Making Structured Wiring Do Your Bidding

So you’ve just moved into a home that has cat5 running throughout. This is called structured wiring and is a great feature for a home. But what if the existing wiring doesn’t work the way you would prefer to setup your network? [Firestorm_v1] has a workaround that lets you reconfigure Ethernet without pulling new cables.

He’s making splitters out of patch cables. Often, Ethernet devices are not using all eight conductors in the cable. Unless you are using Gigabit Ethernet, or running Power over Ethernet, only four of the conductors in each run are being utilized. This means you can create twice as many connections without running new cable or using addition switches. Each splitter has three RJ-45 connectors on it. One of them hooks to the wired jack in the wall while the other two hook to two different devices. You’ll need a second splitter to use on the opposite end of the wall jack, usually this is where the router or switch is located, in order to separate the combined signals.

42 thoughts on “Making Structured Wiring Do Your Bidding

      1. This was relatively common in the early 1990’s in the corporate setting; wires were often run for the handful of computers that they thought they would need in the mid 1980’s, and that number usually doubled in less than ten years.

        It worked fine back then because most networks were only 10mbps, but there were some speed issues and random disconnects when we did it using 100mbps NICs and switches (usually using wires designed for 10mbps).

        I would say that this method is probably OK for 100mbps connections over shorter wires as long as the wires are of high quality, but that for such a slow speed, you might as well just use WiFi.

  1. This is a very handy little hack that has saved me from pulling cable a few times. There are a few different ways to do it, but the way listed here is probably the cheapest. The only problem with it is that the non-switch end (where it terminates to the wall jack) will need a pair of custom-length Cat5 cables that can’t be swapped out without making a new splitter cable.

    The solution that I usually use is actually quite simple. Instead of using four RJ-45 jacks, I use 4 RJ-45 keystones (which are slightly more expensive), or two jacks and two keystones.

    There are a couple other easy ways to do it, too. You can also do a crossover cable within a patch cable (or a patch cable with multiple drops). I’ll go take some pictures and post back in a minute…

  2. I can guarantee that you will lose some performance doing this. Crosstalk will be an issue having multiple signals in one cable and splicing wires like that creates what is called near end crosstalk at the splice.

    It’s a good hack though that well get you buy when you have no other options.

    PS: I am a network engineer ;)

      1. Gigabit requires a higher standard of cabling (Cat5e, as opposed to Cat5) that has reduced crosstalk. Cat6 goes a step farther by physically separating the pairs with a + shaped piece of plastic that runs the entire length of the cable to reduce crosstalk.

        In theory, crosstalk is a problem. In practice, as long as you stay within the specs of the protocols you’re usually just fine. There are several other things (such as error checking and ICMP (also CSMA/CD in the old days)) in place in the TCP/IP stack that help to provide a little buffer where lower layer problems occur, but they obviously degrade performance a bit.

        Random tidbit: It’s possible to run gigabit through Cat3 (in very short runs since all wires are untwisted and signal is very prone to crosstalk, obviously). I’ve streamed HD video through a gigabit link on a 20′ Cat3 cable with no ‘visible’ problems. There was packet loss, yes, but not enough to cause any real problems. Would I suggest using Cat3 for gig links? Hell no.

      2. It is a bit of a problem, which is why you should be using cat6+ for gigabit, but the main contention with what the user in the post is doing is the splice, which is really gonna hurt because of the lack of NEXT performance.

    1. i guess cat 5 is a bit to tight-fisted.
      i have set up my installation using s/stp cables.

      i dont get any performance drawbacks or crosstalk.

      i allways use 2 pairs for ethernet and 2 pairs for isdn (s0-bus) with no problems at all.

      the best thig to do: patchpanel and walljacks. then you dont need to cut any wires.

      1. Totally agree with you regarding patch panels and wall jacks.
        I got Cat7 wire running through the whole house and it would be quite complicated to deal with about 40 of those directly connected to the switches. Especially since the Cat7 wire that’s running through walls is pretty hard to bend.
        Nevertheless I am assuming that Firestorm_v1 is at least using wall jacks. The article is mainly about connecting two devices to a single outlet.

