Cabletron Systems 100BASE-FX, Cabling Manual

Page: 36 / 192

Ethernet Media
4-12 Cabling Types
Crossovers
The 10BASE-T and 100BASE-TX specifications require that some UTP connections
be crossed over. Crossing over is the reversal of the transmit and receive pairs at
opposite ends of a single cable. Each cable that swaps the location of the transmit
and receive pairs at only one end is called a crossover cable. Those cables that
maintain the same pin numbers for transmit and receive pairs at both ends are
called straight-through cables.
The 10BASE-T and 100BASE-TX specifications are designed around connections
from networking hardware to end user stations being made through
straight-through cabling. Because of this, the transmit wires of a networking
device such as a standalone hub or repeater connect to the receive pins of a
10BASE-T or 100BASE-TX end station.
If two similarly-designed network devices are connected using a straight-through
cable, the transmit pins of one device are connected to the transmit pins of the
other device. In effect, the two devices will both attempt to transmit on the same
pair of the cable between them.
To overcome this, a crossover must be placed between two like devices on a
network, forcing the transmit pins of one device to connect to the receive pins of
the other device. When two like devices are being connected to one another using
UTP cabling, an odd number of crossover cables, preferably one, must be part of
the cabling between them.
Figure 4-6. Straight-Through vs. Crossover Cables
Tx+
Tx-
Rx-
Rx+
Tx+
Tx-
Rx-
Rx+
Path of Transmission
Path of Transmission
Crossover
Straight-Through
Tx+
Tx-
Rx-
Rx+
Tx+
Tx-
Rx-
Rx+
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