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  From: Richard Sharpe <sharpe@ns.aus.com>
  To  : Daryl Tester <dt@picknowl.com.au>
        Jake Hawkes <jake@infinitylimited.net>
  Date: Sat, 25 Nov 2000 20:42:14 +1000

Re: ADSL Enabled Exchange Areas: Updated 20/11/2000

At 12:18 PM 11/26/00 +1030, Daryl Tester wrote:
>Jake Hawkes wrote:
>
>> the way I remember, is that 2400 baud means 2400 symbols a second[1]. A
>> symbol may represent more than 1 bit, and with no "signal trickery", one
>> baud is on bit[2].
>
>2 out of 2 so far.
>
>> Then you get very clever "signal tricksters" doing some hoopy maths, and
>> you get constellation patterns, representing a bunch of bits, the more
>> clever the constellation, the more bits per baud[3].

Well, actually, what happens is that more info is packed into each signal
transition, by employing both aplitude modulation and phase modulation.

>3 out of 3 ... This is why you've graduated and become an
>international jet-setter, and I haven't.  :-)
>
>QAM is "Quad Amplitude Modulation" (from memory)(and giving your

Ummm, it actually stands for Quadrature Amplitude modulation, and, as I
said above, uses a combination of phase and amplitude modulation. See, for
example, page 188 of [1].

>constellation patterns mentioned above), and involves altering
>the signal strength to four discrete levels, giving you an
>additional two bits per baud (2 ^ 2 = 4).  From then on, I get
>rusty, as I haven't kept up with modem standards.

Constellation patters are a way of visualizing what is going on with the
multiple phase angles and aplitudes of the signals going down the wire.

To actually achieve 9600bps or more, full duplex, with the switched
telephone network, requires a bit more than just QAM.

There is echo cancellation required, so a modem can both send and receive
at the same time using approximately the same frequency range.

In addition, much more sophisticated techniques are now used, like trellis
coding and beyond.

>> Then, I believe more trickyness was brought in to do more clever
>> things.
>
>Basically, to pack more bits per baud, but at the expense of
>frailty of data (hence the later schemes introduce error
>correction, and retraining), because the signal becomes
>really susceptible to phase variations and distortion that
>the human ear/brain combo is _really adept_ at filtering
>out.  The problem is worse with long cable runs and corroded
>connections, which unfortunately our country bretheren suffer
>from worse than city folk, which is why they have more problems.
>
>But the upshot is, Telstra say "lines are only guaranteed for
>2400 baud", and people say "2400 BPS in this day and age, that's

So, yes, the basic modulation rate seems to be 2400 baud ... The rest is
packing more bits per baud using a variety of schemes.

>ludicrous", and it degenerates from there.  And is Greg Lehey
>pointed out, any large organisation suffers from left hand/right
>hand syndrome, so once any technical spec. has been webified into
>the latest trendy font, technical accuracy goes out the window[1].
>Telecom used to state "2400 baud"; I've no reason to believe that
>Telstra state it differently.
>
>> theressomethignwrongwithmyspacebar
>
>ATLEASTYOURCAPSLOCKKEYISOK.
>
>
>-- 
>Regards,
>  Daryl Tester,  Software Wrangler and Bit Herder, IOCANE Pty. Ltd.

[1] Understanding Data Communication, Sixth Edition, Gilbert Held.

Regards
-------
Richard Sharpe, sharpe@ns.aus.com
Samba (Team member, www.samba.org), Ethereal (Team member, www.zing.org)
Contributing author, SAMS Teach Yourself Samba in 24 Hours
Author, Special Edition, Using Samba


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