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 * BITS, BAUD RATE, AND BPS                          by Michael A. Banks
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 Here's an excerpt from The Modem Reference, written by Michael A. Banks
 and recommended by Jerry Pournelle in Byte, The Smithsonian Magazine,
 et al.  The right to reproduce this article is granted on the condition
 that all text, including this notice and the notice at the end of the
 article, remain unchanged, and that no text is added to the body of the
 article.  Thanks!  --MB
 
 
 Taking the Mystery Out of Modem Speeds
 (Copyright, 1988, 1990, Michael A. Banks.  All rights reserved.)
 
 
 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.
 
 Determining 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-per-baud
 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, 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.  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
 
 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 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
 bit-rate.
 
 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 CompuServe, DELPHI, and GEnie) are 300, 1200,
 and 2400 bps.  A few older systems--especially Telex systems--
 communicate at 110 bps, but these are gradually going the way of the
 dinosaur.  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 you found this article useful, you may want to pick up a copy of the
 book from which it was excerpted:
 
 THE MODEM REFERENCE
 by Michael A. Banks
 Published by Brady Books/Simon & Schuster
 In addition to explaining the technical aspects of modem operation,
 communications software, data links, and other elements of computer
 communications, the book provides detailed, illustrated "tours" of major
 online services such as UNISON, CompuServe, DELPHI, BIX, Dow Jones News
 /Retrieval, MCI Mail, the PRODIGY service, and others.  It also contains
 information on using packet switching networks and BBSs, as well as dial
 -up numbers for various networks and BBSs.
 
 You'll also find hands-on guides to buying, setting up, using, and
 troubleshooting computer communications hardware and software.  (And the
 book "supports" all major microcomputer brands.)  THE MODEM REFERENCE is
 available at your local B. Dalton's, WaldenSoftware, Waldenbooks, or
 other bookstore, either in stock or by order.  Or, phone 800-624-0023 to
 order direct.
 
 Want the lowdown on getting more out of your word processor?  Read the
 only book on word processing written by writers, for writers: WORD
 PROCESSING SECRETS FOR WRITERS, by Michael A. Banks & Ansen Dibel
 (Writer's Digest Books).  WORD PROCESSING SECRETS FOR WRITERS is
 available at your local B. Dalton's, Waldenbooks, or other bookstore,
 either in stock or by order.  Or, phone 800-543-4644 (800-551-0884 in
 Ohio) to order direct.
 
 Do you use DeskMate 3?  Are you getting the most out of the program?  To
 find out, get a copy of GETTING THE MOST OUT OF DESKMATE 3, by Michael
 A. Banks.  Published by Brady Books/Simon & Schuster, it is available in
 your local Tandy/Radio Shack, WaldenSoftware, or Waldenbooks store now.
 Or, phone 800-624-0023 to order direct.
 
 Other books by Michael A. Banks
 UNDERSTANDING FAX & E-MAIL (Howard W. Sams & Co.)
 THE ODYSSEUS SOLUTION (w/Dean Lambe; SF novel; Baen Books)
 JOE MAUSER: MERCENARY FROM TOMORROW (w/Mack Reynolds; SF novel; Baen
  Books)
 SWEET DREAMS, SWEET PRICES (w/Mack Reynolds; SF novel; Baen Books)
 COUNTDOWN: THE COMPLETE GUIDE TO MODEL ROCKETRY (TAB Books)
 THE ROCKET BOOK (w/Robert Cannon; Prentice Hall Press)
 SECOND STAGE: ADVANCED MODEL ROCKETRY (Kalmbach Books)

 For more information, contact:
 Michael A. Banks
 P.O. Box 312
 Milford, OH  45150