Google Answers Logo
View Question
Q: Ethernet Cabling Specifications ( Answered,   2 Comments )
Subject: Ethernet Cabling Specifications
Category: Computers
Asked by: jonhoover-ga
List Price: $60.00
Posted: 29 Mar 2003 07:11 PST
Expires: 28 Apr 2003 08:11 PDT
Question ID: 182817
I am looking for proof that when wiring an Ethernet 802.3 network for
100 Mbps operation that, although Ethernet only requires 2 pair of
wires, it is not proper to use the other 2 pair of wires in a Category
5 cable for telephone service. It causes interference and degrades the
signal when the other 2 pair are being used for telephone service. So
far I have only been able to find that 100 Mbps Ethernet needs only 2
pair of the 4 pair in a Category 5 Ethernet cable. I cannot find
anything to support the fact that the other 2 pair should not be used
for telephone service.

Request for Question Clarification by pafalafa-ga on 29 Mar 2003 11:18 PST
Category 5 cabling uses unshielded wiring, and has been, in the past
notoriously susceptible to crosstalk and noise issues.  This is very
likely the reason (at least in part) for using only two of the four
available wire pairs.

Recent enhancements to both the cables and cabling standards have made
possible even faster (1 gigabyte) capacity over category 5 cables. 
These newer approaches DO use all four wire pairs.

It's quite possible that enhanced category 5 standards (sometimes
called "5e") could also be used to simultaneously carry voice and

If I'm on target here, then the premise of your question may only be
half-right:  category 5 may be limited to 2 of the 4 pairs, but 5e can
use all four, and may, possibly, use two for POTS transmission.

I can provide a number of targeted documents discussing the
differences between 5 and 5e, but I don't want to post these unless
they would be acceptable to you as an answer to your question.

Request for Question Clarification by pafalafa-ga on 29 Mar 2003 11:44 PST
Also, it may well be that category 5 simply transmits signals too fast
for conventional telephone equipment to handle.  I'm pretty sure that
telephone cable (category 1, I think) has a much slower transmission

Clarification of Question by jonhoover-ga on 29 Mar 2003 16:29 PST
Cat5 and Cat5e work fine for telephone. I am looking more for proof of
the susceptibility to crosstalk and noise issue - if other signals
were on the other four wires. A date for the adoption of 1000 Base T
would also help - IEEE adoption of it. Primarily it is the proof of
the first part of this clarification.


Request for Question Clarification by alexander-ga on 30 Mar 2003 03:11 PST
Are you lookiong for technical reasons why it shouldn't be done, or a
specifications document that prohibits it?

1000BASE-T was formally approved by IEEE on June 28, 1999:

Clarification of Question by jonhoover-ga on 30 Mar 2003 13:44 PST
An IEEE document forbidding it would be great, and a list of technial
reasons would also really help a lot.

Thanks again...

Request for Question Clarification by pafalafa-ga on 31 Mar 2003 07:34 PST
Hello again.

This actually is turning out to be a fascinating...but
elusive...research project.  I can't give a definitive answer yet,
hence this post as a clarification.  But I can tell you this.  Your
concerns notwithstanding, Category 5/5e combined voice/data
transmissions ARE done fairly routinely in some settings.  The folks
who do them routinely are not aware of any reports of problems
associated with the dual use.

The technology for doing this requires the use of a simple voice/data
splitter as seen here:

For the most part, this type of splitting is done in homes that have
Cat 5 cable installed, but it is also used, not infrequently, in
office settings to make use of the extra pairs for phone service.

If you call LanShack, and let them know you have an interest in
purchasing the splitters, they will talk you through some of the
details and customer experience with them.

Hope this is of some use.  I have a few queries out for more technical
details, and will let you know if find out anything additional.

I hope this is useful.  


Request for Question Clarification by pafalafa-ga on 31 Mar 2003 13:26 PST
More information is filtering in from some of my queries.  One
engineer with a ton of cabling experience wrote:

" far as I am aware, there is no reason why you can't use the
unused wires for telephones... It is unlikely that the signal from the
telephone would interfere with the data signal..."
"Having said all that, I would strongly advise against would
be setting the scene for total confusion and blown network cards in
the future..."

If you're interested in the full post, let me know.

Clarification of Question by jonhoover-ga on 31 Mar 2003 16:26 PST
It seems to be halfway decent for 10 Mbps operation, but for 100 Mbps
is quite horrible. We've been dealing with a contractor who wired a
building of ours - about 140 ports - using just one cable per user
location. 2 pair for Ethernet and 2 pair for voice. We have had some
cases where data just does not work - Windows reports a disconnected
cable. We have even tried reterminating cables and no luck.