        Btw: I still consider switches the best solution for doing so ;) Especially since it also works for gigabit.

    2. I can guarantee that you won’t see any performance drop. This is totally within Ethernet specs. There is no difference between running an Ethernet signal down the two unused pairs, and running it down another cable bundled together.

      Also, true PoE will normally work over two pairs. It’s the cheap injectors that won’t. Technically, the power supply has to support PoE mode A.

      I am a network engineer, too. =)

    1. Yea, works flawlessly for 100bt… But expensive little things for a passive device…

      Rules being 1 RJ 45 for 2 network line or 4 phone lines. DO NOT MIX BOOTH ON THE SAME WIRE (i/e 1 net + 2 phone).

    2. I know they’re overpriced for what they are. But 7 quid for something that’s plug and play and hopefully well wired… I think I’d just pay the money!

      It is nice to read the authors way of doing it though and a handy thing to remember in the future if I’m ever in a jam.

  3. This is a great trick, useful too if you are rough with your cat5! I found that in one particular run of cat5 the orange and green pairs would not support a full duplex link, but the blue/brown pairs could handle full duplex fast Ethernet. That saved me from having to replace the whole cable.
    P.S. Power over Ethernet works fine over the same pairs as the data, but it’s probably a bad idea to try for both double data and PoE connections through a single run of cat5. There will be twice as much heat from power lost in the cable, besides the crosstalk mentioned above.

    1. PoE can use the same 2 pairs that data does, or it can use the other 2 pairs. It is implementation-dependent.

      As to heat: No, not a problem. Not now, and not ever, with 802.3af — there’s just not enough current flowing to make an issue out of it.

      And as a general retort to all those worried about crosstalk: The same principles that bundles of keep 4-pair UTP Cat5 from having crosstalk issues amongst themselves, also prevent crosstalk issues within a single cable jacket.

      Namely: The pairs are twisted. That’s all it takes. With 5e, it’s exactly the same, but the pairs are simply twisted at different rates. (And, no, the extra .2mm worth of plastic jacketing separating things in multi-cable bundles doesn’t make a lick of difference at a paltry few hundred MHz.)

      It’s an ugly hack, but the world needs ugly hacks.

  4. Yes, you will obviously loose a little performance when doing this, but crosstalk can be minimized by keeping each pair as twisted as possible (within spec, obviously) when terminating. Also, I wouldn’t try to push this anywhere near the 100 meter length limit that 100Base-TX calls for.

    I just took a few quick pictures of a few other examples where this could be used in cables/adapters that I’ve made that I had laying in my misc cable drawer. I’ve got a total of 3 active runs where I use this at work, all under 150′, and have had zero problems with it (normal throughput compared to the rest of the network, essentially zero packet loss).

    Here’s the pics:

    This plugs directly into a switch.

    The cable really only needs to be a few inches long, but I left it longer in this case. A standard patch cable plugs into the keystone jack, can run through a patch panel, wall jacks, etc, etc until it hits another splitter on the other end.

    Here it is in the switch.

    The preferred splitter on the far end of this (in most cases) would be a trio of RJ-45 keystones so that individual patch cables could be used to connect to the individual devices from there.

    Another variation that could also be used on one end. This cable was used to connect two computers that were stacked on top of each other that had only one available drop. There was no need for a Y-type adapter, so I just wired it with a keystone on one end, one plug in the middle, and the other plug on the end.

    Another way to do this is to skip the adapters/couplers/etc in general, and just wire two connectors to each end of the cable (works best if there are two NICs side-by-side in a device, obviously). I once wired up a cable like this as a double crossover cable, connecting two NICs on one machine to two NICs on another machine.

    It’s almost kinda sad that this hack is pretty much obsolete now that gigabit is basically standard, it definitely saved my ass more than a few times when I ran out of drops and didn’t have time to pull more right then and there.

    1. Also, note the lack of excess wire in the terminations in the pictures I took. Every patch panel I come across (that was wired by someone other than myself) in my line of work usually has at least half an inch to an inch more of untwisted wire on every termination. I’ve got OCD pretty badly… they all have to be perfect.

      1. I can’t find the link to your pictures. Even using Ctrl+F, I only find three posts written by you, none of them containing a link.
        Did you post it on here or somewhere else?

