this article carefully; I guarantee to clear up the confusion and disabuse you of any false concepts ... Bits per second (bps) Bits per second is a measure of the number of data bits (digital 0's and 1's) transmitted each second in a communications channel. This is sometimes referred to as "bit rate." Individual characters (letters, numbers, etc.), also referred to as bytes, are composed of several bits. While a modem's bit rate is tied to its baud rate, the two are not the same, as explained below. Baud rate Baud rate is a measure of the number of times per second a signal in a communications channel varies, or makes a transition between states (states being frequencies, voltage levels, or phase angles). One baud is one such change. Thus, a 300-baud modem's signal changes state 300 times each second, while a 600-baud modem's signal changes state 600 times per second. This does not necessarily mean that a 300-baud and a 600-baud modem transmit 300 and 600 bits per second, as you'll learn in a few lines. Calculating bits per second Depending on the modulation technique used, a modem can transmit one bit--or more or less than one bit--with each baud, or change in state. Or, to put it another way, one change of state can transmit one bit--or more or less than one bit. As I mentioned earlier, the number of bits a modem transmits per second is directly related to the number of bauds that occur each second, but the numbers are not necessarily the same. To illustrate this, first consider a modem with a baud rate of 300, using a transmission technique called FSK (Frequency Shift Keying, in which four different frequencies are turned on and off to represent digital 0 and 1 signals from both modems). When FSK is used, each baud (which is, a gain, a change in state) transmits 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 modem operating at 1200 baud were to use one change in state to send each bit, that modem's bps rate would be 1200. (There are no 1200 baud modems, by the way; remember that. This is only a demonstrative and hypothetical example.) Now, consider a hypothetical 300-baud modem using a modulation technique that requires two changes in state to send one bit, which can also be viewed as 1/2 bit per baud. Such a modem's bps rate would be 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: 300 baud --------------- = 150 bps 2 bauds per bit Now let's move away from the hypothetical and into reality, as it exists in the world of modulation. First, lest you be misled into thinking that "any 1200 baud modem" should be able to operate at 2400 bps with a two-bits-perbaud modulation technique, remember that I said there are no 1200 baud modems. Medium- and high-speed modems use baud rates that are lower than their bps rates. Along with this, however, they use multiple-state modulation to send more than one bit per baud. For example, many 300/1200 bps modems operate at 300 baud and use a modulation technique called phase modulation that transmits four bits per baud. Such modems are capable of 1200 bps operation, 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 1/4 baud per bit (Some 1200-bps modems use a modulation technique that sends two bits per baud, in which case they operate at 600 baud.) Similarly, 2400 bps modems that conform to the CCITT V.22 recommendation (virtually all of them) actually use a baud rate of 600 when they operate at 2400 bps. However, they also use a modulation technique that transmits four bits per baud: 600 baud X 4 bits per baud = 2400 bps or 600 baud ------------------ = 2400 bps 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 1/4 baud per bit Characters per second (cps) Characters per second is a measure of the number of characters (letters, numbers, spaces, and symbols) transmitted over a communications channel in one second. Cps is often the bottom line in rating data transmission speed, and a more convenient way of thinking about data transfer than baud- or bitrate . Determining the number of characters transmitted per second is easy: simply divide the bps rate by the number of bits per character. You must of course take into account the fact that more than just the bits that make up the binary digit representing a character are transmitted when a character is sent from one system to another. In fact, up to 10 bits may be transmitted for each character during ASCII