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From: Gord Hannah
To: All
Date: 2010-08-01 01:00:00
Subject: [7 of 12] Comm Primer

of the monopoly formerly held by AT&T in the United States.

RETRAIN - A retrain is where the two modems suspend operations and
renegotiate the best possible connection all over again.

RPI - (R)ockwell (P)rotocol (I)nterface. RPI is a technology that allows
error correction and data compression (ECC) technologies to be performed in
the PC host computer, rather than in the modem hardware, as has been done
traditionally.

Technically speaking, RPI is an asynchronous HDLC (High Level Data Link
Control) link that operates between a PC and a modem, allowing data
compression and error correction to take place on the host PC. This allows
RPI to operate using standard PC COMM ports, or standard serial cards.  The
V.42bis bit stream presented to the remote modem from an RPI modem looks
exactly like that which would come out of a traditional hardware
implementation of V.42bis, fully synchronous and full duplex. MNP levels
2,3,4,5 are supported as well as V.42 LAP-M, and V.42bis.

To get ECC with RPI, as one would get using a hardware implementation of
ECC, the PC comm application needs either a driver or a software package
that supports RPI; the modem  itself needs firmware that supports RPI.

RTS - (R)equest (T)o (S)end - flow control parameter which indicates a
device is ready to transmit data.

SHANNON/HARTLEY THEOREM - further refines Nyquist Theory by taking into
account the signal-to-noise ratio of the communications channel in addition
to the bandwidth. It is expressed as follows:

Maximum bits    Available
per second   =  bandwidth  x  LOGbase2 ( 1 + Signal-to-noise ratio )

SYMBOL RATE - The proper term for what most mistakenly refer to as baud
rate. A "symbol" is a waveform transmitted by the modem, which contains a
certain number of encoded bits of data to be moved across the link. In
simple terms, the Symbol can be viewed as an analog representation of a
digital number.

Consider a stream of data bits: 1 0 0 1 1 1 0 1 0 0
Now take them two at a time   : 10  01  11  01  00

As can be seen there are 4 possible situations, 00, 01, 10, and 11 which
one could call A, B, C, & D respectively. The symbol for 00 would be A and
for 01 it would be B, and for 10, C and for 11, D (these symbols could be
anything agreed to).

Thus if you detected a C at your receiver you would know that the
transmitter was actually sending you 10 etc. etc. The actual symbols are
not alpha characters, of course. They could be, for example, the signal
constellation points in the non-trellis coded telephone line signal.

Consider this simple constellation:

                                     A
                                     +
                                     ^
                                     |
                                B+-------+C
                                     |
                                     v
                                     +
                                     D

The points at O, 90, 180 and 270 could be the four (4) distinct Symbols.
They would correspond to the A, B, C, D in the example above. We would have
just 4 symbol types to detect. The Symbol would change on every other data
bit and thus the Symbol Rate would be half the Data Rate. For 4800 BPS the
Symbol Rate would be 2400 Symbols per Second.

Now consider 9600 BPS. If we wanted the Symbol Rate to still be 2400, as
this sets the required bandwidth needed on the telephone channel to carry
the modem signal, we must reduce the 9600 rate to 2400.

                                9600
                                ---- =  4
                                2400

This means we must process 4 bits at a time. Four (4) bits can be in 16
combinations as 0000, 0001, 0010 etc. Thus we must have 16 symbol types or
points in the un-coded line signal constellation.

For Example:

                                + + + +
                                + + + +
                                + + + +
                                + + + +

This should be viewed as an expansion of the first constellation example
above where the inner 4 points are "like" the 4 points in the above example
but rotated 45 degrees.

Each point (symbol) would now represent 4 bits as 0000, 0001, 0010, 0011,
0100 etc, etc. Thus, the Symbol Rate is one fourth the data rate.

Now for 12000 BPS and a Symbol Rate of 2400, we see we must process 5 bits
at a time, and for 14400 BPS and a Symbol Rate of 2400 be must process 6
bits at a time. This latter example would give us an un-coded constellation
of 64 points.

The noise levels in the channel determine how many bits are encoded in each
symbol; lower noise levels allow a greater number of bits per symbol.  The
bandwidth of the channel limits how many of these symbols may be sent each
second.

SYNCHRONOUS - Describes a form of data transmission in which blocks of data
are sent at strictly timed intervals. Because timing is uniform, no Start or
Stop bits are required.

TCM - (T)rellis (C)oded (M)odulation. TCM is an optional error correction
method included in the V.32 standard. TCM allows modems to check for
transmission errors with a redundancy bit, which results in fewer errors on
noisy lines. Modems can tolerate twice the noise in the channel than
otherwise, using only QAM. Trellis Encoding works with other error
correction methods such as V.42. Unlike other methods, TCM is true error
correction, in that for small errors, it does not force a retransmit, but
actually fixes them.

Instead of using a simple 16-point constellation, for example, you force
the modem to a higher-density 32-point constellation.  The 32-point

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