[Gllug] [slightly ot] ADSL config fun

John Hearns john.hearns at cern.ch
Thu Sep 19 09:09:16 UTC 2002


On Wed, 2002-09-18 at 19:23, Chris Bell wrote:
> On Wed 18 Sep, John Hearns wrote:
> > 
> > 
> > >with control data packets travelling a parallel route through the BT network.
> > I don't fully understand this statement - I'll kick myself when it is
> > explained.
> > 
>    I was on a course many years ago about (then new) digital techniques, and
> was told that BT (it was the Post Office then) sent two separate data
> streams through their network, one was purely control data, and the other
> was the digitally encoded audio.

I am absolutely not being sarcastic here - but we're discussing slightly
different things.
I'm no phone expert, but you're referring to out-of-band signalling on
the phone network - all good stuff. That's what prevents 'blue boxes'
and the like working these days, very much AFAIK.

ADSL on the other hand works by splitting your existing phone line into
different frequency bands - the analogue phone call still continues,
but the digital ADSL data uses another frequency band. Hence the need
for the filters.

You are quite correct in talking about digital audio and a control data
stream - this is how ISDN works. In basic rate ISDN, there are two
B or 'bearer' channels, running at 64kbps. Can carry voice or data,
plus one permanently open D or data channel running at (????)
I remember some internet access shcemes use this low rate D channel
as a permanently on connection (or I can we wibbling)


As I say - please don't interpret this as me indulging in an
"I know more that you" fest. I don't - but its useful to learn 
together. Experts please chip in here!

Thinking about all this - I guess that's why ISDN and ADSL service can't
exist on the same pair of wires? Or am I wittering again?

Also not the 64kps data rate - which is adequate for one voice quality
call. All data streams, leased lines etc. offered by telcos are a
multiple of 64kps (just pointing this out as interest to people
who don't know this). There is a whole heirarchy of different speeds -
all based on multiplexing up the base 64kbps channels.





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