transfer, whether 7 or 8 data bits are used. This is because what are called start- and stop-bits are added to characters by a sending system to enable the receiving system to determine which groups of bits make up a character. In addition, a system usually adds a parity bit during 7-bit ASCII transmission. (The computer's serial port handles the addition of the extra bits, and all extra bits are stripped out at the receiving end.) So, in asynchronous data communication, the number of bits per character is usually 10 (either 7 data bits, plus a parity bit, plus a start bit and a stop bit, or 8 data bits plus a start bit and a stop bit). Thus: 300 bps ----------------------- = 30 characters per second 10 bits per character 1200 bps ----------------------- = 120 characters per second 10 bits per character 2400 bps ----------------------- = 240 characters per second 10 bits per character Common speeds The most commonly-used communications rates for dial-up systems (BBSs and online services like DELPHI) are 300 and 1200 bps. A few older systems--especially Telex systems--communicate at 110 bps, but these are gradually going the way of the dinosaur. 2400, 4800 and 9600 bps modems are generally available, but few online services or BBSs accommodate them. This will be changing in the near future, however, with the cost of high-speed modem technology decreasing as the demand for it increases. Modems with even higher bps rates are manufactured (19,200 and up) but these are not used with dial-up systems; the upper limit on asynchronous data transmission via voice-grade telephone lines appears to be 9600 bps. The use of higher transmission rates requires special dedicated lines that are "conditioned" (i.e., shielded from outside interference) as well as expensive modulation and transmission equipment. If nothing else, I hope the examples here have shown you just why baud rate is not always equivalent to bps rate. (And if anyone who tries to sell a modem to you tells you otherwise, you'll do well to take your business elsewhere.) When you're considering a particular modem for purchase, look for its bps rate, rather than its baud rate, and use the bps rate to determine how many characters per second the modem can actually send. # Michael A. Banks (KZIN on DELPHI) has published some 700 magazine articles and short stories. He's also written 17 books on topics ranging from telecommunications (most recently, DELPHI: The Official Guide, and The Modem Book, from Brady Books/Simon & Schuster), to science fiction novels.
bill@bilver.UUCP (Bill Vermillion) (08/29/90)
In article <5001@pegasus.ATT.COM-> dmt@pegasus.ATT.COM (Dave Tutelman) writes: ->>In article <849@idcapd.idca.tds.philips.nl> lexw@idca.tds.PHILIPS.nl (Lex Wassenberg) writes: ->>> ->>>Okay, in some recent articles there were these hints about the difference ->>>between "baud" and "bits per second". Now I for one have always thought that ->>>baud and bps is the same. So what's the difference? Has it something to do ->>>with bits that don't carry actual data like start and stop bits? Or is it ->>>more subtle? Thanks to anyone who can inform me (and others). ->>> ________________ -> ->In article <4188@trantor.harris-atd.com> sonny@trantor.harris-atd.com (Bob Davis) writes: ->> ->> Baud rate and Bit rate are two different things (although they ->>are sometimes numerically equal). ->> ->>THE LIGHTS DIM AS THE LEGEND BEGINS... -> ->... [ excellent discussion of transmission symbols that carry ->... more than one bit ] ->... BUT ... -> ->> THE BIT RATE *ALWAYS* IS EQUAL TO THE BAUD RATE TIMES THE ->> NUMBER OF INFORMATION BITS CARRIED PER BAUD TIME BY THE MODEM. Try this - bit rate is equal to baud rate time bits per baud times channels per carrier. Take a 7 cycle carrier, phase modulate it up to 6 bits per baud, and that give you 42 bits/second. Take 400 of these channels and put them down a phone line (400 * 7 = 2800hz - enough to fit inside the 3kHz bandwith of a voice grade line) and you wind up with 6 * 7 * 400 or 16800 bps on what is nominally a 7 baud (or 42 baud?) signal. Do you know what that scheme is. That's a Telebit Trailblazer - getting up to 18003 bps over a phone line with no compression. Bits and baud are a LONG way apart in that machine. -- Bill Vermillion - UUCP: uunet!tarpit!bilver!bill : bill@bilver.UUCP
davet@tsdiag.ccur.com (Dave Tiller N2KAU) (08/29/90)
