Ethernet Cross-Over Cabling

Ethernet Cross-Over Cabling

Network devices can be categorized as either data circuit equipment (DCE) or data terminating equipment (DTE). DCE equipment connects to DTE equipment, similar to the male and female end of a garden hose. DCE equipment usually is a type of concentrator or repeater, like a hub. DTE equipment is usually equipment that generates traffic, like a workstation or host.

Sometimes, it is necessary to connect like equipment. Connecting like devices can be accomplished by altering the twisted-pair media, and taking transmit and receive wires and reversing them. This is commonly called a "cross-over" cable. Figure 1-2 shows an RJ-45 connector with its pinouts. Pins 4, 5, 7, and 8 are not used.

The pinouts are a bit different in a Gigabit scenario because all the pins are used. In addition to the pinouts for 10 Mbps/100 Mbps aforementioned, two additional changes are necessary: pin 4 to 7, and 5 to 8.

A crossover cable can link DCE to DCE, and DTE to DTE. The exception to connecting like devices is that some devices are manufactured to be connected together. An example would be that some hubs and switches have an uplink or Media Dependent Interface (MDI) port. There is typically a selector that allows the user to toggle between MDI and MDI-X (X for crossover), with MDI-X intentionally reversing the pin out of transmit and receive similar to a crossover cable. A setting of MDI-X allows two DCE devices, such as two hubs or switches, to connect to each other using a typical straight through wired twisted-pair cable.