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Reference Guide for the Model RP114 Web Safe Router
Network and Routing Basics B-11
Uplink Switches, Crossover Cables, and MDI/MDIX Switching
In the wiring table above, the concept of transmit and receive are from the perspective of the PC,
which is wired as Media Dependant Interface (MDI). In this wiring, the PC transmits on pins 1 and
2. At the hub, the perspective is reversed, and the hub receives on pins 1 and 2. This wiring is
referred to as Media Dependant Interface - Crossover (MDI-X). When connecting a PC to a PC, or
a hub port to another hub port, the transmit pair must be exchanged with the receive pair. This
exchange is done by one of three mechanisms:
• Uplink switch
Most hubs provide an Uplink switch which will exchange the pairs on one port, allowing that
port to be connected to another hub using a normal Ethernet cable.
• Crossover cable
A crossover cable is a special cable in which the transmit and receive pairs are exchanged at
one of the two cable connectors. Crossover cables are often unmarked as such, and must be
identified by comparing the two connectors. Since the cable connectors are clear plastic, it is
easy to place them side by side and view the order of the wire colors on each. On a
straight-through cable, the color order will be the same on both connectors. On a crossover
cable, the orange and blue pairs will be exchanged from one connector to the other.
• Auto MDI/MDI-X switching
Some Ethernet switch products, such as the Model RP114 router, are able to sense the polarity
of a connection and automatically adapt to the proper mating polarity.
Cable Quality
A twisted pair Ethernet network operating at 10 Mbits/second (10BASE-T) will often tolerate low
quality cables, but at 100 Mbits/second (10BASE-Tx) the cable must be rated as Category 5, or
"Cat 5", by the Electronic Industry Association (EIA). This rating will be printed on the cable
jacket. A Category 5 cable will meet specified requirements regarding loss and crosstalk. In
addition, there are restrictions on maximum cable length for both 10 and 100 Mbits/second
networks.