Ethernet wiring question

October 10, 2006 2:22 PM

An electrician doing some work for my employer has taken a standard 4 pair cat5 cable and split off from each of the 8 wires to feed 2 different wall mounted ethernet jacks. Should this work? So far, it isn't. One jack appears electrically inert, the other will register a link, but I can't ping or get a DHCP reply through it.
posted by Steve3 to computers & internet (17 comments total)

Might work for a phone line ... not for data though! Time to fire the contractor...
posted by JakeWalker at 2:25 PM on October 10, 2006

I agree. Generally no. It might sort-of work but those twisted pairs need to stay paired and twisted. We do often split up cables but only for short runs, typically in a patch panel.
posted by chairface at 2:28 PM on October 10, 2006

You boss is an idiot, hire a proper network engineer, not an electrician for this job.
posted by b1tr0t at 2:33 PM on October 10, 2006

It will work, and I have done this on multiple occassions with runs up to 300 feet.

The trick is knowing the twist ratio of the pairs, and using that to reduce the interference. Each pair in a cat-5 is twisted at a different twist rate, some tightly wound, and others loosely wound. When an electrical current runs through a twisted pair, an electrico-magnetic field will be generated perpendicularly to the twist. If the twist is close on the two pairs in use for ethernet, then this electromagnetic field will cause cross-talk, and impede your ability to get ethernet to work.

If you are going to have this work, you need to make sure that the Orange pair is on pins 1/2, Green is on 3/6 on the first connection.

On the second connection, use the blue pair on pins 1/2, and the brown pair on 3/6.

This should work. This is not the ideal, but with modern equipment (the 802.3 spec is over twenty years old now) that is much more tolerant to external noise, this should be sufficient for up to 100MB. I won't guarantee gigabit connections, but I would hazard a guess that gigabit would be fine up to 300 feet as well.

To those that disagree, this is much less disruptive than POE, which basically uses the same concept.
posted by stovenator at 2:38 PM on October 10, 2006 [1 favorite]

Cat 5 is 4 pair; you only need 2 pairs for each network connection, so it is possible to sent two physically separate network connections down the same Cat 5 cable.

I know this for certain as I'm typing this from a computer that is connected using one connection from a pair of "double headed" patch cables that I made up myself. It's a kludgy work around but it can be useful occasionally.

That said, if you are going to do it, do it with patch cables NOT inside the jack. And I wholeheartedly concur that any cabling contractor who pulls this kind of crap should be dumped immediately as it is obviolus that they don't know what they are dooing (demonstrated by the fact that the connections don't work). There are established standards for wiring jacks, any contractor worth using will follow one of them.
posted by Quinbus Flestrin at 2:40 PM on October 10, 2006

Oh, and to be clear. I agree that the contracter, if they pulled new cable, is an idiot for doing it this way. If new cable was not pulled, or cannot be pulled, then it isn't the contractor's fault. However, the question was "Should this work?", to which the answer is "Yes, it can work, if done properly".
posted by stovenator at 2:49 PM on October 10, 2006

It was existing cabling from this conference room to the patch panel. What was new was the conference room table- a new one that has phone, data and power jacks in the center.

He tried to get the same cable pull to present in a wall jack and the jack in the table, via this "split every wire" idea. We can sacrifice the wall jack to get the table working... assuming the untwisting that's been done isn't fatal to the data connectivity.

He got the phone and electric working properly, but apparently mucked up the data end of it.
posted by Steve3 at 2:56 PM on October 10, 2006

That's staggeringly inept. Yes, it's possible but it's almost always a bad idea. In addition to hideous crosstalk and the normal even-looser wires floating around problems, when you go to gigabit you'll just have to re-wire the whole thing again. Either pull another run from wherver or just put a cheap and cheerful switch somewhere.

Really, the right way is to re-run the cut cable alongside a new one.
posted by Skorgu at 3:22 PM on October 10, 2006

As several previous posters have noted, this can be done if you're carefull. However, 1000BASE-T uses all four pairs of a UTP cable (Category 5 or better), so the most common variant of Gigabit ethernet will not operate over the cable that you describe. When asked, I always recommend pulling another cable.
posted by RichardP at 3:43 PM on October 10, 2006

I've done it and it works fine (with the caveats noted above) but only for temporary installations. It shouldn't be done for permanent ones ever.
posted by thatwhichfalls at 4:07 PM on October 10, 2006

I'm not exactly sure what you are describing. Some comments here assumed that two pair go to one jack and two pair go to the other jack. That gives you two independent jacks with two data paths. But from your description it sounds like he ran all eight wires in parallel to both jacks. In this case you have one data path and can only use one jack at a time.
posted by JackFlash at 4:55 PM on October 10, 2006

there is no way that both jacks will work at the same time.

EVER.

if it's one or the other... well, see above.


If they split the pairs up, with 4 going to one, and the other 4 going to the other, that would work, and is the way it SHOULD be done.
posted by hatsix at 5:42 PM on October 10, 2006

JackFlash:

No, it's not 2 pair to jack 1, and 2 pair to jack 2.

He started with an existing run of 8 wires. They were disconnected from the existing jack, and each of the 8 was crimped into what GIS tells me is a 3 wire crimp splice. Each of the 8 crimp splices has a wire running to jack 1, to jack 2, and back down the original wiring run.
posted by Steve3 at 7:20 PM on October 10, 2006

Each of the 8 crimp splices has a wire running to jack 1, to jack 2, and back down the original wiring run.

No, this will not work. You might want to check and make sure that any other wiring this electrician has done is checked by someone else. Data needs a home run, which anyone that has hooked up a 2 phase appliance should understand.

Hire someone to fix the original jack, and buy a cheap switch if it is too difficult to run a new network connection to the table.
posted by bh at 8:02 PM on October 10, 2006

There is an outside chance that one of the jacks might work at 10baseT speeds, but not reliabily. Neither jack should be expected to run at 100baseTX speeds, and gigabit speeds are far beyond out of the question.

To fix this, cut the cable of well below the "3 wire crimp splices". Find the other end of the cable, and crimp the proper ends on. Install a small switch (hubs only really exist at 10baseT speeds anymore) at the two-port end, and run a short cable to the second termination point (unless a hub with several ports in one location is sufficient).

The core problem is that ethernet cables are not just wires, they are carefully constructed waveguides. 10BaseT is fairly tolerant of bad wiring. 100BaseTX is less tolerant of poor installation, and gigabit over copper is extremely intolerant of poorly built cables.
posted by b1tr0t at 8:13 PM on October 10, 2006

er, transmission line. IANAEE
posted by b1tr0t at 11:53 PM on October 10, 2006

I like the D-Link DES-1005D 5-port switch and its 8-port cousin the DES-1008D. I've used a lot of them, they're cheap and wall-mountable and they work.
posted by flabdablet at 6:47 AM on October 11, 2006