Registered jack

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Short description: Telecommunication network interface

Left to right, modular connectors:
  • eight-contact 8P8C plug used for RJ45S, RJ49, RJ61 and others.
  • six-contact 6P6C plug used for RJ25
  • four-contact 6P4C plug used for RJ14
  • four-contact 4P4C handset plug
  • 6P6C jack, may be used for RJ11, RJ14, and RJ25.

A registered jack (RJ) is a standardized telecommunication network interface for connecting voice and data equipment to a service provided by a local exchange carrier or long distance carrier. Registration interfaces were first defined in the Universal Service Ordering Code (USOC) system of the Bell System in the United States for complying with the registration program for customer-supplied telephone equipment mandated by the Federal Communications Commission (FCC) in the 1970s.[1] They were subsequently codified in title 47 of the Code of Federal Regulations Part 68.[2][3][4] Registered jack connections began to see use after their invention in 1973 by Bell Labs.[5] The specification includes physical construction, wiring, and signal semantics. Accordingly, registered jacks are primarily named by the letters RJ, followed by two digits that express the type. Additional letter suffixes indicate minor variations. For example, RJ11, RJ14, and RJ25 are the most commonly used interfaces for telephone connections for one-, two-, and three-line service, respectively. Although these standards are legal definitions in the United States, some interfaces are used worldwide.

The connectors used for registered jack installations are primarily the modular connector and the 50-pin miniature ribbon connector. For example, RJ11 uses a six-position two-conductor connector (6P2C), RJ14 uses a six-position four-conductor (6P4C) modular jack, while RJ21 uses a 25-pair (50-pin) miniature ribbon connector.

Naming standard

The registered jack designations originated in the standardization process of telephone connections in the Bell System in the United States, and describe application circuits and not just the physical geometry of the connectors. The same modular connector type may be used for different registered jack applications. Modular connectors were developed to replace older telephone installation methods that used hardwired cords or bulkier varieties of telephone plugs.

Strictly, Registered Jack refers to both the female physical connector (modular connector) and specific wiring patterns, but the term is often used loosely to refer to modular connectors regardless of wiring, gender, or use, commonly for telephone line connections, but also for Ethernet over twisted pair, resulting in confusion over the various connection standards and applications. For example, the six-position physical connector, plug and jack, is identically dimensioned and inter-connectable, whether it is wired for one, two, or three lines. These are the RJ11, RJ14, and RJ25 interfaces. The RJ standards designations only pertain to the wiring of the (female) jacks, hence the name Registered Jack. It is commonplace, but not strictly correct, to refer to the unwired connectors or the (male) plugs by these names.

The nomenclature for modular connectors is based on the number of contact positions and the number of wires connected. 6P indicates a six-position modular plug or jack. A six-position modular plug with conductors in only the middle two positions is designated 6P2C; 6P4C has four conductors in the middle positions, and 6P6C has all six equipped.

History and authority

Registration interfaces were created by the Bell System under a Federal Communications Commission order for the standard interconnection between telephone company equipment and customer premises equipment. These interfaces used newly standardized jacks and plugs, primarily based on miniature modular connectors.

The wired communications provider (telephone company) is responsible for delivery of services to a minimum (or main) point of entry (MPOE). The MPOE is a utility box, usually containing surge protective circuitry, which connects the wiring on the customer's property to the communication provider's network. Customers are responsible for all jacks, wiring, and equipment on their side of the MPOE. The intent was to establish a universal standard for wiring and interfaces, and to separate ownership of in-home (or in-office) telephone wiring from the wiring owned by the service provider.

In the Bell System, following the Communications Act of 1934, the telephone companies owned all telecommunications equipment and they did not allow interconnection of third-party equipment. Telephones were generally hardwired, but may have been installed with Bell System connectors to permit portability. The legal case Hush-A-Phone v. United States (1956) and the Federal Communications Commission's (FCC) Carterfone (1968) decision brought changes to this policy, and required the Bell System to allow some interconnection, culminating in the development of registered interfaces using new types of miniature connectors.

Registered jacks replaced the use of protective couplers provided exclusively by the telephone company. The new modular connectors were much smaller and cheaper to produce than the earlier, bulkier connectors that were used in the Bell System since the 1930s. The Bell System issued specifications for the modular connectors and their wiring as Universal Service Order Codes (USOC), which were the only standards at the time. Large customers of telephone services commonly use the USOC to specify the interconnection type and, when necessary, pin-assignments, when placing service orders with a network provider.

