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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. These interface standards are most commonly used in North America, though some interfaces are used world-wide.

The physical connectors that registered jacks use are of the modular connector type, except RJ21X which is a 25-pair Amphenol connector. For example, RJ11 uses a 6 pin 2 conductor (6P2C) modular plug and jack.

Left to right, RJ connectors: an eight-pin plug (8P8C, used for RJ49, RJ61 and others, but often called "RJ45" because of its outward semblance to the true RJ45), six-pin RJ25 plug, four-pin RJ14 plug (often also used instead of two-pin RJ11), and a four-pin handset plug (also popularly, though incorrectly, called "RJ22", "RJ10", or "RJ9"). The middle two can be plugged 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 used for RJ11, RJ14 or even RJ25, all of which are actually names of interface standards that use this physical connector. The RJ11 standard dictates a 2-wire connection, while RJ14 spells out a 4-wire configuration, and 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.

Registered jacks were originally the subject of Bell System Universal Service Ordering Codes. A USOC is a code one can use on an order for telephone service to specify the kind of service ordered. For example, to order a new telephone extension installed the subscriber might specify the USOC "RJ11W" in order to get a 6P2C jack for a conventional wall mounted single line telephone installed. People sometimes use "USOC" to refer to the service specification itself, though the USOC is just the name of it. With respect to registered jacks, the complete specification was registered (at one time with the US government) -- hence the name. Though it's awkward to refer to a standard for a jack as a jack, it is nonetheless the conventional terminology. The registered jack is the type of physical interface and the USOC is a name for that type of physical interface.

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

Twisted pair

While (unlike the 8P8C modular plug used in Ethernet twisted-pair cabling) the plugs are generally used with a flat cable, the long cables feeding them in the building wiring and the phone network before they 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).

International use

The modular jack was chosen as a candidate for ISDN systems. In order to be considered, the connector system had to be defined under international standards. In turn this led to ISO 8877. Under the rules of IEEE standards Project 802, international standards are to be preferred over national standards so the modular connector was chosen for IEEE 802.3i, the original 10BASE-T twisted-pair wiring version of Ethernet.

Registered jack types

The most familiar registered jack is probably the RJ11. This is a 6 position modular connector wired for one phone line, and is what you see in most homes and offices in North America for single line telephones.

RJ14 and RJ25 are also fairly common, using the same size connector as RJ11, but with two and three phone lines, respectively, connected.

Essentially all one, two, and three line telephones made today (2006) are meant to plug into RJ11, RJ14, or RJ25 jacks, respectively.

The true RJ45(S) is an extremely uncommon registered jack, but the name "RJ45" is also used quite commonly to refer to any 8P8C modular connector.

Many of the basic names have suffixes that indicate subtypes:

  • C: flush-mount or surface mount
  • W: wall-mount
  • S: single-line
  • M: multi-line
  • X: complex jack

For example, RJ11 comes in two forms: RJ11W is a jack from which you can hang a wall telephone, while RJ11C is a jack designed to have a cord plugged into it. (You can plug a cord into an RJ11W as well, but it usually isn't as aesthetic as a cord plugged into an RJ11C).

Common types

  • RJ11C/RJ11W: 6P2C, for one telephone line (6P4C with power on second pair)
  • RJ14C/RJ14W: 6P4C, for two telephone lines
  • RJ25C/RJ25W: 6P6C, for three telephone lines

Uncommon types

  • RJ2MB: 50-pin miniature ribbon connector, 2-12 telephone lines with make-busy
  • RJ12C/RJ12W: 6P4C, for one telephone line ahead of the key system
  • RJ13C/RJ13W: 6P4C, for one telephone line behind the key system
  • RJ15C: 3-pin weatherproof, for one telephone line
  • RJ18C/RJ18W : 6P6C, for one telephone line with make-busy arrangement
  • RJ21X: 50-pin miniature ribbon connector, for up to 25 lines
  • RJ26X: 50-pin miniature ribbon connector, for multiple data lines, universal
  • RJ27X: 50-pin miniature ribbon connector, for multiple data lines, programmed
  • RJ31X: 8P8C (although usually only 4C are used), allows an alarm system to seize the telephone line to make an outgoing call during an alarm. Jack is placed ahead of all other equipment.
  • RJ38X: 8P8C, similar to RJ31X, with continuity circuit
  • RJ41S: 8P8C keyed, for one data line, universal
  • RJ45S: 8P2C + keyed, for one data line with programming resistor
  • RJ48S: 8P8C, for four-wire data line (DDS)
  • RJ48C: 8P8C, for four-wire data line (DSX-1)
  • RJ48X: 8P8C with shorting bar, for four-wire data line (DS1)
  • RJ49C: 8P8C, for ISDN BRI via NT1
  • RJ61X: 8P8C, for four telephone lines
  • RJ71C: 12 line series connection using 50 pin connector (with bridging adapter) ahead of customer equipment. Mostly used for call sequencer equipment.Urie

"Unofficial" (incorrect) plug names

These "RJ" names do not really refer to truly existing FCC RJ types:

  • "RJ9", "RJ10", "RJ22": 4P4C or 4P2C, for telephone handsets. Since telephone handsets do not connect directly to the public network, they have no Registered Jack code whatsoever.
  • "RJ45": 8P8C, informal designation for T568A/T568B, including Ethernet; not the same as the true RJ45/RJ45S
  • "RJ50": 10P10C, for data
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