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Registered jack

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A registered jack (RJ) is a standardized physical interface for connecting telecommunications equipment (commonly, a telephone jack or computer networking equipment). The standard designs for these connectors and their wiring are named RJ11, RJ14, RJ45, etc.

Numbering and pinouts were set forth by the Bell System in Universal Service Ordering Codes (USOCs), and were introduced in the 1970s by AT&T. They are also registered with the U.S. Federal Communications Commission (FCC), under 47 CFR §68.502.

A USOC is a code one can use on an order for telephone service to specify the kind of service ordered. For example, if you order a new telephone extension installed, you might specify the USOC "RJ11W" in order to get a 6P jack for a conventional wall-mounted single line telephone installed. People sometimes use "USOC" to refer to the service specification itself, though used literally, the USOC is just the name of it.

It is important to note that a USOC does not always define exactly a connector to use.

Contents

Common usage

The most familiar registered jacks are the 6-position connectors known variously as RJ11 (2 conductor/1 pair), RJ12 or RJ25 (6 conductor/3 pair), and RJ14 (4 conductor/2 pair); and the RJ45 (8 conductor/4 pair), all sometimes simply called RJ connectors or modular phone plugs/jacks. These are commonly used in building wiring for telephone and local area networks. They were originally invented and patented by Bell Telephone Laboratories (patent filed 6 July 1973; [U.S. Patent 3,860,316] issued 14 January 1975), and replaced the hard-wired connections on most Western Electric telephones around 1976. Thus, they are also sometimes called Western jacks and Western plugs.

Left to right, RJ connectors: an eight-pin RJ45 plug, six-pin RJ25 or RJ12 plug, four-pin RJ11 or RJ14 plug, and a four-pin RJ22 (RJ10 or RJ9) handset plug. The middle two plug into the same standard six-pin jack, pictured.
Enlarge
Left to right, RJ connectors: an eight-pin RJ45 plug, six-pin RJ25 or RJ12 plug, four-pin RJ11 or RJ14 plug, and a four-pin RJ22 (RJ10 or RJ9) handset plug. The middle two plug into the same standard six-pin jack, pictured.

Naming confusion

There is much confusion over these connection standards. The six-position plug and jack commonly used for telephone line connections may be called an RJ11, RJ14 or even RJ12 (RJ25), all of which are actually names of interfaces that use this physical connector. The RJ11 standard dictates a 2-wire connection, while RJ14 spells out a 4-wire configuration, and RJ12 or RJ25 uses all six wires.

Plugs and jacks of this type are often called modular connectors, which originally distinguished them from older telephone connectors, which were very bulky or wired directly to the wall and therefore not accommodating of modular systems. A common nomenclature for modular connectors is e.g. "6P" to indicate a six-position modular plug or jack. Sometimes the nomenclature is expanded to indicate the number of positions that contain conductors. For example, a six-position modular plug with conductors in the middle two positions and the other four positions unused is called a 6P2C. RJ11 uses a 6P plug; furthermore, it often uses a 6P2C.

An alternative terminology sometime used is e.g. 6x2. Another alternative is e.g. 6/2.

Physical compatibility

There is also confusion over the physical compatibility. As designed, they are physically compatible in that a four-pin plug will fit a six- or eight-pin socket, connecting to the center four of the conductors, and a six-pin plug will fit an eight-pin socket, connecting to the center six conductors. However, plugs from different manufacturers may not have this compatibility, and some manufacturers of eight-pin jacks now explicitly warn that they are not designed to accept smaller plugs without damage.

RJXX jacks are also available keyed, with an extra side tab to fit in an appropriate keyed jack, to make it intentionally incompatible with standard jacks.

RJ11 is found commonly on almost all phone and fax equipment.

Twisted pair

While (with the exception of the RJ45) the plugs are generally used with a flat cable, the long cables feeding them in the building wiring and the phone network before it are normally twisted pair. Wiring conventions were designed to take full advantage of the physical compatibility ensuring that using a smaller plug in a larger socket would pick up complete pairs not a (relatively useless) two half pairs but here again there has been a problem. The original concept was that the centre two pins would be one pair, the next two out the second pair, and so on until the outer pins of an eight-pin connector would be the fourth twisted pair. Additionally, signal shielding was optimised by alternating the “live” (hot) and “earthy” (ground) pins of each pair. This standard for the eight-pin connector is the USOC-defined pinout, but the outermost pair are then too far apart to meet the electrical requirements of high-speed LAN protocols. Two variations known as T568A and T568B overcome this by using adjacent pairs of the outer four pins for the third and fourth pairs. The inner four pins are wired identically to RJ14. (See: Category 5 cable.)

History and Authority

Registered Jacks were introduced by the Bell System in the 1970s, replacing much bulkier connectors that were in use before. The Bell System gave the specifications for them (both the modular connectors and the wiring of them) Universal Service Ordering Codes (USOC), and that was the only standard at that time.

When the US telephone industry was opened to more competition in the 1980s, the specifications were made a matter of US law, ordered by the Federal Communications Commission (FCC) and codified in the Code of Federal Regulations, 47 CFR 68, subpart F.

In January 2001, the FCC turned over responsibility for standardizing connections to the telephone network to a new private industry organization, the Administrative Council for Terminal Attachment (ACTA). The FCC removed Subpart F from the CFR and added Subpart G, which delegates the task to the ACTA. The ACTA published a standard called TIA/EIA-IS-968 which contained the information that was formerly in the CFR. The current version of that standard, called TIA-968-A, specifies the modular connectors at length, but not the wiring. Instead, TIA-968-A incorporates a standard called T1.TR5-1999 by reference to specify the wiring. Note that a Registered Jack name such as RJ11 identifies both the physical connectors and the wiring (pinout) of it (see above).

List of designations

Common types

Unofficial types

Uncommon types

This list is [Incomplete listsincomplete]; you can help by [http://encycl.opentopia.com/ expanding it].

Suffixes

External links

USOC registered jacks

  RJ9 | RJ11 | RJ12 | RJ14 | RJ21 | RJ25 | RJ45 | RJ48 | RJ50 | RJ61  

 

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