Data Cabling Techniques
44
Diagram A is correct because the proper pairs are twisted together.
Diagram B is incorrect because the wires for the receive pair, pins 3 and 6,
are not twisted together. Diagram B wiring could result in excessive
common mode noise and an unacceptably high data error rate.
If you pair pins incorrectly, for example as shown in Figure 21, Diagram B,
your network may have a high data error rate. In a straight-through cable,
the transmit pins, 1 and 2, are paired, as are the receive pins, 3 and 6.
In a crossover cable, shown in Figure 22, pins 1 and 2, TD+ and TD-, are
paired, as are pins 3 and 6, RD+ and RD-. When pins 1 and 2 are crossed
over, they connect to pins 3 and 6 respectively; and pins 3 and 6, when
crossed over, connect to pins 1 and 2, respectively.
Figure 21: Hub-to-MAU
Wiring Usable
(Diagram A) and
Unusable
(Diagram B)
Not Used
RD-
Not Used
Not Used
RD+
TD-
TD+
Not Used
Not Used
RD-
Not Used
Not Used
RD+
TD-
TD+
Not Used
RJ45 Pin
RJ45 Pin
1
2
3
4
5
6
7
8
1
2
3
4
5
6
7
8
1
1
Pair twisted as per Level 3, 4 or 5 cable.
1
A
Not Used
RD-
Not Used
Not Used
RD+
TD-
TD+
Not Used
Not Used
RD-
Not Used
Not Used
RD+
TD-
TD+
Not Used
RJ45 Pin
RJ45 Pin
1
2
3
4
5
6
7
8
1
2
3
4
5
6
7
8
This cable will not work for 10BASE-T.
Note that 3 and 6 are twisted, but are not a pair.
B
