ExSimGuy and AVPIN are definitely on the right track, I would say. Here's the theory:
OK, why pairing? Send a modulated signal up a long wire at 10 Mbps, even at a couple of volts and you generate a detectable electro-magnetic field. This will be picked up in the adjacent wires and a signal can be detected on those wires. The higher the frequency, the higher the strength of the field, which explains why this problem manifests itself at high net speeds. The way these networks work is as follows: a packet gets sent from one computer to another on the network. Think of the packet as a train. A train has an engine, some carriages and a caboose. Imagine that the IP address of the computer you are sending to is printed on the front of the train. Underneath that is printed the return address (ie the "from" computer"). The carriages are the data and the caboose is there to denote the end of the packet. Off it goes, round the network, and it visits each computer in turn. A computer sees the train approaching and reads the number off the front. If the computer address and train address do not match, the packet is ignored and allowed to pass
through the card unmolested. If the address does match, the computer effectively reads the data (carriages) from the packet, reverses the addresses and sends the train on its way again. The "from" computer intercepts this train (because its address is now uppermost on the front), realises that the data has been received correctly, and it can now send another packet. From this you will see that there are "up" and "down" lines with almost identical signals flashing up and down the cable. If you twist pairs of wires together, the EMF in one wire is
exactly counteracted by the EMF generated in its twin. Only 4 of the pins out of 8 in an RJ45 plug are used, so you've got to get the right ones.
I can't remember which ones they are, offhand, but this order always worked for me: hold the plug with the "open" end (the end the cable goes into) facing your chest, gold pins uppermost. Mentally label the pins 1 to 8, left to right. The standard cable we used had 4 twisted pairs as follows:
Orange + White/Orange (O-W/O)
Green + White/Green (G-W/G)
Blue + White/Blue (Bl-W/Bl)
Brown + White/Brown (Br-W/Br)
Insert the wires in the following order:
Pin Colour
--- ------
1 W/O
2 O
3 W/G
4 Bl
5 W/Bl
6 G
7 W/Br
8 Br
I know it doesn't look right at pins 3, 4, 5 and 6, but that is down to the internal wiring of the card. That's the way it is. If The colours of the wires in your cable don't match those above, find out, as AVPIN said, which cables are paired, then map those colour pairs to the table above. For example, if yellow and red are paired, map yellow to pin 1, red to pin 2, etc, etc, but you've got to get that "twist" at 3, 4, 5 and 6 right.
It is possible to tell how good your cables are with a tester. You may be able to hire one, or borrow one from your friendly local computer retailer/network guru. They are quite expensive, so be prepared to offer a check as deposit. He might allow you to bring your leads in for testing. On the other hand he might "generously" allow you to buy some of his fine wares at a grossly inflated price!!
