The other posted response from this was good, but there are a couple of other points that may need to be covered, due to the nature of the question.

Going back to the basics, all 8 conductor network patchcords (and cabling) have the conductors divided up into four pairs, with the wires of the pairs twisted together. This is many refer to this cable as “four-pair” cable, or “twisted-pair” cable. By twisting these pairs together, the idea is to minimize the amount of signal that “bleeds” through from one pair to another. As previously noted, Ethernet devices transmit on one “pair” and recieve on another “pair”.

Standards created to regulate this stuff have determined that, if you look into a computer port on the back of the computer, the first two conductors from the left (1 & 2) and the two pins just “outside” the center two conductors (3 & 6) are the active pins. A straight through cable makes sure that 1 & 2 are a twisted pair all the way through the cable, and 3 & 6 are a different twisted pair. The remaining pairs (4 & 5) and (7 & 8) are unused in 10 and 100Base-t, but used in 1000Base-t…still in pairs. The cross-over cable also keeps the signals in pairs, but reverses the pairs on one end.

In addition to allowing two pc’s (or station NIC’s) to directly connect, a crossover cable also allows two hubs, switches, or routers (Head-end NIC’s) to be direcetly connected to each other, which allows hubs to be cascaded, or a small hub in a workstation to be connected to a larger switch back in the telecom room.

Here’s a good web site for your question:

http://www.duxcw.com/digest/Howto/network/cable/cable5.htm

Basically, this is how it looks:

*Pin 1 on sideA (TX+) goes to Pin3 on sideB (RX+)

*Pin 2 on sideA (TX-) goes to Pin6 on sideB (RX-)

*Pin 3 on sideA (RX+) goes to Pin1 on sideB (TX+)

*Pin 6 on sideA (RX-) goes to Pin2 on sideB (TX-)

So if you really think about it, when you use a normal land-line telephone, you are hooking up two telephones with a cross-over connection. You can’t have two people talking on one wire that both transmit (TX). You have to have one side that has a wire that transmits (TX), and another side that receives (RX), and vice-versa. The “+” and “-” in the example above only relates voltage.

Another good web site to look at is:

http://www.cisco.com/univercd/cc/td/doc/product/lan/cat3200/3200/amappexa.htm

This web site can be a little confusing, but it can answer your question with regards to, ?why crossover cables would be used as opposed to straight through cables. Also, what do crossover cables do that straight through cannot?? Just look under the title ?Twisted-pair Cable and Connector Pinouts / 10BaseT / Straight-through and Cross-over? sections. This explains OK for people starting to try to understand what this whole convoluted idea of ?pin-out? is. However, I wouldn?t follow their example of the diagram of their pin-out for a cross-over and straight-through. Use the first web site I gave you.

Hope this helps.

We have quite a bit of info on crossover cables on SearchNetworking.com. You might want to take a look at these articles:

Ten cabling tips in 10 minutes
http://searchnetworking.techtarget.com/originalContent/0,289142,sid7_gci1007161,00.html

Network cable, lesson 3: CAT5 UTP crossover cable
http://searchnetworking.techtarget.com/tip/1,289483,sid7_gci993181,00.html

Good luck!

One other minor piece of information. The number of twists per foot is different in each pair. This also helps eliminate cross-talk bleedover from the wires.

Pair 1 – wires 4,5 (blue, blue white)
Pair 2 – Wires 1,2 (orange white, orange)
Pair 3 – Wires 3,6 (green white, green)
Pair 4 – wires 7,8 (brown white, brown)

This is AT&T 568B standard wiring.

You can also use a crossover cable to access another computer the NIC card