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RJ-11 Definition

RJ-11 is the standard connector used to connect telephones, facsimile machines and related products in the United States, Japan and many other countries. Its main application for computers is connecting modems with telephone wall jacks for dial-up Internet connections.

RJ is an acronym for Registered Jack¹, because this interface is registered with the U.S. Federal Communications Commission (FCC). RJ-11 is the most widely used of the RJ series of connectors (which ranges from RJ-10 through RJ-61). The series is mainly for use with telephone equipment, and the various models have a similar appearance but differ mostly with regard to the number of electrical conductors and width.

The RJ-11 connectors, which are usually made from transparent plastic, consist of mating plugs and receptacles. Each has four contacts (pins) and can accommodate two sets of twisted pair wire. Twisted pair wire is a thin-diameter copper wire (22 to 26 guage) commonly used for telephone cabling. The wires are twisted around each other to minimize crosstalk (i.e., interference from adjacent twisted pairs).

RJ-11 connectors are also sometimes used to link computers into very simple networks. However, most networks, including Ethernet, use RJ-45 connectors, which have a similar appearance but are larger and have eight contacts. Ethernet is by far the most commonly used LAN (local area network) architecture.

RJ connectors were originally invented by Bell Labs (a subsidiary of AT&T, the former U.S. telecommunications monopoly) around 1973, and they soon began replacing the hard-wired (i.e., using screws) connections that had been standard for telephones until then.

The inflexibility of such connections had long been of relatively little concern because telephone equipment was owned and installed by the telephone company. However, in July 1977 the FCC instituted a certification program that permitted any telephone equipment meeting its standards to be connected to the telephone lines. Hundreds of manufacturers soon began competing with Western Electric, AT&T's manufacturing subsidiary, producing and aggressively marketing a wide range of compatible products including answering machines, fax machines, speaker phones, modems and uniquely styled telephones. The modular nature of the RJ-11 connectors added greatly to the flexibility of such equipment and made it practical for consumers to purchase and install it themselves.

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¹Technically, a jack is the socket, or female, portion of a two-piece connector. However, sometimes the term is used more broadly, as in this case, to refer to both pieces.