When the U.S. telephone industry was reformed to foster competition in the 1980s, the connection specifications became federal law, ordered by the FCC and codified in the Code of Federal Regulations (CFR), Title 47 CFR Part 68, Subpart F,[2] superseded by T1.TR5-1999.[3]

In January 2001, the FCC delegated responsibility for standardizing connections to the telephone network to a new private industry organization, the Administrative Council for Terminal Attachments (ACTA).[4] For this delegation, the FCC removed Subpart F from the CFR and added Subpart G. The ACTA derives its recommendations for terminal attachments from the standards published by the engineering committees of the Telecommunications Industry Association (TIA). ACTA and TIA jointly published the standard TIA/EIA-IS-968,[6] replacing the CFR information.

TIA-968-A, the current version of that standard,[6] details the physical aspects of modular connectors, but not the wiring. Instead, TIA-968-A incorporates the standard T1.TR5-1999, "Network and Customer Installation Interface Connector Wiring Configuration Catalog",[3] by reference. With the publication of TIA-968-B, the connector descriptions have been moved to TIA-1096-A.[4] A registered jack name, such as RJ11, still identifies both the physical connectors and the wiring (pinout) for each application.


6P4C crimp-on style connector commonly used for RJ11 and RJ14

The most widely implemented registered jack in telecommunications is the RJ11. This is a modular connector wired for one telephone line, using the center two contacts of six available positions. This configuration is also used for single-line telephones in many countries other than the United States. It may also use a 6P4C connector, to use an additional wire pair for powering lamps on the telephone set. RJ14 is similar to RJ11, but is wired for two lines and RJ25 has three lines. RJ61 is a similar registered jack for four lines, but uses an 8P8C connector.

The RJ45S jack is rarely used in telephone applications but a standard version of the 8P8C modular connector used by RJ45S is used in Ethernet networks and the connector is often referred to as RJ45 in this context.

Officially recognized types of registered jacks
Code Connector Usage
RJA1X 225A adapter Connector for a modular plug to a four-prong jack
RJA2X 267A adapter Connector for splitting one modular jack to two modular jacks
RJA3X 224A adapter Connector for adapting a modular plug to a 12-prong jack
RJ2MB 50-pin 2–12 telephone lines with make-busy arrangement
RJ11(C/W) 6P2C Establishes a bridged connection for one telephone line (6P4C if power on second pair)
RJ12(C/W) 6P6C Establishes a bridged connection for one telephone line with key telephone system control ahead of line circuit
RJ13(C/W) 6P4C Similar to RJ12, but behind the line circuit
RJ14(C/W) 6P4C For two telephone lines (6P6C if power on third pair)
RJ15C 3-pin weatherproof For one telephone line for boats in marinas
RJ18(C/W) 6P6C For one telephone line with make-busy arrangement
RJ21X 50-pin Multiple (up to 25) line bridged T/R configuration
RJ25(C/W) 6P6C For three telephone lines
RJ26X 50-pin For multiple data lines, universal
RJ27X 50-pin For multiple data lines, programmed
RJ31X 8P8C Allows an alarm system to seize the telephone line to make an outgoing call during an alarm. Jack is placed closer to the network interface than all other equipment. Only 4 conductors are used.
RJ32X 8P8C Like RJ31X, this wiring provides a series tip and ring connection through the connecting block, but is used when the customer premises equipment is connected in series with a single station, such as an automatic dialer.
RJ33X 8P8C This wiring provides a series tip and ring connection of a KTS line ahead of the line circuit because the registered equipment requires CO/PBX ringing and a bridged connection of the A and A1 lead from behind the line circuit. Tip and ring are the only leads opened when the CPE plug is inserted. Typical usage is for customer-provided automatic dialers and call restrictors.
RJ34X 8P8C Similar to RJ33X, but all leads are connected behind the line circuit.
RJ35X 8P8C This arrangement provides a series tip and ring connection to whatever line has been selected in a key telephone set plus a bridged A and A1 lead.
RJ38X 8P4C Similar to RJ31X, with a continuity circuit. If the plug is disconnected from the jack, shorting bars allow the phone circuit to continue to the site phones. Only 4 conductors are used.
RJ41S 8P8C, keyed For one data line, universal (fixed loop loss and programmed)
RJ45S 8P8C, keyed For one data line, with programming resistor
RJ48C 8P4C For four-wire data line (DSX-1)
RJ48S 8P4C, keyed For four-wire data line (DDS)
RJ48X 8P4C with shorting bar For four-wire data line (DS1)
RJ49C 8P8C For ISDN BRI via NT1
RJ61X 8P8C For four telephone lines
RJ71C 50-pin 12-line series connection using 50-pin connector (with bridging adapter) ahead of customer equipment. Mostly used for call sequencer equipment.

