215 lines
9.9 KiB
Plaintext
215 lines
9.9 KiB
Plaintext
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Here's an excerpt from The Modem Reference, written by Michael A.
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Banks and recommended by Jerry Pournelle in Byte, The Smithsonian
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Magazine, et al.
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The right to reproduce this article is granted on the condition
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that all text, including this notice and the notice at the end of
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the article, remain unchanged, and that no text is added to the
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body of the article.
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Thanks! --MB
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BITS, BAUD RATE, AND BPS
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Taking the Mystery Out of Modem Speeds
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by Michael A. Banks
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(Copyright, 1988, Michael A. Banks. All rights reserved.)
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Modem transmission speed is the source of a lot of
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confusion, even among otherwise informed computer and modem
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users. The root of the problem is the fact that the terms "baud"
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and "bits per second" are used interchangeably and
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indiscriminately. I strongly suspect this is a result of the
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fact that it's easier to say "baud" than "bits per second,"
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though misinformation has a hand in it, too.
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If you've ever found yourself confused by the relationship
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between bits and baud rate, or if you think that a modem's baud
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rate is the same as the number of bits or characters it transmits
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per second, please read this article carefully; I guarantee to
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clear up the confusion and disabuse you of any false concepts ...
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Bits per second (bps)
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Bits per second is a measure of the number of data bits
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(digital 0's and 1's) transmitted each second in a communications
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channel. This is sometimes referred to as "bit rate."
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Individual characters (letters, numbers, etc.), also
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referred to as bytes, are composed of several bits.
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While a modem's bit rate is tied to its baud rate, the two
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are not the same, as explained below.
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Baud rate
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Baud rate is a measure of the number of times per second a
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signal in a communications channel varies, or makes a transition
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between states (states being frequencies, voltage levels, or
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phase angles). One baud is one such change. Thus, a 300-baud
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modem's signal changes state 300 times each second, while a 600-
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baud modem's signal changes state 600 times per second. This
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does not necessarily mean that a 300-baud and a 600-baud modem
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transmit 300 and 600 bits per second, as you'll learn in a few
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lines.
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Determining bits per second
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Depending on the modulation technique used, a modem can
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transmit one bit--or more or less than one bit--with each baud,
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or change in state. Or, to put it another way, one change of
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state can transmit one bit--or more or less than one bit.
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As I mentioned earlier, the number of bits a modem transmits
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per second is directly related to the number of bauds that occur
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each second, but the numbers are not necessarily the same.
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To illustrate this, first consider a modem with a baud rate
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of 300, using a transmission technique called FSK (Frequency
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Shift Keying, in which four different frequencies are turned on
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and off to represent digital 0 and 1 signals from both modems).
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When FSK is used, each baud (which is, a gain, a change in state)
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transmits one bit; only one change in state is required to send a
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bit. Thus, the modem's bps rate is also 300:
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300 bauds per second X 1 bit per baud = 300 bps
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Similarly, if a modem operating at 1200 baud were to use one
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change in state to send each bit, that modem's bps rate would be
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1200. (There are no 1200 baud modems, by the way; remember that.
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This is only a demonstrative and hypothetical example.)
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Now, consider a hypothetical 300-baud modem using a
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modulation technique that requires two changes in state to send
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one bit, which can also be viewed as 1/2 bit per baud. Such a
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modem's bps rate would be 150 bps:
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300 bauds per second X 1/2 baud per bit = 150 bps
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To look at it another way, bits per second can also be
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obtained by dividing the modem's baud rate by the number of
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changes in state, or bauds, required to send one bit:
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300 baud
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--------------- = 150 bps
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2 bauds per bit
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Now let's move away from the hypothetical and into reality,
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as it exists in the world of modulation.
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First, lest you be misled into thinking that "any 1200 baud
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modem" should be able to operate at 2400 bps with a two-bits-per-
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baud modulation technique, remember that I said there are no 1200
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baud modems. Medium- and high-speed modems use baud rates that
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are lower than their bps rates. Along with this, however, they
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use multiple-state modulation to send more than one bit per baud.
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For example, 1200 bps modems that conform to the Bell 212A
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standard (which includes most 1200 bps modems used in the U.S.)
