A registered jack-45 (RJ45) coupler is a device with two female RJ45 jacks that joins two wires with Ethernet® plugs together. The primary use for an RJ45 coupler is to turn two short Ethernet® computer networking cables into one long cable. While this can be a convenient way to make a long cable on the fly, the RJ45 couplers need to be built to carry the same bandwidth as the cables in the network, and can be sources of interference and performance degradation if used incorrectly.
RJ45 couplers are typically small, cheap devices shaped like elongated cubes with an RJ45 jack at either end. Inside, a regular RJ45 coupler has a series of wires that connect each of the eight pins on the two ports together. Since each data stream is kept on the same part of the wire, the signal coming out of the coupler is the exact same as the one going into it, making the second wire plugged into it an exact copy of the first wire.
In addition to the traditional RJ45 coupler, crossover RJ45 couplers are also available. These devices look just like a regular RJ45 coupler, but with the send and receive wires crossed, and can be used in peer-to-peer networking where two computers are directly connected to each other. Connecting the two computers' Ethernet® cables with a crossover coupler allows the first computer to send data to the second computer's receive ports and to receive data from the second computer's send ports. While these devices are extremely useful in linking two computers together, unintentionally using a crossover coupler instead of a regular RJ45 coupler will cause a network to stop working.
Couplers and Ethernet® cables are typically defined by their "category." This rating refers to both the type of wire used in the cable or coupler and to how often it is twisted together, and wires with higher "cat" ratings can typically carry higher densities of data. Using a Cat 5 coupler in a Cat 6 network can slow the network's performance down over the wire that uses the inferior coupler. In addition, even if the coupler has the same rating as the network, joining two wires together is never as electrically sound as just having one long wire. Although many networks use RJ45 couplers, they always carry the additional risk of a negative impact on network performance.