In article <849@idcapd.idca.tds.philips.nl> lexw@idca.tds.PHILIPS.nl (Lex Wassenberg) writes: > >Okay, in some recent articles there were these hints about the difference >between "baud" and "bits per second". Now I for one have always thought that >baud and bps is the same. So what's the difference? Has it something to do >with bits that don't carry actual data like start and stop bits? Or is it >more subtle? Thanks to anyone who can inform me (and others). A good question, Lee. I'll give it a shot. The term 'baud' refers to the number of symbol transitions per second (note that the unit 'per second' is built into the name baud. Bauds/s is bogus). Bps refers to the actual number of data bits sent per second. These two numbers can be the same, or the bps figure can be greater than the baud rate. How can this be? Well, there's more than one way to modulate a signal, and there's more than one way to represent data than 0's and 1's. Here's an example: If I choose to use phase modulation, I could easily use the following: Phase change dibit ------------ ----- 0 deg 00 90 deg 01 180 deg 10 270 deg 11 In this scheme, each phase transition (baud symbol) represents _two_ data bits! Therefore, if I decide to sent 2400 transitions/sec (baud rate=2400) the actual bps rate will be 4*2400=4800. Neat, huh? Taken to an extreme, you can simultaneously change the phase and amplitude of a signal, stretching the number of data bits/symbol higher and higher. Another method to squeeze data over a narrow bandwidth channel is to use multiple tones, instead of just two for mark/space. Telebit's PEP protocol starts out with _512_(!) discrete tones and negotiates down from there depending on line quality. If all the tones were used for data, at 19200 bps the actual baud rate would be 37.5!!! I'm pretty sure there's lots of error check/correction stuff as well, so don't expect this is really the case. On a related note, the FCC was quite visionary when they declared that the baud rate of HF data transmissions can't exceed 300 _baud_. If we can squeeze more bits/baud out of our signal, all the better. With some new DSP modems, I can forsee 4800-9600 bps over a 3 kHz voice channel to almost anywhere in the world. I hope this explanation helps, mail me directly if you have any questions. -- David E. Tiller davet@tsdiag.ccur.com | Concurrent Computer Corp. FAX: 201-870-5952 Ph: (201) 870-4119 (w) | 2 Crescent Place, M/S 117 UUCP: ucbvax!rutgers!petsd!tsdiag!davet | Oceanport NJ, 07757 ICBM: 40 16' 52" N 73 59' 00" W | N2KAU @ NN2Z
jgarland@kean.ucs.mun.ca (08/30/90)
<<<<<<<<<Multiple Discussions of Bit/Baud defn's deleted>>>>>>>>> As a psychologist I can't really follow all the esoteric discussions going on here but--again as a psychologist--I can't help wondering *what's* going on here. Why is there no standard def'n of "baud?" Why is it open to such interpretation? [This whole discussion reminds me of discussions in psychology w.r.t. scaling techniques or factor analytic techniques...but it doesn't seem right that it should. I mean *everybody* knows that psychologists are fuzzy. Aren't *you* talking about quantifiable, measureable phenomena?] Is there a senior member of the telecommunications fraternity who can enlighten the rest of us? Just curious. John Garland jgarland@mun Bitnet jgarland@kean.ucs.mun.ca Internet
dmt@pegasus.ATT.COM (Dave Tutelman) (09/04/90)
In article <130108@kean.ucs.mun.ca> jgarland@kean.ucs.mun.ca writes: > >As a psychologist I can't really follow all the esoteric discussions >going on here but--again as a psychologist--I can't help wondering >*what's* going on here. Why is there no standard def'n of "baud?" Why >is it open to such interpretation? Nope! There IS a standard definition. The fact that some choose to ignore it is their problem. The fuzzy one is actually bits per sec, but I'll get to that. >Is there a senior member of the telecommunications fraternity who can >enlighten the rest of us? And I thought nobody would ask >:-)> First of all, let me compliment Bob Davis on a fine discussion of the history of multiple-bits-per-symbol. Also, let me withdraw my follow-up to his formula; it was right and I was misinterpreting it. That said: BAUD = the number of symbols per second being transmitted or received. BIT = a binary digit. REAL SIMPLE !!! Points worth noting: - As Bob pointed out, many modems transmit