[Asterisk-Users] New $89 VOIP phone

[asterisk at bsius.com]

Chris Shaw [chriss at watertech.com] said:

> You're kidding right?
> 
> There's a reason why category 5 cable is twisted the way it is... to
eliminate 
> or greatly reduce RF crosstalk on the wires... Now what would happen if
you 
> split those wires between 2 different signals and kept them tightly packed

> together?
> 
> Can we say random data corruption, mysterious errors and terrible
performance? 
> Bingo!
> 
> Again, this kind of thing might be ok for a small home network, but you
can't
> seriously suggest it for a business...

Usually generic cabling standards are used to specify twisted pair cabling
for a commercial building. These generic cabling standards are TIA/EIA 568-A
and 568-B (ATT 258A) in the US and ISO/IEC 11801 internationally. These
standards generally specify 4-wire pairs per connection.

Those standards do not attempt to address what kind of signals will be run
on the different categories of cable (cat 3-7 for TIA/EIA and Class A-F). It
is the responsibility of the applications standards group IEEE 802.3, ATM,
etc... These groups tend to use existing generic cabling standards so as not
to create their own and have to promote these new standards.

With that being said, as 100BASE-TX uses only 2 out of the 4 available pair
it is entirely feasible to utilize the remaining 2 pair for traditional
analog teleco, or another 100BASE-TX signal. This could limit your ability
to expand to newer technology that may require all 4 pairs.

Unused wire pairs can affect the crosstalk from the 1,2 to the 3,6 wire
pairs (orange/green in 568B) that are used in 2-wire pair Ethernet slightly.
They can have a larger influence on external noise susceptibility and
emission of LAN signals on the cable into the air. The unused wire pairs do
function as a poor shield, but not enough to cause major heartache. And
according to ANSI/TIA/EIA 568-B, when terminating jacks all pairs must be
twisted to within 13mm to the point of termination. I've seen very few
installations where this is followed, most ever time these test out fine for
NEXT, though there is no reason not to wire things according to the
standard.

I doubt though, that you'll find this enough to cause any major issues with
the Ethernet signals, especially if you're under 100', crosstalk becomes
more of an issue the father the signal has to travel.

In any case, the quality of the transmission is as much, if not more,
dependant on the Ethernet transceiver as it is the signal line. Poor quality
Ethernet transceivers will cause as many, if not more, problems as poor
quality line. Take, for instance, Cirrus Logic's CS8952T which is capable of
transmitting well over 160M on CAT-5 in a "noisy" EMI/RF industrial
environment.

As far as your statement about random data corruption, mysterious errors and
terrible performance. Data corruption should never be an issue when you're
using protocols that provide error correction (IP). If using UDP, most all
UDP applications provide some sort of error correction or discarding of
invalid packets, since UDP should always be used in instances where data
integrity is not important this should be of little concern.

FWIW this type of situation can be found at a lot of universities (two
100BASE-TX signals on a single CAT-5 cable. Though most, if not all, have
stopped this practice.

Personally, I currently have several installations where I am running two
Ethernet signals over a single CAT-5 cable (all 4 pair), through testing has
uncovered no ill effects of this installation. We have also, in the past,
been forced to utilize existing CAT-3 cabling where it was "impossible" to
run certified cable. The existing cable tested out properly and has been
running trouble free ever since.

As far as suggesting it for a "business" environment, probably not. When
possible it's always best to run the best (or better) cable for the
application. However, as we all know, budget or physical limitations may get
in the way. If this is the case, just make it clear up front of the possible
issues that could be encountered and establish the procedures and costs
associated with remedying the issues if they do arise.


With all of that out of the way, if you are putting two 100BASE-TX signals
over a single CAT-5 cable, it should be obvious that you will not be able to
use PoE from the head end. I've never tried it, but I would imagine that
your equipment would not care too much for the 48VDC on it's signal lines
and it more than likely would not work with the 802.3af standard anyway.
But, if you can afford the PoE switches/injectors in the first place,
running another CAT-5 drop should really be the least of your worries. You
can still use a PoE injector just before the device itself, or use a wall
wart transformer.

As far as the suggestion of using Ethernet cable splitter. This is possible,
however I've seen many products that claimed to be splitters but were
actually hubs or something to that effect. IMHO they are all a waste of
money, because an additional keystone jack is under a dollar and would make
a tidier installation in the long run anyway.

If it were me wiring this, I would break out pins 1-2 and 3-6 (orange/white
- orange and green/white - green) to the same corresponding pins on jack 1,
standard 568B.

Then pins 4-5 and 7-8 (blue - blue/white and brown/white - brown) to pins
2-1 and 3-6 (orange - orange/white and  green/white - green) on jack 2.

Crude ASCII ART follows (make sure you're using a monospaced font to view
this):

  JACK 1   1  2  3  4  5  6  7  8
  PC #1    |  |  |        |
           |  |  |        |
           |  |  |        |
           |  |  |        |
           |  |  |        |
           |  |  |        |
568B Port  1  2  3  4  5  6  7  8
  Wall              |  |     |  |
                    |  |     |  |
              +-----+  |     |  |
              |        |     |  |
           +--|--------+     |  |
           |  |  +-----------+  |
           |  |  |        +-----+
           |  |  |        |
           |  |  |        |
           |  |  |        |                         
  JACK 2   1  2  3  4  5  6  7  8                         
  PC #2

-Bill