LinuxSA Mailing list archives

  From: Daryl Tester <dt@picknowl.com.au>
  To  : Jake Hawkes <jake@infinitylimited.net>
  Date: Sun, 26 Nov 2000 12:18:03 +1030

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

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].

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
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.

> 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
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.

"Who knows what men lurk in the heart of eval?"

[1] Not that I claim to be 100% accurate - my grey cells decay at
    a faster rate than I am able to replace them.

-- 
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