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Re: connection speed when using pon

On Sunday 30 January 2005 21:22, John Hasler wrote:
>Gene writes:
>> No, these are meaningfull numbers alright, and thats the actual
>> speed the data is moving.
>But they do not take into consideration the compression done by the
> modem.

Oh, but it does.  If it weren't for the compression used in the 
modems, the working baud rate would be limited to 1/4 or less of 
whats achieved.  Both v42.bis and MN5 play a role in this, a very 
heavy role.  Even at 9600 baud, v42.bis is needed just to get that.  
The rest of it is of course in the odd phase shifted keying as a way 
to pack more than one bit per cycle of the audio tone that is 
actually sent down the wire.  That phase shift keying is the major 
reason you don't hear the individual tones of the code anymore, but a 
quite raucous, nearly white noise occupying the whole 300-2500 hz 
bandwidth of a voice grade line, if the line is listened in on.

Some bandwidth is of course used in error corrections, and when the 
error corrections begin to impinge on the data rate, thats when the 
modems will call a temporary halt, tell us users hey we're busy hold 
up a few seconds, while they get together and negotiate a new, more 
robust but slower modulation method.  Conversely, if the error rate 
drops to zilch for a while, they'll stop, and renegotiate a faster 
modulation method, up to the theoretical maximum of 57k in the 
download direction on a really good line.

Much of this error correction is also because modern modems are 
capable of compareing what they send to whats comeing back slightly 
later in the form of echos, and adjusting their internal digital 
filter taps to cancel the echos that inevitably come back from poor 
junctions and terminations in every box that cable goes thru between 
you and the CO.  Once at the CO, then its generally into radios or 
microwave stuff and in pretty good shape, but that 'last mile' of 
copper is, figuratively speaking, pure hell for a signal as complex 
as these modems can generate.  Its got to be a lot better than what 
it takes for you to recognize your friends voice for a modem to 
function correctly.

Todays 56k dual mode modem that runs on a couple of watts from a wall 
wart, has more computing power in it than your dads whole computer 
had 15 years (or even less) ago.

>I suggest that the OP look at the pppstatus package.
>John Hasler

Cheers, Gene
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by Gene Heskett are:
Copyright 2005 by Maurice Eugene Heskett, all rights reserved.

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