On Mar 22, 1996 at 21:28, Paul Edwards of 3:711/934.9 wrote:

 PE> Just run it by me again.  Why is there two numbers shown, one 64
 PE> and one 16?  The # states is not something that both modems need
 PE> to agree on?  BFN.  Paul.

One might be what the USR is capable of using, the other might be what was
actually negotiated.  Who knows, it's not explicit about how trellis coding
is negotiated - but yes, they must both agree on it, otherwise how on earth
is one modem meant to understand what the heck the other one is saying
during the data stream?

I just downloaded/checked that on-line manual that Bill says is so good ..
I can't find the answer in there, either.  BILL!!  :-)

I *assume* (since V.34 isn't clear at *all* on this point), that the *call*
modem sends MP with *all* bits set according to what it supports, and the
*answer* modem returns MP with only *one* bit set, according to which one
it wants to use (of those supported by the call modem).

The whole MP/MP' sequence exchange is *very* ambiguous and unclear, and
you'd probably need the rationale, working notes etc. of the ITU-T working
group in order to know the real answer.  Unless it's covered in a previous
ITU-T spec.

Look at table 20 (page 34), the description of the MP sequence Type 0 - see
for bits 29 and 30, it says:

    Trellis encoder select bits:
        0 = 16 state, 1 = 32 state, 2 = 64 state, 3 = reserved for ITU.
    Receiver requires remove-end transmitter to use selected trellis encoder.

Who here is the "receiver"?  The receiver of the MP sequence, the
demodulation (receiving) side of the modem sending the MP sequence, or .. ?
 If that last line is to be taken literally, and the "receiver"
is the demodulation side of the modem sending the MP sequence, then it
would seem to indicate that all three forms of trellis coding are
*mandatory*.

How many wires are there in a telephone cord - two or four?  How difficult
would it be to make a split cable that ran between two modems to enable
them to negotiate a connection directly, but have the split section run off
to one or more A/D converters and thence into a PC that could capture the
conversation between the modems, allowing us to see once and for all
exactly what the two modems were saying?

Cheers..


    - dave
    d.begley{at}ieee.org

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