Ethernet cable (Was: Sun3 valuations?)

Bill Gunshannon bill.gunshannon at
Tue Jan 23 13:09:53 CST 2018

>From Wiki:
"Transceivers should be installed only at precise 2.5-metre intervals. This distance
was chosen to not correspond to the wavelength of the signal; this ensures that the
reflections from multiple taps are not in phase. These suitable points are marked
 on the cable with black bands."

It cites "IEEE Standard 802.3<>-1985. IEEE<>. p. 121."


From: cctalk <cctalk-bounces at> on behalf of Pete Turnbull via cctalk <cctalk at>
Sent: Tuesday, January 23, 2018 1:54 PM
To: cctalk at
Subject: Re: Ethernet cable (Was: Sun3 valuations?)

On 23/01/2018 16:48, Grant Taylor via cctalk wrote:
> On 01/23/2018 09:10 AM, Bill Gunshannon via cctalk wrote:
>> If you didn't locate the transceivers on those black marks you would
>> have had terrible performance as that affects collisions.  Timing
>> (among other things like grounding) was very important with that
>> version of ethernet hardware.
> It's my understanding that the marks (black bands or other markings on
> the sheath) were exactly one wavelength apart.  Which from what I
> remember ~> understand from my ham radio days is quite important.

No, that's quite wrong.  The marks are every 2.5m (8.2 feet) wich IIRC
is 1/19th of a wavelength apart, the point being to try to minimise the
likelihood of connections being made where they'd suffer constructive
(additive) interference and to prevent transceivers being too close

Pete Turnbull

More information about the cctalk mailing list