Fun With Ethernet Cables

So you mope round the office bemoaning the fact that the bozo who specified the network infrastructure didn’t put enough network points in. That bozo may or may not be you.

Cheer up. Its time to have some fun with Ethernet cables!

Cat 5, Cat5e or Cat 6, all Ethernet cables consist of 4 twisted pairs of wires. The twists are important, and quite clever. The twists ensure that any current inducted in the pairs of wires from external signals will more or less cancel out as they will be alternately inducted in one direction then the other as the wires twist. This is why you can nearly always get away with using unshielded cables (UTP) instead of the more expensive shielded cables (STP).

Oi! I thought you said this was going to be fun?

Hold on. The point I am making is that there is nothing very magical about Ethernet cables. They are just 4 twisted pairs and the twists provide each pair with pretty good protection from interference, even with the other pairs. The fun bit is that you actually only need two pairs for 100Mb network connections (three if you use PoE) and only one pair for old style analogue phone connections (some, but not all, digital phones need two) . This leaves some scope for fun and games.

You have probably seen “cable economisers” (or “doublyups” as one network engineer I knew insisted on calling them) on sale. These enable you to get two 100Mb network connections down one cable by using the two unused pairs for a second connection. This is probably the least worthwhile use of the cable. If you are really stuck for network points it would be better to just bung a cheap gigabit switch on the end. That way you get more bandwidth and more ports for about the same money. So long as you don’t use VLANs or subnets in a way that requires physical separation this is easier and better.

What about Gigabit? I like Gigabit.

Gigabit may be where it falls down a bit because standard Gigabit Ethernet (1000Base-T) uses all 4 pairs in the cable. There are two possible ways round this:

  • If your equipment supports Ethernet@Wirespeed then the Gigabit connection will come up even when only 2 pairs are connected. Bandwidth will be impaired but should still be faster than 100Mb. Test (and load test) this well before using it as it relies on both ends being able to work out what is doing on and deal with it correctly. There is a risk that it will be slow, unreliable or not work at all.
  • If your equipment supports the 1000Base-TX standard (and not all equipment that says it does actually does!) then you can get Gigabit over 2 pairs, however those 2 pairs must be Cat 6 (at least that is what the spec says).

I’m confused. How many pairs do I need?

OK. Here is a quick summary:

Analogue phone 1 pair
Digital phones 1-2 pairs Depends on type
10BASE-T / 100BASE-TX 2 pairs
10BASE-T / 100BASE-TX with PoE 3 pairs
1000BASE-T 4 pairs With or without PoE
1000BASE-T Ethernet@Wirespeed 2 pairs Impaired bandwidth. Not universally supported
1000BASE-TX 2 pairs Not widely supported

So what can you do with one cable?

  • 4 x analogue phones (or digital, if your digital phones use a single pair)
  • 1 x 100Mb network and 2 x analogue phones (This is a good combination. Add a network switch and you are serving two users with phone and network through one cable!)
  • 2 x 100Mb network (with genuine separation of the two networks), i.e. a cable economiser.

Even if your digital phones need two pairs you can still get 2 x digital phones or 1 x 100Mb network and 1 x digital phone on each cable, which is better than nothing.

So what can you do with two cables?

Consider a scenario where an office reorganisation has put six people into an office with only two network points. They have a phone and a PC each plus a multifunction device that sits directly on the network (and optionally piggybacks onto one of the user’s phones for outgoing faxes). Normally it would take 13 network ports to accommodate all that (or 7 if you use a switch) but you can do it by putting 1 network and 2 phones on one point and 4 phones on the other. Add an 8 port switch for the network and that’s it. Of course, the PCs have to share bandwidth on the uplink (which will probably only be 100Mb) but it is still better than trying to use WiFi, which will probably provide even less bandwidth and be much less stable.

If the users really do need a gigabit connection to share then you can investigate options for getting gigabit down 2 pairs. If that fails then the only option will be to use all 4 pairs on one cable for the network reducing the number of phones to 4. Some of the users in the office will need to share a phone extension. At this point it becomes clear that this is only a short term solution. You will need to put some more cables in sooner or later.

How do you do it?

There may or may not be an adaptor available that does what you want. I used to make my own boxes for this sort of thing. You may have to do likewise. The key thing you need to do is make the same splitter on both ends of the cable (although not necessarily in the same type of housing). Punchdown connector boxes are ideal but you can also solder the cables. I wouldn’t recommend using choc-block. It is just a matter of pulling the cable apart and connecting the pairs up as you see fit. While you can use pairs as you see fit, remember that the pairings themselves are sacrosanct. If you try to make an ad-hoc pair out of two wires that are not physically twisted together then there will be horrible interference and nothing will work. Also, try to avoid leaving any pairs untwisted over any appreciable distance as this can also allow interference to creep in.

I still don’t see the fun?

The fun is in being a bit more inventive than the average IT guy and demonstrating that there is more to life than the stuff they teach you on networking courses. Plus you get to play with the punchdown tool. If you have a more perverse idea of fun, you can try explaining to your boss how you used a clever trick to save the time, cost and disruption of getting the network installers back just to install a few extra network points and how this means you deserve a bonus.

And the Cat 5/5e/6 specs allow this?

Of course not. If you are specifying or installing a certified Cat 6 network you should do it properly to ensure full compliance and compatibility with future network standards.

This is just a quick bodge to get you out of trouble with an already installed network when money is tight and certification isn’t an issue. Depending on your network and budget you may or may not want to leave these bodges in place for the long term.

Note: Images on this page are from the Wikimedia commons. They are copyright to their original creators. Click on them for licence details.

Note: Article updated 1/6/2010 to cover the difficulties using gigabit.


April 5, 2010. IT, Sensible.

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