I notice my 10/100 Ethernet wire uses 4 wires (2 twisted pairs). I'm curious what each wire is used for. I understand one pair is for Tx (transmit) and the other for Rx (receive) but why does it need 2 wires for each? I assume bits of data cross on one, but what of the other?
This is an ethernet only cable. A full patch lead has 8 cables. Yours is STILL an ethernet cable. The additional blue pair would be telephone and the brown pair was unused, but is commonly used for power over ethernet now. ANY data connection requires 2 cables to balance the signal. In effect one is pushing voltage while the other is pulling and vice versa. One is signal, in effect the other is a ground, bur the idea is that by using twisted pair cables the inductive voltage created in one polarity on one is cancelled by the inductive voltage returning on the other in opposite polarity. The same rule applies to external interference from other equipment as these are also cancelled. If it used one cable and a common ground you can never guarantee this immunity to interference.
