In order to understand more complex network technologies it is important to have a strong understanding of the basics.
So here we are Layer 1 – Ethernet, the most commonly used access medium for LAN network. An Ethernet cable consists of a Cat5/Cat5e/Cat6 cable and a RJ45 connector. The cable comprises of 4 twisted copper pairs totaling 8 pins.
The 10Base-T and the 100Base-TX standard both use 2 pairs, one pair to transmit and one pair to receive.
Note that in a straight through setting, the transmit pair occupies pins 1 and 2. The receive pair occupies 3 and 6. Pairs 4,5 and 7,8 are left unused. In order to connect two PCs together, a crossover cable should be used by basically connecting the transmit pair from one side to the receive pair on the other side and vice versa.
The mappings in a crossover cable are Pins 1-3, 2-6, 3-1, and 6-2.
Modern Cisco switches and router interfaces support Auto-MDIX (Auto medium-dependent interface crossover) which automatically detects and swaps the receive and transmit pairs when a wrong cable is used.