MULTI-LEVEL pulses, not just 2-level. If a "pulse" (or "symbol", or discrete signal) can take on one of 8 values, it is capable of transmitting 3 bits per pulse (because it takes three bits to name one out of eight values). - Baud IS A RATE. It doesn't make sense to talk about "300 bauds per second", any more than "knots per hour". The correct usage is "300 bauds" for 300 symbols per second. - The information rate or bit rate provided by a modem is, as Bob correctly posted, THE BAUD RATE TIMES THE NUMBER OF INFORMATION BITS PER SYMBOL. ^^^^^^^^^^^ I have highlighted the word "information", because it was the thing that overruled my original objection to the formula. Some of the potential information (potential bits) can be taken up by the modem itself. Examples include synchronizing the bit stream, or performing error coding (some modems actually do this). These bits are not information bits at the interface, and mustn't be counted in the modem's bit rate. Soapbox time, for a fine point. I posted earlier that the well-known (and well-worn) 300-baud modem was typically used at 240 bits/sec, because 2 bits are added per character for synchronization purposes. Since not everyone agreed (:-), let me explain further: The so-called "300 baud modem" does not actually transmit discrete symbols. It transmits a binary level, and will support transitions of that level up to 300 times per second. But a "bit" is a DISCRETE binary digit, so some time quantization must be imposed on this binary level. In order for the bits to be "information", the quantization at the receiver must be the same as that at the transmitter; otherwise the binary digits would be garbage. This is called "synchronization". There are lots of ways of synchronizing this "asynchronous" stream. Few are more recent than the 60s (this was my field in the 60s), and the most common one goes back to the earliest days of the teletypewriter (very early 20th century). That technique is to transmit a "1" level for a while, followed by a "0" level, then each of the next "n" bits are a sampling of that level at a known later time. The one-to-zero transition is the clocking moment to reference for all subsequent pulses. These levels are commonly called a "stop pulse" and a "start pulse" respectively. They are frequently and incorrectly called stop and start BITS. Note: - The two synchronizing levels are NOT INFORMATION. Rather, they allow a transmitter and receiver to agree on how to break up a time-continuous binary signal into bits. - The more stable the clocks at the transmit and receive ends, the more bits you can send without needing to "resync" (that is, transmit a stop and start pulse). Today's crystal oscillators would easily permit thousands of bits per "character", allowing the 300 baud modem to come very close to 300 bits per second. - For convenience, hardware modularity, and a fair dollop of just-plain-history, most REAL USERS of the 300 baud modem just resynchronize every octet (8 bits). Thus my comment that its typical use is 240 bits/sec. >Just curious. Hope the first part helped. I'm sure the last part (the "soapbox") didn't :-( (But it helped me :-) Dave +---------------------------------------------------------------+ | Dave Tutelman | | Physical - AT&T Bell Labs - Lincroft, NJ | | Logical - ...att!pegasus!dmt == dmt@pegasus.att.com | | Audible - (201) 576 2194 | +---------------------------------------------------------------+
bobmon@iuvax.cs.indiana.edu (RAMontante) (09/04/90)
dmt@pegasus.ATT.COM (Dave Tutelman) <5022@pegasus.ATT.COM> : | | - Baud IS A RATE. It doesn't make sense to talk about "300 | bauds per second", any more than "knots per hour". The | correct usage is "300 bauds" for 300 symbols per second. I claim that it ought to be "300 Baud", not "300 bauds". By way of analogy, we speak of "50 MegaHertz", not "50 Megahertzes". Also, much as I hate to say this, it may be that binary-digital types will evolve a variant definition of "Baud" that suits their need better than the analog-EE one. Namely, BAUD == INFORMATION BITS PER SECOND. Certainly many people use it this way, and most modems are marketed using this definition. There is precedent for it, too --- computerish types treat the prefix "kilo" (abbreviated "K" or sometimes "k") as meaning 2^10 == 1024, although the formal SI definition is 10^3 == 1000, and similarly for "mega", "giga", etc.