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.
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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."
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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