      2. Nope, posted it on here. Comment is ‘awaiting moderation’ it says. Possibly because I’ve got links and it’s a rather long post (about as long as all of my other comments on this page combined).

        Will reply to this with direct links to pictures to see if that will work for now.

  5. Certinaly not a bad hack and a quick way to increase your port count; however, to me the performance issues (especially when we are talking about streaming HD as a way of life these days) would be a concern. I think I’d stick with a $20 unmanaged gigabit switch if I really needed the extra ports.

    That said it is wonderful to see somebody that does care about how their cabling looks. When I first saw the picture I had asssumed it was a commercial product.

  6. Fortunately for those who often have to deal with old or insufficient cabling there’s a cheap alternative. A search for “RJ45 Ethernet Splitter” will find them for less than $2 each.

    Place one of those on both ends of a Cat5 cable and you have a dual run. It’s definitely not as pretty as the author’s but it saves a bunch of time. :-)

  7. I did the same thing for a few years because I didn’t want to run another cable to my barn (through a too-small conduit). Everyone said I’d have problems with cross-talk. I never did. Ethernet checksums and retransmission F.T.W.

    Now, I moved my firewall/router into the barn (that’s where the wimax antenna is and the DSL was), so there’s only the “trusted” network cable going over the physical wire. No splitters=gigabit. Of course this is all academic as there’s no way I’m getting anywhere near gigabit to the internet.

  8. My house came wired with cat5, but just for phone. I split the pairs at the junction box: two for network, two for phone. I replaced the wall plates with combo rj11/rj45 plates, and hooked the pairs at the box to a patch panel. It’s worked great for 6 years!

  9. My house uses that sparingly, and i’ve already helped at least a couple of my friends implement this at theirs.. Around here, they’re known as “hydra ethernet” or “kerberos cables”.. :)

  10. Beware of cheaper cables too. Some manufacturers don’t wind the “unused pairs” with the same number of twists per inch that the data pairs have. The results in the blue and brown pairs having lower bandwidth and more interference. Fortunately for short runs of cable this won’t be a problem for 100 Mbit links.

    less twists per inch = less copper = cheaper cable

  11. Not really new, I’ve done this for years. But don’t solder, that’s asking for trouble.

    Instead, take two keystone jacks and wire them to one RJ-45 plug.

    Or make up two cables of appropriate length and terminate them in a single plug to attach to the wall jack.

    Yes, it uses more cable and yes it means two (& sometimes too) long cables at times, but the reliability is much better.

  12. We’re using a similar trick for home automation — our cat5 cables carry a single Ethernet link, as well as ground, 12v, and two data lines for an arduino on either end. Definitely a hack, but it works and is a lot cheaper than the alternatives.

    I’ll get around to documenting our set-up one day ;)

  13. I had to do a very similar technique to run a second phone line upstairs at my old office. The existing line had only one pair connected, with the second pair simply hanging loose. Wire up the second pair in the phone box (boy was THAT a rats nest, I have no idea how we got the phones to work at all) and run upstairs to wire up the jack the same way.

    One 4-conductor phone wire (I had one that was only 2) and a splitter to Line A and Line B, and everything was great. Except the old man couldn’t remember to use Line B for outgoing calls, and he didn’t want to set up call forwarding. -.-

  14. At first I was wondering why not just put a switch at each end but then I read the article and I can see where you might want to use it to secure one network from another. If you had a renter in a room in your house and wanted to put them directly on a port at the ISP’s access point but then have other hardware in your own network behind a router for example. Nice piece of info, thanks.

  15. I just did this last week in my apartment. I needed to get abetter wireless signal (since the router was in wall recessed box and in a closet in the bedroom). I replaced the router with a GB switch in the wall and moved the router/VOIP adapter to the kitchen (where the phone is). Since there was only one connection and this is the only non-gigabit device on the network, i used the 2 pairs to get the signal from the modem to the router and then the output from the router on the other pair back to the closet GB switch.
    Works perfectly and my wireless signal now is almost too good (it is seeping out more into the hall and parking garage. I may have to turn down the strength to keep from anyone having such good signal outside).

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