For the exam, you should be well prepared to choose which type of cable (straight-through or crossover) is needed in each part of the network. The key is to know whether a device acts like a PC NIC, transmitting at pins 1 and 2, or like a switch, transmitting at pins 3 and 6. Then, just apply the following logic:
Crossover cable: If the endpoints transmit on the same pin pair
Straight-through cable: If the endpoints transmit on different pin pairs
Table 2-3 lists the devices and the pin pairs they use, assuming that they use 10BASE-T and 100BASE-T.
Table 2-3 10BASE-T and 100BASE-T Pin Pairs Used
| Transmits on Pins 1,2 | Transmits on Pins 3,6 |
| PC NICs | Hubs |
| Routers | Switches |
| Wireless access point (Ethernet interface) | — |
For example, Figure 2-12 shows a campus LAN in a single building. In this case, several straight-through cables are used to connect PCs to switches. In addition, the cables connecting the switches require crossover cables.
Figure 2-12 Typical Uses for Straight-Through and Crossover Ethernet Cables
Using the wrong cabling pinout with a UTP cable happens to be one of the more common implementation mistakes. Back in 1998, with the introduction of Gigabit Ethernet, the IEEE added a new feature to Ethernet that defines how any device can use electrical pulses to sense if the cable has the wrong cable pinout. For instance, if the link needs a crossover cable, but the installer connected a straight-through cable, this feature can sense the incorrect pinout, and then redirect the electrical signals to the correct pairs to compensate so that the link works. The Ethernet standard calls this feature automatic medium-dependent interface crossover (auto-MDIX).
Auto-MDIX allows sites to use straight-through pinouts on all cables. For instance, the entire cable plant could use a straight-through pinout. On the links that need a crossover pinout, the auto-MDIX on the switch port will sense the use of the straight-through pinout and then internally swap the pairs used by the transceiver to make the link work. (Additionally, most installations use four-pair UTP cables that support Gigabit Ethernet so that all links support 10-, 100-, or 1000-Mbps Ethernet.)
1000BASE-T (Gigabit Ethernet) differs from 10BASE-T and 100BASE-T as far as the cabling and pinouts. First, 1000BASE-T requires four wire pairs. Second, it uses more advanced electronics that allow both ends to transmit and receive simultaneously on each wire pair. However, the wiring pinouts for 1000BASE-T work almost identically to the earlier standards, adding details for the additional two pairs.
The straight-through cable for 1000BASE-T uses the four wire pairs to create four circuits, but the pins need to match. It uses the same pinouts for two pairs as do the 10BASE-T and 100BASE-T standards, and it adds a pair at pins 4 and 5 and another pair at pins 7 and 8, as shown in Figure 2-13.
Figure 2-13 Four-Pair Straight-Through Cable to 1000BASE-T
Just as with 10BASE-T and 100BASE-T, 1000BASE-T (Gigabit Ethernet) uses straight-through cable pinout for some links but crossover cables in other cases. The Gigabit Ethernet crossover cable crosses pairs A and B in the figure (the pairs at pins 1,2 and 3,6) and also pairs C and D (the pair at pins 4,5 with the pair at pins 7,8). You need a crossover cable pinout in the same cases listed earlier in Table 2-3—for instance, between two switches.
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