10BASE-T and 100BASE-T use two pairs of wires in a UTP cable, one for each direction, as shown in Figure 2-9. The figure shows four wires, all of which sit inside a single UTP cable that connects a PC and a LAN switch. In this example, the PC on the left transmits using the top pair, and the switch on the right transmits using the bottom pair.
Figure 2-9 Using One Pair for Each Transmission Direction with 10- and 100-Mbps Ethernet
For correct transmission over the link, the wires in the UTP cable must be connected to the correct pin positions in the RJ-45 connectors. For example, in Figure 2-9, the transmitter on the PC on the left must know the pin positions of the two wires it should use to transmit. Those two wires must be connected to the correct pins in the RJ-45 connector on the switch so that the switch’s receiver logic can use the correct wires.
To understand the wiring of the cable—which wires need to be in which pin positions on both ends of the cable—you need to first understand how the NICs and switches work. As a rule, Ethernet NIC transmitters use the pair connected to pins 1 and 2; the NIC receivers use a pair of wires at pin positions 3 and 6. LAN switches, knowing those facts about what Ethernet NICs do, do the opposite: Their receivers use the wire pair at pins 1 and 2, and their transmitters use the wire pair at pins 3 and 6. The switch effectively reverses the transmit and receive logic of the endpoint device.
To make the preceding logic work, the UTP cable must use a straight-through cable pinout convention. The term pinout refers to the wiring of which color wire is placed in each of the eight numbered pin positions in the RJ-45 connector. An Ethernet straight-through cable connects the wire at pin 1 on one end of the cable to pin 1 at the other end of the cable; the wire at pin 2 needs to connect to pin 2 on the other end of the cable; pin 3 on one end connects to pin 3 on the other, and so on. Figure 2-10 shows the concept of straight-through pinout with two pairs—one pair at pins 1,2 and another at pins 3,6, as used by 10BASE-T and 100BASE-T.
Figure 2-10 10BASE-T and 100BASE-T Straight-Through Cable Pinout
They effectively reverse or cross over the transmit and receive functions, so that for links between a switch and an endpoint device, the cable can use the straight-through pinout shown in Figure 2-9.
A straight-through cable works correctly when the nodes use opposite pairs for transmitting data, as seen in Figure 2-9. However, when connecting two devices that transmit on the same pins, you then need another type of cabling pinout called a crossover cable pinout. The crossover cable pinout crosses the pair at the transmit pins on each device to the receive pins on the opposite device.
While that previous sentence is true, this concept is much clearer with a figure such as Figure 2-11. The figure shows what happens on a link between two switches. The two switches both transmit on the pair at pins 3 and 6, and they both receive on the pair at pins 1 and 2. So, the cable must connect a pair at pins 3 and 6 on each side to pins 1 and 2 on the other side, connecting to the other node’s receiver logic.
Figure 2-11 Crossover Ethernet Cable Pinouts
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