Path: utzoo!attcan!uunet!decwrl!hayes.fai.alaska.edu!accuvax.nwu.edu!nucsrl!telecom-request
From: bcsaic!carroll@beaver.cs.washington.edu (Jeff Carroll)
Newsgroups: comp.dcom.telecom
Subject: Re: Crosstalk on Two lines on One Four-Wire Cable
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Date: 13 Sep 90 17:59:50 GMT
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In article <11864@accuvax.nwu.edu> tcora@pica.army.mil (Tom
Coradeschi) writes:

>The phone on my desk and the PhoneNet drop into my office use two pair
>from the same line. I've honestly never noticed ANY noise in the phone
>line - and I spend a LOT of time on the phone - and never noticed any
>AppleTalk problems due to the phone being in use. 

>So, I guess what I'm wondering is - what's the problem? Is it possible
>that the problems others experience, or think they experience, are due
>to other factors? Poorly terminated lines, bad grounds, etc?

As others have pointed out, the problem in the particular case of the
guy who started the thread is that he probably wasn't using twisted
pair in his premises wiring. Just running four wires can cause all
kinds of undesirable electromagnetic coupling between wires that
aren't supposed to couple.

With all due respect to Mr. Higdon, however, it is quite possible for
crosstalk problems to arise in telco cabling. Two of the most likely
culprits that come to mind are

(a) improperly insulated cable (e.g., rural areas where
paper-insulated cable is still in service, and the insulation is
getting wet. I have been in a number of rural and semi-rural areas,
including until recently my home on the outskirts of Bellevue, WA,
where crosstalk gets worse as the weather gets wetter.)

and (b) misadjusted transmission levels, that is, cases in which a
signal is transmitted too loudly, such that the signal coupled to
adjacent pairs rises noticably above the thermal noise floor. Cables
in which pairs carrying modem signals are operated at the same
transmission levels as those carrying voice signals are liable to
exhibit crosstalk problems. Good design practice calls for setting
data lines at a lower transmission level than voice lines, precisely
in order to avoid crosstalk. These days, there are a lot of people
(myself included) who operate modems over voice lines, which may
explain some reports of crosstalk problems.

	This phenomenon can work both ways. There seems to be an
interoffice trunk somewhere along the most common path between the 641
exchange (south and east Bellevue) and the 525 exchange (University
district, Seattle) here in 206 on which the transmission level is set
so low that I had to resort to manually connecting my autodial modem
when dialing in from home, because the modem couldn't find the carrier
from the answering modem in Seattle.


	Jeff Carroll
	carroll@atc.boeing.com