W8SDZ@SIMTEL20.ARPA (Keith Petersen) (06/18/88)
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-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, 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 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 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.friedl@vsi.UUCP (Stephen J. Friedl) (06/19/88)
Keith Petersen posts this article, of which he is not the author: > 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. [note: the article was a good one but my nit-picker is bored] I believe that "baud rate" is incorrect. My understanding is that a state change as above is a "symbol" and "baud" means "symbols per second". I contend that saying: > 300 bauds per second X 1 bit per baud = 300 bps Is like saying: "To calculate time to get to Omaha from Los Angeles: 70 MPH per hour x 1500 miles = .... (Geography flames should join the speed-limit flames on their way to /dev/null, please). Shouldn't it be: 300 baud X 1 bit per symbol = 300 bps Anybody have any comment? I contend that "baud rate" is redundant in the same way that "MPH per hour" unless you are talking about acceleration. When you want to say "baud rate", say "bit rate". -- Steve Friedl V-Systems, Inc. (714) 545-6442 3B2-kind-of-guy friedl@vsi.com {backbones}!vsi.com!friedl attmail!vsi!friedl Nancy Reagan on the Mac-II architecture: "Just say Nu"
berger@clio.las.uiuc.edu (06/20/88)
It's cute, but I wish it had stuck to the topic. Confusing people
with statements like "There are no 1200 baud modems" wrongly
reduces the credibility of experts that know better (what's a
202? Amateur radio operators use them, and they're still common
in industry).
Mike Berger
Department of Statistics
Science, Technology, and Society
University of Illinois
berger@clio.las.uiuc.edu
{ihnp4 | convex | pur-ee}!uiucuxc!clio!bergerpjh@mccc.UUCP (Pete Holsberg) (06/20/88)
In article <721@vsi.UUCP> friedl@vsi.UUCP (Stephen J. Friedl) writes:
...Keith Petersen posts this article, of which he is not the author:
...> 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.
...
...[note: the article was a good one but my nit-picker is bored]
...
...I believe that "baud rate" is incorrect. My understanding is
...that a state change as above is a "symbol" and "baud" means
..."symbols per second". I contend that saying:
...
...> 300 bauds per second X 1 bit per baud = 300 bps
...
...Is like saying:
...
... "To calculate time to get to Omaha from Los Angeles:
...
... 70 MPH per hour x 1500 miles = ....
...
...(Geography flames should join the speed-limit flames on their
...way to /dev/null, please).
...
...Shouldn't it be:
...
... 300 baud X 1 bit per symbol = 300 bps
...
...Anybody have any comment? I contend that "baud rate" is redundant
...in the same way that "MPH per hour" unless you are talking about
...acceleration. When you want to say "baud rate", say "bit rate".
...
...--
...Steve Friedl V-Systems, Inc. (714) 545-6442 3B2-kind-of-guy
...friedl@vsi.com {backbones}!vsi.com!friedl attmail!vsi!friedl
...
...Nancy Reagan on the Mac-II architecture: "Just say Nu"
By the definition of "baud" you are correct. A baud IS a rate. On the
other hand, we use that terminology lots. Eg., "interest rate" and
"interest" are used interchangeably. The amount of interest is almost
useless without stating the period of time over which it is to be
calculated. But that's not the same as "MPG/hr".