Driving in the Right Lane (Pair) on the Road

Driving in the Right Lane (Pair) on the Road

Refer to the simple network of Figure 4-1 early in this chapter. That network consisted of two PCs, each with a networking card, and a cable between them. So far, you've learned the basics about wires, cables, and connectors. However, there's one last important thing about basic LAN data transmission that you need to know, and it relates to which wires are used for actual data transmission by Ethernet LANs.

Ethernet NICs in PCs try to send data over the twisted pair that uses pins 1 and 2 of an RJ-45 connector. These same NICs expect to receive data on the twisted pair that uses pins 3 and 6. However, without the right kind of cabling, two Ethernet NICs cannot communicate. Figure 4-9 depicts the effect when both NICs try to send using the twisted pair that uses pins 1 and 2.