(Rather than clutter the figure with more cabling, I just drew
the two pairs of wires; the wires do indeed sit inside a single cable, with
RJ-45 connectors on each end.)
The cable in Figure 4-9
puts one end of a wire in pin 1 of one connector, and the other end into pin 1
of the other connector. Pin 2 on one end of the cable connects to pin 2 on the
other side; and so on, for all eight wires. A cable with the wires connected in
this manner is called a straight-through cable.
Okay, back to the problem illustrated in Figure 4-9. As you can see from the bubbles in the figure,
both PC's NICs send on the twisted pair at pins 1 and 2. That electricity goes
over the wires and enters the other NIC on pins 1 and 2. But, the NICs aren't
receiving data on pins 1 and 2! That's because Ethernet NICs try to send on pins
1 and 2, and they receive data on the pair at pins 3 and 6. In this case, both
PCs send, but neither receives data.
The solution is to use a cross-over cable. Cross-over cables connect the wire
at pin 1 on one end of the cable with pin 3 on the other end; the wire at pin 2
with pin 6 on the other end; the wire at pin 3 with pin 1 at the other side; and
the wire at pin 6 with pin 2 at the other side. The result: The PCs can receive
the data sent by the other device! Figure
4-10 shows the basic idea.
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