A recent Ask the Expert question on SearchNetworking.com asked about the number of wire pairs in Category 5 unshielded...
twisted pair cable. The answer is four pairs, or eight individual wires, but there is a lot more information that network administrators should know about Cat 5 UTP.
Cat 5 cable is called unshielded twisted pair cable because there is no shielding around the pairs of wire that carry data signals. In co-axial cable, for instance, an insulating material surrounds the internal conductor, and then a shield, made up either of metal foil or braided wire surrounds the assembly. This shield stops interference from getting to the signal carrying wire by draining the interference to the terminals of the wire, which are grounded; the interference (or noise) goes to ground where it cannot do any damage to your data transmission.
In UTP cable, on the other hand, the noise reduction function is handled by the twist. Twisting actually makes the noise that is picked up in both wires of a twisted pair to cancel itself, thus rendering your cables relatively noise-immune. That's why you should not untwist the cable pairs when you terminate them in a modular connector, any more than 1/4 inch. The less twist, the more noise can get into your data circuits.
If you're going to build your own Ethernet cables, and many network admins do, then you need to know what the wires inside the cable do. A good reference for this can be found at the APT Communications Website. The thing to remember is that you have to pick one of the wiring schemes that are shown in this article (Either EIA/TIA 568A or B) and stick with it. EIA is the Electronic Industries Association, and TIA is the Telecommunications Industries Association, both proponents for wiring standards. The 568 A standard is supposed to be the preferred one for wiring of Ethernet cables, but many products coming from wire and cable companies seem to follow the B version, according to APT Communications. If you have patch cords and installed cables that follow the B version, you have to make up new cables that follow that same standard, or else your network will have crossed cable pairs and won't work.
Remember also that the pairs are always of the same color, but one of the pairs is solid, while the other is striped. So, for example, in EIA/TIA 568B, pair number 2 is made up of one orange solid wire and one orange/white striped wire.
Knowing your color codes and understanding the purpose of the cable design will solve a lot of problems in your network installations.
David Gabel is Executive Technology Editor of TechTarget.