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uplink port

A port on a network hub or switch that is used to connect to other hubs and switches rather than an end station. See MDI port.

MDI port

(Medium Dependent Interface port) Also called an "uplink port," it is a port on a network hub or switch used to connect to other hubs or switches without requiring a crossover cable. The MDI port does not cross the transmit and receive lines, which is done by the regular MDI-X ports that connect to end stations (the X stands for "crossed"). The MDI port connects to the MDI-X port on the other device. There are typically one or two ports on a device that can be toggled between MDI (not crossed) and MDI-X (crossed).

Automatic MDI/MDI-X
Gigabit Ethernet introduced automatic line sensing in the ports, which detects whether the crossover is needed or not. Referred to as "auto MDI/MDI-X," "universal cable recognition" and "auto sensing," it eliminates having separate MDI and MDI-X ports, selector switches and special crossover cables on the equipment. See crossover cable.

Crossing Over!
The whole idea is to put the transmitted data onto the receive line of the receiving machine. Auto MDI/MDI-X equipment does the correct configuration automatically.

On hubs and switches that do not have auto MDI/MDI-X capability, one port typically provides the option of not crossing the lines. On this 10/100 switch, the MDI/MDI-X designations are represented with graphics that show the non-crossover and crossover configurations.

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