28 May 2011


Here is a picture of unshielded twisted pair (UTP) cable.  It is clear that each pair of wires is twisted around one another.  It is not entirely clear from this picture that the rate at which each pair is twisted is different.  It is also not obvious from this picture that the twisted pairs are then twisted around the other pairs.  Even though it may not be obvious, it happens to be true.

At the ends of such cables one expects plugs or sockets.  RJ-45 plugs are the ones to use with the cable depicted above.  Below is a picture of two unused RJ-45 plugs (from the top and bottom).  For the sake of comparison an RJ-11 telephone plug (attached to a telephone cable) is shown on the left.
To attach the cable one would separate the individual wires, push them into the plug in the correct order and then make the connection permanent by crimping it.  Crimping pushes the pieces of copper (that can be seen on the pug in the centre of the picture above) through the wire inserted below it.  This keeps the wire in place and provides an external contact via the copper strip that still protrudes slightly.  Crimping also pushes the sleeve (on the other end of the plug as the copper) in so that it helps to hold the cable in place.

The order in which wires are arranged depends on the wiring scheme used at a particular installation.  The following diagram depicts the two alternatives.  T568A starts with the green and white wire in position 1 and ends with the solid brown wire in position 8.  In contrast, T568B starts with the orange and white wire in position 1, but also ends with the solid brown wire in position 8.
     T568A                 T568B
1                         1                   
2                         2                   
3                         3                   
4                         4                   
5                         5                   
6                         6                   
7                         7                   
8                         8                   

The 'strange' use of blue in both standards in positions 4 and 5 is intended to ensure compatibility with telephone wiring.  Although the RJ-11 plug has four pins, ordinary telephone connections only use the middle two.  An RJ-11 plug physically fits into an RJ-45 socket and hence installations that use either of these wiring schemes can use the same wiring for telephone or network services.  (In some cases it may even be and/or.)

The next picture shows an RJ-45 socket from the rear.
The 'A' stamped on it indicates that it has been colour coded for the T568A wiring scheme.  In order to install it (using T568A) one merely places each wire into the groove next to its corresponding colour - and then pushes it down into the groove with something known as a pushdown tool.  As the wire is pushed down the copper contacts of the socket cut through the insulation of the wire.  In addition, the contacts are shaped in such a manner that they will retain the wire in the groove and maintain the contact.