An uplink port is a special port (i.e., connector) on a network switch or hub that reverses the transmit and receive circuits of any twisted pair cable connected to it. It is also referred to as an MDI (medium dependent interface) port.
Uplink ports eliminate the need for crossover cables. A crossover cable is identical to a conventional twisted pair copper wire patch cable (also called straight-through cable) except that the wires on the cable are crossed over so that the receive signal pins on the RJ-45 connector on one end are connected to the transmit signal pins on the RJ-45 connector on the other end. Its purpose is to allow the direct connection of two like devices (i.e., two hubs, two switches or two computers).
An uplink port is required for linking two like devices directly with a standard patch cable because the wire pairs would be connected transmit-to-transmit and receive-to-receive, and thus the devices would not be able to communicate with each other.
When connecting two like devices, the uplink port on only one of them is used. If the uplink ports on both are connected with straight-through cable, the result is the same as using two conventional ports, and thus the devices would be unable to communicate.
Some network devices have switches that allow the user to select the mode of operation for the uplink ports. For example, in the case of a small network in which it is not necessary to connect two hubs, such switch enables a port to be used to connect an additional computer.
An uplink port superficially resembles any other port on a hub or switch. However, it is typically labeled as such.
Created October 28, 2005.