UTP Cabling Pinouts for 10BASE-T and 100BASE-T
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So far in this section, you have learned about the equivalent of how to drive a truck on a 1000-
acre ranch, but you do not know the equivalent of the local traffic rules. If you worked the
ranch, you could drive the truck all over the ranch, any place you wanted to go, and the police
would not mind. However, as soon as you get on the public roads, the police want you to
behave and follow the rules. Similarly, so far this chapter has discussed the general principles
of how to send data, but it has not yet detailed some important rules for Ethernet cabling: the
rules of the road so that all the devices send data using the right wires inside the cable.
This next topic discusses conventions for 10BASE-T and 100BASE-T together, because they
use UTP cabling in similar ways (including the use of only two wire pairs). A short comparison
of the wiring for 1000BASE-T (Gigabit Ethernet), which uses four pairs, follows.
Straight-Through Cable Pinout
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
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.
To allow a PC NIC to communicate with a switch, the UTP cable must also use a straightthrough
cable pinout. 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 straightthrough
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. Also, it uses the wires in one wire pair
at pins 1 and 2, and another pair at pins 3 and 6.
Figure 2-10 10BASE-T and 100BASE-T Straight-Through Cable Pinout
Figure 2-11 shows one final perspective on the straight-through cable pinout. In this case,
PC Larry connects to a LAN switch. Note that the figure again does not show the UTP
cable, but instead shows the wires that sit inside the cable, to emphasize the idea of wire
pairs and pins.
Figure 2-11 Ethernet Straight-Through Cable Concept
A straight-through cable works correctly when the nodes use opposite pairs for transmitting
data. However, when two like devices connect to an Ethernet link, they both transmit
on the same pins. In that case, you then need another type of cabling pinout called a crossover
cable. 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 2-12. 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. The top of the figure shows the
literal pinouts, and the bo ttom half shows a conceptual diagram.
Figure 2-12 Crossover Ethernet Cable
Choosing the Right Cable Pinouts
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
For example, Figure 2-13 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 2-13 Typical Uses for Straight-Through and Crossover Ethernet Cables
NOTE If you have some experience with installing LANs, you might be thinking that you
have used the wrong cable before (straight-through or crossover) but the cable worked.
Cisco switches have a feature called auto-mdix that notices when the wrong cable is used
and automatically changes its logic to make the link work. However, for the exams, be
ready to identify whether the correct cable is shown in the figures.