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From: Gord Hannah
To: All
Date: 2011-01-15 01:00:04
Subject: [2 of 12] Comm Primer

The data was taken two bits at a time and thus could be as: 00, 01, 10, and
11. Accordingly, the 4 possible phase changes could be directly related to
these (4) bit sequences. Now, in this example, the changing of the phases
was in line with every other bit, and so at half the bit rate. It was
therefore said to have a Baud Rate of half the Bit Rate.

So for 1200 bits per second data, the Baud Rate was 600. This extrapolation
was actually used in ITU-T Recommendation V.26. Given this fact, it is hard
to fault anyone for following it.

That is basically the end of the extrapolation, or at least it should have
been!  It was finally recognized as being inappropriate and is no longer
used. If you did a search of Recommendation V.32bis or V.34 you would find
that the word Baud is not used in any context. It is simply not found in
the recommendations at all. Why then are so many people saying things like
the baud rate of V.32bis is 2400? Simply, it is because magazine articles
and so-called experts continue to use this poor and improper extrapolation.
It is also why so many people try to explain it in different ways and
always seem to be in conflict with each other. Frankly, there is NO correct
explanation or description.

BIT - Acronym for (Bi)nary Digi(t) (ie. 0 or 1). As used in the context of
this forum the bit is a basic unit of data.

BPS - (B)its (P)er (S)econd - the unit of measure which refers to the data
transfer rate between communications devices. The unit by which raw modem
throughput is rated.

BYTE - Typically a group of 8 binary digits, but can vary from 6 to nine
bits depending on the platform. It is a basic unit of memory used to store
data in a computer.  Additionally, it is the basic element size used when
expressing ASCII characters such as you see typed here. [See OCTET]

CAS - IBM and DCA standard for computer-fax modem interface.

CCITT - Comite Consultatif International Telegraphique et Telephonique
(French for The International Telegraph and Telephone Consultative
Committee). Now known as ITU-T (International Telecommunications Union-
Telecommunication Standards Sector).

CLASS - "Classes" refer to the standards defined by technical subcommittee
TR-29.2 of the Telecommunications Industry Association (TIA) in the USA.  A
"Class" (or, formally, Service Class) is a set of capabilities of a fax
MODEM (not a fax machine) which is controlled by an extension of the Hayes
AT Command Set.  Class 1 defines only a few commands (about 10), and
provides the minimal set of hardware features in the modem necessary to
support fax transmission and reception. Class 2 defines a much larger set
of commands (mostly parameters), and requires the modem to implement the
ITU-T T.30 fax protocols in the modem firmware.

Because the "Class" of a device refers only to the way that fax software
talks to the modem and not to how the device works across the phone line
with other fax devices, the Class has no bearing on interoperability
(assuming, of course, the modem firmware and fax software are correctly
implemented!).  Class 1 and Class 2 modems and other Group 3 fax devices
can communicate with each other across the phone line, in any combination.

   Class1 - Electronic Industries Association/Telecommunications Industry
   Association standard for minimal computer-fax modem interface. EIA-578

   Class2.0 - EIA/TIA standard for extended computer-fax modem interface.
   EIA-592 specifies Class 2.0 "Class 2" is a non-standard based on an
   obsolete draft.

CO - Central Office, the telephone company's central switching station
for a given area.

CPS - (C)haracters (P)er (S)econd - unit of measure for the conjugate data
transfer rate, in bytes, between modems. From a user's perspective, this is
the bottom line measure of a modem's throughput performance.

CRC - (C)yclic (R)edundancy (C)hecking {or Checksum} - an error-detection
technique consisting of a cyclic algorithm performed on each block or frame
of data by both sending and receiving modems or transfer protocols.

CTS - (C)lear (T)o (S)end - flow control parameter which indicates that a
device is ready to receive data.

DCE - (D)ata (C)ommunication {or Circuit-Terminating} (E)quipment - applies
to the device that establishes and controls the data link via the telephone
network (eg. a modem). When referring to data flow rates, it is the rate of
raw data flow between two modems.

DS-0 - The base unit of digital transmission capacity. 1 communication
channel = 1 simultaneous voice grade equivalent with a communication
capacity of 64 thousand bits per second (64Kbps)

DS-1 (T1) - The equivalent of 24 multiplexed voice grade channels (DS-0s).
1.544 million bits per second (1.5Mbps)

DS-1C - The equivanlent of 2 multiplexed T1 channels. 3.15Mbps

DS-2 (T2) - The equivalent of 4 multiplexed T1 channels. 6.312 million bits
per second (6.3Mbps)

DS-3 (T3) - The equivalent of 28 multiplexed T1 channels. 44.736 million
bits per second (45Mbps)

DS-4 (T4) - The equivalent of 6 multiplexed T3 channels. 274.176 million
bits per second (274Mbps)

DSP - (D)igital (S)ignal (P)rocessor - a semiconductor optimized for
handling auio or video signals.

DSVD - (D)igital (S)imultaneous (V)oice and (D)ata

DSZ -  a shareware X,Y, and Zmodem protocol driver marketed by Omen
Technology that is most noted for its proprietary extensions/improvements
to the base Zmodem protocol.

DTE - (D)ata (T)erminal {or Terminating} (E)quipment - The device that
generates or is the final destination of data (eg. a computer). When
referring to data flow rates, it references to the rate of raw data flow
between a computer and modem.


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