Crimping a Cat-5 loopback
A cat-5 loopback is a very simple device which simulates the behaviour of an active network card. Loopbacks are very useful when running wiring over long distances, as they use no power, require no interaction at the other end when testing a cable run, and are cheap enough that it doesn't even matter whether they are collected again or not.1
Loopbacks are created by wiring the upstream twisted pair to the downstream twisted pair. The easiest way to do this is inside an RJ45 plug - no cable required. Cut about an inch of cat-5 off the end of your spool, and untwist the pairs - this is enough for 4 loopbacks. Fold the wires in half, and insert both ends into the plug, connecting pin 1 to pin 3, and pin 2 to pin 62, then crimp. Though it's not necessary, I recommend using the same colour for both wires, to aid in describing individual loopbacks over the 'phone. Most RJ45 plugs are transparent, so it's quite easy see which is (for instance) the brown or green loopback.
8=== | |\__________
7=== | C |
6===xxxxxxxxx | o |
5=== x | r G | Cable.
4=== x | d r |
3===ooooooooxoooooooo | i | __________
2===xxxxxxxxx o | p |/
As seen from the wiring side of the RJ45; you can't see the
catch, because it's on the other side of the plug.
Loopbacks appear to most ethernet devices as a switched-on ethernet device at the other end of the link. When one is plugged into a network card, hub or switch, its link light should switch on. Devices capable of more than one speed (10/100 switches, and network cards, for instance) should come on at the highest rate supported by the device (It's talking to itself, after all). If the device is capable of noticing low-quality cabling, it may negociate a lower speed - this is a clue that there is something wrong with the cable.
Most dedicated cable testers have a loopback mode, where instead of being attached to both ends of the cable, the tester is attached to one end, and a loopback plugged into the other. Many network cards can go into loopback testing mode, where they either test their own hardware (with a loopback plugged straight into the card), or act as a rudimentary cable tester (with a loopback on the other end of the cable). While a network card can't tell you what's wrong with a cable (for instance it may be wired incorrectly, or one of the wires may be damaged), but it can tell you whether the cable is working or not - the results of the loopback test should be near perfect. If more than 1% of the packets are being corrupted, there's something wrong with the cable.
As well as testing single cables, loopbacks are very useful in running several cables at once to a patch panel - wire all the wall sockets, put a colour-coded loopback in each one, and then after all the wires have been run to the patch panel (and tested using a loopback cable tester), connect them to a hub. 'phone your accomplice, and have them remove loopbacks one at a time.
1 - When telephone companies are testing local loops, they use a similar device - it connects a resistor across each pair, so they can tell from the exchange if the pair is working, open, or shorted. They usually instruct customers who have had their loop tested in this manner to throw the loopback in the bin - it costs much less to buy a new one than to send someone to pick it up.
2 - Less common are fully-connected loopbacks. These test all four wire pairs, and can test if the link is wired correctly for ethernet and voice, or gigabit ethernet. In addition to the megabit ethernet pairs, these also connect pin 4 with pin 7, and pin 5 with pin 8.