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From: Gord Hannah
To: All
Date: 2011-02-15 01:00:00
Subject: [8 of 12] Comm Primer

constellation offers higher encoding density with its "quintbits" (2^5=32),
as opposed to the 16-point constellation with its "quadbit" structure
(2^4=16).

With a given fixed symbol rate (e.g., 2400), the 16-point constellation can
deliver data exchange rates of 9600 bps (2400X4). The more-complicated
32-bit constellation offers an intrinsically higher data rate capacity of
12,000 bps (2400 symbols/sec X 5).  It's also more susceptible to channel
noise because you have to push the individual decision points closer
together, and any minor noise perturbation is more likely to cause the
distant receiver to make a decoding error.

However, instead of using the higher density of the 32-point constellation
for speed purposes, suppose you use its added complexity for noise immunity
purposes to derive a performance boost.  TCM does just that.

Instead of operating at 12,000 bps, V.32 TCM transmission remains at 9600
bps with twice as many constellation points as it needs.  The extra
constellation bits produce a redundant bit.  This fifth bit is used as an
embedded "checksum" to produce intelligent transmission. Decision rules are
used in both the transmitter and the receiver to produce an orderly
transmission that is designed to maximize noise immunity.

The redundant bit serves an error-detection function similar to the ninth
bit used for parity in 8-bit PC memory systems.  The memory bit doesn't
prevent errors from occurring; however, it flags them and prevents them
from propagating.

With TCM, the fifth bit acts to help the remote receiver make a high-
probability choice among possible alternatives when  channel noise has
distorted the incoming signal.  It doesn't absolutely guarantee error free
communication, but helps reduce the probability of errors. The net result
is about 4 decibels (a little more than double) the S/N ratio of a non
encoded system.

TSB38/37A - A network model description and set of testing procedures which
are defined in Technical Standard Bulletins of the Telecommunications
Industry Association. The TSB37A test suite is used with modem performance
test procedures given in TSB38.  It is widely used today for comparisons
between modems. It models the network as two independent local loops with a
four-wire trunk connection between them.  Impairments primarily found in
the digital network are specified in proportion to their believed presence
in the network.   For instance, up to three PCM links are included in the
tests. ADPCM is also represented.

There are also minimal tests that represent the older analog equipment in
the network. Each loop and trunk connection specified is assigned a
Likelihood of Occurrence percentage (LOO).  By multiplying the scores for
the loop combination and the trunk, an overall LOO can be assigned to a
given test.  A LOO of 4% indicates that the results of that test represent
4% of the expected connections in the actual network.

UART - (U)niversal (A)synchronous (R)eceiver (T)ransmitter - a
semiconductor which interfaces serial ports to the processor buss.

    FIFO - (F)irst-(I)n-(F)irst-(O)ut - describes the data handling
    technique utilized by high-performance buffered UARTs such as the
    NS16550AFN.

V.8 - Procedures for starting sessions of data transmission over the
general switched telephone network

V.17 - ITU-T recommendation of Facsimile operation at up to 14400bps.

V.18 - Operational and inter-working requirements for modems operating in
the text telephone mode

V.21 - ITU-T recommendation for modem communications at 300bps. NOTE: this
recommendation is widely utilized outside of the United States and Canada
where the Bell 103 protocol is more prevalent.

V.22 - ITU-T recommendation  for modem communications at 1200bps. NOTE:
This recommendation is compatible with the Bell 212A protocol which is
prevalent in the US and Canada for devices which operate at 1200bps.

V.22bis - ITU-T recommendation for modem communications at 2400bps.
Includes an automatic link negotiation fall-back to 1200bps and
compatibility with Bell 212A/V.22 modems.

V.25 - ITU-T recommendation which specifies a 2100Hz answer tone which is
different from the Bell answer tone used in the US and Canada.

V.25bis - ITU-T recommended command language for modems.

V.27ter - ITU-T modulation scheme used in Group 3 Facsimile for image
transfer at 2400 and 4800 bps. It is also used for 4-wire leased line
modems.

V.29 - ITU-T modulation scheme used in Group 3 Facsimile for image transfer
over dial-up lines at 9600 and 7200 bps.

V.32 - ITU-T recommendation for modem communications at 9600 and 4800bps.
Incorporates fall-back/fall-forward logic for operation on impaired phone
lines.

V.32bis - ITU-T recommendation which extends the V.32 connection range to
incorporate link rates of 4800, 7200, 9600, 12000, and 14400bps.
Incorporates logic that falls back to the next lower speed when line
quality deteriorates, and fall back further as necessary. Also incorporates
fall forward and rapid rate negotiation logic.

V.32terbo - 19200bps, with fall back to 16800bps. Designed by AT&T, and is
public domain, so any manufacturer can use this standard and put it into
their modems. USR has further extended this to support a proprietary link
rate of 21600bps.

V.32turbo - A modulation protocol developed by Rockwell which extended V.32
link rates to 12000bps.

V.34 - ITU-T recommendation for data signalling rates of up to 33.6kbps for
use on the general switched telephone network and on leased point-to-point
2-wire telephone-type circuits.

V.42 - ITU-T recommendation for modem communications that defines a

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