I would imagine crosstalk, interference, etc are all factors, but I
posted the question just to see what someone could come up with. At
any rate, the building was specified after the ratification of 1000
Base T, so the electrician should have known that a new building
should be outfitted for the most current twisted pair technology.

I'd be interested in that engineer's full response.

Anything else you get in would be appreciated...

Subject: Re: Ethernet Cabling Specifications
Answered By: pafalafa-ga on 31 Mar 2003 17:18 PST
Hello Jonhoover,

Here's the full text of the note I mentioned earlier from, a network cabling help site:

Hi David
Thanks for the kind words about the website, I do try to keep it
updated when I get time.
With regard to your question, as far as I am aware, there is no reason
why you can't use the unused wires for telephones, in fact there is
talk of using the brown pairs (pins 7 & 8) for low voltage power in
some applications, i.e. CCTV cameras. It is unlikely that the signal
from the telephone would interfere with the data signal because they
work on completely different frequencies.
Having said all that, I would strongly advise against it. Unless there
are stringent documentation rules in place, and a guarantee that these
cables will never be used for Gigabit Ethernet (which uses all four
pairs), you would be setting the scene for total confusion and blown
network cards in the future. The whole idea of a structured cabling
system is to set up a communications media which uses a common wiring
system, over a common topology with common connectors. By introducing
different rules for some cables you are undermining the whole
principle of a structured cabling system, you can no longer assume
that plugging a device into one end of a cable assures total
compatibility with any system designed to run over a standard cabling
system. I have been to sites where this has happened and although it
worked in the original configuration, it added a great deal of extra
expense on the customer when he wanted to distribute his DSL line
around the building.
Telephones use a nominal 50V DC which can pulse up to about 70 Volts,
putting this across a network card could cause failure of the card.
OK, this isn't the end of the world, but if someone other than
yourself is trying to get things working and doesn't know about the
use of the same cable for a telephone, they could spend a lot of time
trying to figure out why this thing isn't working. Well I'm sure you
get the picture.
I hope this helps to answer your question. and please let me know how
you get on!
Ian Patrick
Data Cottage
Networking & Internetworking Solutions
Network Cabling Help website:


I hope this information meets your needs, but if anything is not
clear, please post a Request for Clarification before rating this
question.  I'd be glad to provide more detailed information, as

Subject: Re: Ethernet Cabling Specifications
From: ajisp-ga on 31 Mar 2003 08:22 PST

we are using the spare 2 pair in CAT5e pair for PSTN lines and no problem

Subject: Re: Ethernet Cabling Specifications
From: t568a-ga on 17 Apr 2003 14:35 PDT
I would bet the problem lies along the cable path and not that you are
transmitting phone on 2 pairs and data on two pairs.  (This practice
is not recommended and will void must MFG warranties.)  The cables
maybe longer than 295ft or to close to sources of EMI.  Fluorescent
lights can be evil.  Check to make sure your patch cords are maid Cat
5 or 5e with stranded wire (they should be very flexible)and not be
longer than 20 feet.  Field made patch cord should be throw in the
trash!!  The problem you run into running voice and data over the same
cable is where the pairs are split.  If they are split behind the
faceplate then look for more than 1/2 inch of untwist or kinks.  If
you see this problem then it should be re-terminated.  The proper way
to split pairs is past the faceplate with a plug in adapter.  Also
make sure your data jacks are Cat 5 or 5e rated.  If they don't say
cat 5 or 5e then they are not.  If there still is a problem call a
cabling company to stop by and test your cables with a 100Mhz tester. 
It will measure for length, cross talk, EMI, and pair placement.  They
can print the test results out.  They you have something to show your
Good Luck.

Important Disclaimer: Answers and comments provided on Google Answers are general information, and are not intended to substitute for informed professional medical, psychiatric, psychological, tax, legal, investment, accounting, or other professional advice. Google does not endorse, and expressly disclaims liability for any product, manufacturer, distributor, service or service provider mentioned or any opinion expressed in answers or comments. Please read carefully the Google Answers Terms of Service.

If you feel that you have found inappropriate content, please let us know by emailing us at with the question ID listed above. Thank you.
Search Google Answers for
Google Answers  

Google Home - Answers FAQ - Terms of Service - Privacy Policy