RJ11 and RJ45 are two specifications for wiring telephone cable to connectors. These designs were sponsored by the Federal Communications Commission, who registered and codified their wiring patterns and labelled each scheme with a number. "RJ" stands for "Registered Jack."
The terms RJ11 and RJ45 do not refer to the cable, or even the connectors at each end of a telephone cable. They refer to the wiring scheme connecting the cable to its plugs, and in the sockets housing those plugs. Despite the narrow definition of the codes, each wiring scheme is closely associated with a specific plug format. The RJ11 is what most Americans would recognise as a standard telephone plug. It is made of plastic with a spring clip at the top. The underside contains six grooves. These correspond to contact strips within the socket into which the plug fits. The RJ45 plug is larger, but it has the same appearance. This plug has eight grooves. A variation on the designed has an extra strip of plastic on one side. This is called a "keyed RJ45 plug." The strip prevents a plug wired for specific equipment being inserted into a non-compatible RJ45 socket.
A standard telephone cable has two wires. These are called "tip" and "ring." The two wires form a complete circuit. Tip is the positive line; ring is the negative line. This means that despite the number of pins inside each type of connector, only two need to have contacts to pass signals into the socket of the equipment.
An RJ11 plug has six pins and an RJ45 plug has eight. In both cases, only two contacts connect to pins. In telecom terminology, the RJ11 is a 6P2C (six pins, two contacts) connector and the RJ45 is an 8P2C (eight pins, two contacts). The other pins are not necessary. The active pins are the two central pins in each jack type.
The four spare pins of the RJ11 jack open up the possibility of connecting more than one phone line to the same connector. The RJ14 jack is the same design as the RJ11, except that it has four contacts to enable two lines to connect to one telephone. The RJ25 jack has six contact in a connector that looks the same as the RJ11. This connects three phone lines to the same telephone.
The same plug and socket design of the RJ45 connector is used for connecting data cable in networks. This usage follows the Ethernet guidelines and the plug contains eight pins and eight contacts. Because it does not follow the RJ45 wiring specification, the Ethernet plug, is not officially an RJ45 connector, however, because it looks the same, the Ethernet plug is widely referred to as an RJ45.
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