Newsgroups: sci.electronics.misc
Subject: Re: crosstalk in ethernet cables
References: 
Allan Adler  writes:

> The ethernet cables I've seen have 4 pairs of twisted wires. Of those
> pairs, as I understand it, one carries a signal of some kind and the
> other serves to prevent crosstalk between that wire and the other
> wires. My guess would be that this anti-crosstalk wire is simply
> grounded but I am told that this is not the case and some other
> method is used.

Your quess was not entirely correct. In the most commonly
used Ethernet implementations which use unshielded twisted
pair cable there are two pairs used for data (one for
transmitting, another for receiving). This applies to
most commonly used standards 10Base-T and 100Base-TX.
The two remaining pairs are then just connected to ground through
resistor networks which gives optimal termiantion for noise
signals (there are few diffeent way how this is done,
all methods work quite similarly).

Please note that there are also Etherner schemes which use
all four pairs. One such standard was 100Base-T4 for 100 Mbit/s
Ethernet ment for lower quality than CAT-5 cabling. This
specification is not used much nowadays, because new wirings
are CAT-5 and support nicely 100Base-TX specification.
Another scheme which uses all four pairs is the copper version
of gigabit ethernet. 


-- 
Tomi Engdahl (http://www.iki.fi/then/)
Take a look at my electronics web links and documents at 
http://www.epanorama.net/