>It is my understanding that copper lines top out at about 38K, the
resistance
>in wire to conducting electrical current is what determines this (even the
>Feds can't regulate natural physics - no matter how hard they try - they do
>try to regulate advertising though, which is what keeps the modem companies
>from claiming 56K without that little disclaimer - unfortunately they don't
>explain the disclaimer)
>Bill
(adding to Trevor's already posted remarks)
"Basically, the signaling levels of the modems are in all cases just
princely. But as it turns out, it is possible to pass digital data at
64kbps rate between the ISP and the teleco central office switch such that
at times the output of the ANALOG line cards on the analog loop side- to the
customer, drives a bit too far in amplitude - exceeding the analog signals
levels managed by the FCC Part 68 specification at no greater than -12 dBm
averaged over any nominal three second interval."
"A copper wire exhibits certain inductive characteristics when a varying
signal is passed through it. Most notably, it acts as an antenna and
transmits electric and electrostatic fields just like a transmitter - quite
through the insulation actually. These fields are very small, but if you
increase the signal noise sufficiently, they can induce currents in wires
that are held in close proximity. At the bundle <a large group of POTS
lines>, they are all in VERY close proximity. The result is that traffic
from one wire can magically show up on another. This is termed crosswalk
and there is much ado about Near End Xtalk (NEXT) and Far End Xtalk (FEXT)
etc. in telco engineering circles. The FCC Part 68 rules limit signal
strength in the bundle, and ostensibly crosswalk."
(copied from Boardwatch, March 1997: 56k Modems)
Also see
http://www.fcc.gov/Bureaus/Engineering_Technology/Documents/cfr/1997/47cfr68
.pdf - Code of Federal Regulations, Connection of terminal equipment to the
telephone network.
Boy, the junk I find cleaning... too much paper ;-)
--
Kai Radicke -- kai@radiohead.net
IRC: irc.mcs.net, #inet-access
1966 MGB -- 1974 Triumph TR6
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