Wiring help?

Robin Laing Robin.Laing at drdc-rddc.gc.ca
Tue Apr 4 15:04:14 UTC 2006


Tim wrote:
> On Tue, 2006-04-04 at 01:31 -0500, Mike McCarty wrote:
> 
>>So, if one needs one or two 5 foot cables, then the prepared cable
>>is cheaper. If one needs twenty 5 foot cables, then unprepared
>>cable is cheaper.
> 
> 
> Not to mention that if you need to pass cables through tiny holes in
> walls, you do need to be able to put a plug or socket onto a wire.  That
> probably requires a tool (I'm yet to see a connector that could be
> easily done without one).
> 
> 
>>One thing I've wondered: On what basis should one choose between
>>using EIA 568A and EIA 568B? Electrically, they are the same.
>>I've been out of the cable making "business" for several years,
>>so I don't know why there are two standards for this, anyway.
> 
> 
> I wonder if the way some cables are formed makes it easier to wire it
> one way or another?  Hand wrangling the wires into the right place to
> crimp on a plug can be a right pain.  You've got untangle wires, cross
> some over others, and get them all to go into the plug the same
> distance.  It would have been a lot easier to do that if they'd come up
> with a wiring arrangement different from the current specs.  i.e. A pair
> on 1 & 2, the next on 3 & 4, the next on 5 & 6, the last on 7 & 8,
> instead of having the break apart pairs and straddle others.  I
> seriously doubt that would have degraded noise rejection.
> 

This to me is one reason I think purchasing normal patch cables is 
cheaper.  Making patch cables is normally non-billable time.

As I understand it, the reason for the pair arrangement is based on 
the older 4 pin phone connector and cross talk reduction between 
pairs.  This is described in the Google link provided by Ed Greshko.

The important parts.
===================================

4.0 The AT&T "Standard"

Before the TIA/EIA standard was created in 1985 AT&T the giant 
telecommunications company had been developing at its research labs 
newer and faster computer networks. These networks were designed to 
run over existing telecommunications infrastructure, this used USOC as 
its termination method (described later in this document). To provide 
backward compatibility for a single line phone AT&T created its own 
way of terminating cables for UTP networks; this specification was 
named 258A. 258A started to become well known and widely used 
(especially in the USA) and UTP networks became more and more popular.



7.3 Comparing 568A to USOC

Once again using the telephone standard USOC as a reference we can see 
that with the 568A standard both the blue pair and the orange pair 
appear in the same order, so both existing one and two line phones can 
be used with a 568A system.

8.0 Which Specification is Preferred?

If 568A and 568B specifications are technically identical and both are 
international standards which one is preferred? The information here 
is sketchy and I have not received a response from many standards 
organizations, from what I can understand is that although 568B is 
widespread (especially in the USA) all new installations should be 
carried out using the TIA/EIA developed 568A. Standards Australia – 
The Australian equivalent the ISO says, "There is no reason to change 
existing 568B installations to 568A although all new installations 
should be implemented with 568A." More recent information, "As 
published in the EIA Commercial Building Draft 9.0 as the preferred 
sequence for termination of UTP data cabling… This is also the 
preferred option for AS/NZS 3080."

=============================================

This is an interesting article.  One of those questions that sits at 
the back of your mind but never take the time to look up.


-- 
Robin Laing




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