What Is an RJ45 Port?

Correct Usage

An RJ45 jack is really a telephone connector. The US Federal Communication Commission, who standardized the connectors used in telecommunications in the USA, endorsed the RJ45. The FCC lists a number of connectors and registers each with a number. "RJ" stands for "Registered Jack." The RJ nomination refers to the wiring plan of both the socket and the plug, and not the physical appearance of the plug. The original RJ45 has eight pins inside and two connectors. The last two pins are used for a programmable resistor. The plug looks the same as a standard US telephone plug (which is the RJ11), except it is bigger.

The most widely used standards defining the physical properties of networks are called Ethernet. Of all the cable types recommended for Ethernet standards, the most common is Unshielded Twisted Pair. This cable contains eight wires configured in pairs with the two wires of each pair twisted around each other. The size and shape of the RJ45 connector was deemed suitable for terminating UTP cable. The eight pins connected to the eight wires in the cable. However, the RJ45 connector only has two contacts. Thus, it was redesigned by the Electronic Industries Alliance for the purposes of Ethernet usage to have eight contacts.

The alteration of the design of the connector from an eight-pin, two-contact (8P2C) plug to an eight-pin, eight-contact (8P8C) plug took it away from the original specification of the RJ45. But the telephone RJ45 is still in circulation and the definition of the RJ45 jack was not changed in line with networking requirements. As the RJ45 designation refers to the wiring plan and not the appearance of the plug, the Ethernet socket is not really an RJ45. However, the two plugs look the same, and so everyone calls the network plug an RJ45. The real RJ45 is rarely used.

Although the eight pins of the plug connect to the eight wires of the UTP cable, only four of those cables ever carry any charge. There are the transmit circuit and the receive circuit. Each circuit is made up of two wires -- a positive and a negative. All computers listen on the receive wires for incoming data. They do not send data out on the same wires. Data leaving a computer travels down the transmit wires. Networking devices have this wiring reversed, so they listen on the transmit circuit and transmit on the receive circuit.