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operate at 300 baud and use a modulation technique called phase
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modulation that transmits four bits per baud. Such modems are
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capable of 1200 bps operation, but not 2400 bps because they are
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not 1200 baud modems; they use a baud rate of 300. So:
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300 baud X 4 bits per baud = 1200 bps
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or
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300 baud
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------------------ = 1200 bps
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1/4 baud per bit
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Similarly, 2400 bps modems that conform to the CCITT V.22
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recommendation (virtually all of them) actually use a baud rate
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of 600 when they operate at 2400 bps. However, they also use a
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modulation technique that transmits four bits per baud:
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600 baud X 4 bits per baud = 2400 bps
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or
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600 baud
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------------------ = 2400 bps
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1/4 baud per bit
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Thus, a 1200-bps modem is not a 1200-baud modem, nor is a
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2400-bps modem a 2400-baud modem.
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Now let's take a look at 9600-bps modems. Most of these
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operate at 2400 baud, but (again) use a modulation technique that
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yields four bits per baud. Thus:
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2400 baud X 4 bits per baud = 9600 bps
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or
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2400 baud
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------------------ = 9600 bps
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1/4 baud per bit
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Characters per second (cps)
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Characters per second is the number of characters (letters,
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numbers, spaces, and symbols) transmitted over a communications
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channel in one second. Cps is often the bottom line in rating
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data transmission speed, and a more convenient way of thinking
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about data transfer than baud- or bit-rate.
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Determining the number of characters transmitted per second
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is easy: simply divide the bps rate by the number of bits per
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character. You must of course take into account the fact that
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more than just the bits that make up the binary digit
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representing a character are transmitted when a character is sent
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from one system to another. In fact, up to 10 bits may be
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transmitted for each character during ASCII transfer, whether 7
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or 8 data bits are used. This is because what are called start-
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and stop-bits are added to characters by a sending system to
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enable the receiving system to determine which groups of bits
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make up a character. In addition, a system usually adds a parity
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bit during 7-bit ASCII transmission. (The computer's serial port
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handles the addition of the extra bits, and all extra bits are
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stripped out at the receiving end.)
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So, in asynchronous data communication, the number of bits
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per character is usually 10 (either 7 data bits, plus a parity
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bit, plus a start bit and a stop bit, or 8 data bits plus a start
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bit and a stop bit). Thus:
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300 bps
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----------------------- = 30 characters per second
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10 bits per character
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1200 bps
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----------------------- = 120 characters per second
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10 bits per character
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2400 bps
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----------------------- = 240 characters per second
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10 bits per character
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Common speeds
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The most commonly-used communications rates for dial-up
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systems (BBSs and online services like CompuServe, DELPHI, and
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GEnie) are 300, 1200, and 2400 bps. A few older systems--
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especially Telex systems--communicate at 110 bps, but these are
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gradually going the way of the dinosaur. 4800 and 9600 bps
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modems are generally available, but few online services or BBSs
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accommodate them. This will be changing in the near future,
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however, with the cost of high-speed modem technology decreasing
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as the demand for it increases.
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Modems with even higher bps rates are manufactured (19,200
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and up) but these are not used with dial-up systems; the upper
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limit on asynchronous data transmission via voice-grade telephone
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lines appears to be 9600 bps. The use of higher transmission
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rates requires special dedicated lines that are "conditioned"
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(i.e., shielded from outside interference) as well as expensive
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modulation and transmission equipment.
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#
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If you found this article useful, you may want to pick up a
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copy of the book from which it was excerpted:
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THE MODEM REFERENCE
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by Michael A. Banks
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Published by Brady Books/Simon & Schuster
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ISBN # 0-13-586646-4 $19.95
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In addition to explaining the technical aspects of modem
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operation, communications software, data links, and other
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elements of computer communications, the book provides detailed,
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illustrated "tours" of major online services such as UNISON,
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CompuServe, DELPHI, BIX, Dow Jones News/Retrieval, MCI Mail, and
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others. It also contains information on using packet switching
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networks and BBSs, as well as dial-up numbers for various
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networks and BBSs.
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You'll also find hands-on guides to buying, setting up,
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using, and troubleshooting computer communications hardware and
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software. (And the book "supports" all major microcomputer
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brands.)
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For more information, contact:
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Michael A. Banks
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P.O. Box 312
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Milford, OH 45150
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