I am told by a customer that his IT guy at work has told him I can run 2 jacks off one Cat5 cable for ethernet since it really only requires the use of 2 pair.

I found one web site that seems to suggest that the transmit and receive are carried on 1&2, and 3&6. Can anyone confirm?

A regular 10Base-T or 100Base-TX connection only requires those two pairs, and yes, they are carried on pins 1/2 and 3/6 of the modular jack, usually connected using the white/orange and white/green pairs (see the recent 568A vs. 568B thread in this area for the debate over which way around).

The newer 1000Base-T networks require all four pairs, however, as do some 100Base-T variants which were intended to run over CAT3 cable. You're probably not likely to run into the latter though.

While this will work in a pinch it is NOT something that you want to install permanently. As with all IT geeks they often don't know much beyond the keyboard and putting this idea in a customer's head is plain wrong.

-Hal

You can get adapters that plug into the 8P8C (RJ45 for the non-pedantic) jacks which have two 8P8C jacks on them with the appropriate pairs split out. Using two of these adapters at both ends will allow you to run two ethernet connections over one cable.

Or you could make your own, using three 8P8C jacks.

This avoids the need to modify the permanently-installed wiring.

Usage of these types of adapters is commonly done, when the cost to run another cable is prohibitive. I've seen it done at two different, very large, companies.

Somehow I get the impression that the customer here is trying to save money be doing this on a new installation.

Like I said, if there is no other way that's all you can do. If you are installing new wiring don't plan on this as being a way to reduce the cable runs, install 1 drop per jack and do it properly.

-Hal

Same here. Standards are in place for a reason. Sure, you or your customer can split cable pairs out and "make it work". If you let the customer tell you how things are to be done, well you are already headed toward defeat. Do it right or don't do it at all is my motto.

Re-reading the original post, I'm getting the impression that this might be a planned new installation too.

My take is, I hope, the pragmatic one. If this is a completely new system, then pull two cables and have done with it. The cost of the cable compared to labor (both for initial install and any possible later wiring) is minimal. It's really no harder to pull two CAT5 cables than one.

However, if there is already a single link in place and a second is needed, if pulling another cable is going to involve considerable disruption and time, and if there's no chance that the existing link will need the extra pairs in the forseeable future, then I don't see a problem with "borrowing" the blue/brown pairs and simply transfering them to a second jack.



Well, I like to be a pedant. And I'm sure Ed will agree with me!

Perhaps it is a new installation. Someone would have to be *really* cheap to do this to avoid running a second cable.

I suppose the optimist in me thought it must be an existing installation!

Thanks for the replies guys. As to new vs. existing install.... yes and yes. We originally pulled 12 drops in a second floor residential addition. A couple of the drops were specified as doubles, and that info was lost by the time it got to the installers. It was caught by the homeowner and I told him we would pull those additional cables. In the mean time he talked to his IT guy at the office who told him ethernet only uses 2 pair, so he decided don't worry about pulling additional cables and while we're at it, why not just make all the jacks doubles, just in case. So that's where we're at.

None of this is for anything specific, it is all just for future sake, as per owner. As far as I'm concerned there is no difference between pulling a jack out and plugging the port, or stealing a pair if needed, just that the modification is made in the reverse order as to what is needed, just that in doing this way is what the owner wants. Commercial, I would put up a lot more resistance to veering off the standard.

I would have used external adapters if only because they might want to upgrade to gigabit which uses all 4 pairs. But then again only a hardcore IT geek wants to run gigabit in their house

Truth be told, I mixed 100baseT and POTS telephone in the same cat5 cable for a residential installation I did for a friend of mine (surface wiring, none of it was fished). The IEEE 802.3 specs state that this is acceptable. (It is not acceptable for 100baseT4--was any 100baseT4 equipment actually ever sold?). It works fine. Running another cable was not an option due to the surface wiring.

Actually this is something I've always wanted to test anyway, since armchair electrical engineer wannabees always pontificate that the high ring voltage may interfere with the ethernet signals.

It works fine, but I wouldn't do it that way for anything but a residential installation with those contstraints, where I know that they'll never want to use gigabit ethernet over that cable, and where any potential failure is only going to cause their son's PC to not have internet access. (Not a big deal in the grand scheme of things).

POTS ringers will spike ethernet, mechanical bells worse than electronic noisemakers. In a home with few incoming calls and good tolerance for retransmitted packets it is not a big deal but in an office where the phone rings a lot and traffic is high on the LAN it will make a huge difference in throughput.
Of course a small wiring error will probably let the smoke out of your LAN card

The IEEE 803.2 specs specifically say that mixing POTS and 100baseT4/100baseT2 in the same cable is disallowed.

"100BASE-T2 supports three types of service in adjacent pairs of the same cable: 100BASE-T2, 10BASE-T
and digital phone service compliant with the ISDN-BR U and S/T interfaces. Analog phone service is no
supported since the noise generated during off-hook transitions and ringing source from older PBX equip
ment can cause bit errors to occur."

They make no such notation for either 100baseTX or 10baseT. For 10baseT the following is stated:

"14.4.4 Noise environment
The noise level on the link segments shall be such that the objective error ratio is met. The noise environ-
ment consists generally of two primary contributors: crosstalk from other 10BASE-T circuits; and externally
induced impulse noise, typically from telephone ringing and dialing signals, and other office and building
equipment."


As far as miswiring goes, the following is stated. This was copied form the 100base2 section, but similar text is found for other specs (10baseT, 100baseTX, etc).

"32.10.2.4 Telephony voltages
The use of building wiring brings with it the possibility of wiring errors that may connect telephony voltages
to 100BASE-T2 equipment. Other than voice signals (which are low voltage), the primary voltages that may
be encountered are the battery and ringing voltages. Although there is no universal standard, the following
maximums generally apply.
Battery voltage to a telephone line is generally 56 Vdc applied to the line through a balanced 400 source
impedance.
Ringing voltage is a composite signal consisting of an AC component and a DC component. The ac component
is up to 175 V peak at 20 Hz to 60 Hz with a 100 source resistance. The DC component is 56 Vdc with 300
to 600 source resistance. Large reactive transients can occur at the start and end of each ring interval.
Although 100BASE-T2 equipment is not required to survive such wiring hazards without damage, applica-
tion of any of the above voltages shall not result in any safety hazard.
NOTEWiring errors may impose telephony voltages differentially across 100BASE-T2 transmitters or receivers.
Because the termination resistance likely to be present across a receiver™s input is of substantially lower impedance than
an off-hook telephone instrument, receivers will generally appear to the telephone system as off-hook telephones. There-
fore, full-ring voltages will be applied for only short periods. Transmitters that are coupled using transformers will simi-
larly appear like off-hook telephones (though perhaps a bit more slowly) due to the low resistance of the transformer coil."

It has been my experience that connecting an Ethernet card to a Merlin phone system won't damage either. (Merlins use RJ45 plugs and people do get confused...)

Back in the '90s through early 2000's, we would connect Voicelines from one CAT 3 UTP 4 Pair cable across two jacks at certain stations, as follows:

1. The Blue + Orange + Green pairs to one jack,
2. The Brown pair to another jack.

The Brown Pair was for a Fax Machine option, whereas the B-O-G were for the Voiceline (Telephone Set).

This was the only "Sharing" of stuff we ever did.

Just wanted to throw this in!

Scott