[RFI] Ethernet RFI

Bill Coleman aa4lr at arrl.net
Sun Jan 30 09:09:56 EST 2005

On Jan 22, 2005, at 8:41 PM, Ford Peterson wrote:

> One of my pet peeves is when computer technicians wire cat5 cables 
> incorrectly.  There is a right way and a wrong way to do it.  The 
> right way is the most difficult.  Guess which way most people do it?
> The 8 conductors in that cable are actually 4 pairs of twisted cables. 
>  Each pair is a "hot" and a ground return.  When you hook up the 
> cables to keep the colors all the same on each pin at each end, you 
> will have conductivity between the correct pins, and the network will 
> work, but you completely lose any shielding benefits of the twisted 
> pairs.  You should not be hearing anything from that cable.

For a standard 10/100 Base-T Ethernet run in CAT 5 (or CAT 3) cable, 
only two of the four pairs are used for signal. One is a differential 
pair running to the hub, the other a differential pair running from the 
hub. The other two pairs are typically not used, but do provide some 
shielding to the run.

I have about 300 feet of surplus CAT 3 (!) cable running through my 
house, and it all runs 100 Mb/s with no errors, and no interference. 
It's all installed and wired correctly, and I'm proud to say I did it 

Bill Coleman, AA4LR, PP-ASEL        Mail: aa4lr at arrl.net
Quote: "Not within a thousand years will man ever fly!"
             -- Wilbur Wright, 1901

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