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Re: 56K Line Speed - physics not Feds

To: MGBDude@aol.com
Subject: Re: 56K Line Speed - physics not Feds
From: Trevor Boicey <tboicey@brit.ca>
Date: Sun, 14 Feb 1999 14:38:38 -0500
MGBDude@aol.com wrote:
> 
> It is my understanding that copper lines top out at about 38K, the resistance
> in wire to conducting electrical current is what determines this

  No, that's not true, the traces on your computer
motherboard are copper and think of the frequencies they
run at.

  The limit is simply because your virtual phone channel was
made to carry voice. Whether your station is frequency or
time multiplexed, either way you end up with certain bandwith
and certain signal to noise ratio that is designed to be
good enough for voice. (and it is)

  When used to carry data, there are some formulas from guys
called Nyquist and Shannon that compute what the maximum
data carrying ability of a channel is based on S/N and
bandwith, and it is what determines how fast your modem
can operate.

  For analog to analog connections, symmetrical 33.6k is about what
you are going to get at best, if you are calling someplace
fairly close and calling on a clean circuit.

  The new 56k technologies cheat a little. They don't change
the laws of physics, but what they do is allow the
ISPs to connect their end directly to the multiplexer
at the phone company.

  So with a pure analog conection you get an analog
signal, multiplexed, then routed through the phone
company switches, then demultiplexed, then back to
the other end.

  With 56k-capable ISPs, you skip a few steps that allows
you to get a better S/N and/or more bandwith. From this
extra channel capacity, you squeeze out the extra cps
up to 50+K.

> - so unless you have fibre optic lines in your area that is going to be about
> the best you can do. 

  It's really the phone system design that kills you,
nothing to do with copper.

  DSL service also runs on a copper line, but it's a virtual
circuit that allows you to use the entire bandwith of the
copper to yourself, plus whatever S/N ratio you can get from
four miles of cable snaking around the city. The exact rate
you get depends on your provider, but my corporate ADSL
is 4M and my home is 2.2M.

  As well, ethernet cables are generally copper, and they run
at 100M all day. It's not a physics problem, it's more a problem
of using a system that was designed to carry voice signals,
and trying to use it to carry data.


-- 
Trevor Boicey, P. Eng.
Ottawa, Canada, tboicey@brit.ca
ICQ #17432933 http://www.brit.ca/~tboicey/

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