Many of the basic names have suffixes that indicate subtypes:

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

For example, RJ11 comes in two forms: RJ11W is a jack from which a wall telephone can be hung, while RJ11C is a jack designed to have a cord plugged into it. A cord can be plugged into an RJ11W as well.

RJ11, RJ14, RJ25 wiring

6P6C connector showing the location of pin 1

All of these registered jacks are described as containing a number of potential contact positions and the actual number of contacts installed within these positions. RJ11, RJ14, and RJ25 all use the same six-position modular connector, thus are physically identical except for the different number of contacts (two, four and six respectively) allowing connections for one, two, or three telephone lines respectively.

Cords connecting to an RJ11 interface require a 6P2C connector. Nevertheless, cords sold as RJ11 often use 6P4C connectors (six position, four conductor) with four wires. Two of the six possible contact positions connect tip and ring, and the other two conductors are unused.

The conductors other than the two central tip and ring conductors are in practice variously used for a second or third telephone line, a ground for selective ringers, low-voltage power for a dial light, or for anti-tinkle circuitry to prevent pulse dialing phones from sounding the bell on other extensions.


Observing the male connector from the cable opening, with prong facing downward, the pins are numbered 1–6, left to right:

Position Pair Tip or
± RJ11 RJ14 RJ25 U.S. Bell System colors[upper-alpha 1] 25-pair color code[upper-alpha 2] German colors[upper-alpha 3] Australian colors Dutch colors[8]
1 3 T + T3 Pair 3 wire 1 or Pair 3 wire 1
white or orange
Pair 3 Wire 1
Pair 3 wire 1
Pair 3 wire 1
Not used
2 2 T + T2 T2 Pair 2 Wire 1 Old
Pair 2 Wire 1
Pair 2 ext. bell
Pair 2 ext. bell
Pair 3 wire 1
3 1 R R1 R1 R1 Pair 1 Wire 2 Old
Pair 1 Wire 2
Pair 1 wire A
Pair 1 wire B
Pair 2 ext. bell
4 1 T + T1 T1 T1 Pair 1 Wire 1 Old
Pair 1 Wire 1
Pair 1 wire B
Pair 1 wire A
Pair 1 wire B
5 2 R R2 R2 Pair 2 Wire 2 Old
Pair 2 Wire 2
Pair 2 ground
Pair 2 ground
Pair 1 wire A
6 3 R R3 Pair 3 Wire 2 or Pair 3 Wire 2
blue or brown
Pair 3 Wire 2
Pair 3 wire 2
Pair 1 Wire 1 Old
Not used
  1. While the old solid color code was well established for pair 1 and usually pair 2, several conflicting conventions exist for pair 3. The colors are from a vendor of silver-satin flat 8-conductor cable that is claimed to be standard. At least one other vendor of flat 8-conductor cable uses the sequence blue, orange, black, red, green, yellow, brown and white/slate.
  2. 25-pair color codes established in the 1950s for polyethylene-insulated conductor (PIC) cable.[7]
  3. This color scheme originates in the national standard DIN 47100 (withdrawn).

Provisioning of power

Some telephones such as the Western Electric Princess and Trimline telephone models require additional power (~6 V AC) for operation of the incandescent dial light. This power is delivered to the telephone set from a transformer by the second wire pair (pins 2 and 5) of the 6P4C connector.


Female RJ21 connector; Pin assignment is coordinated with the table.
Even-count color code
Color Pin
Wire white blue stripe.svg White/blue 26 1 Wire blue white stripe.svg Blue/white
Wire white orange stripe.svg White/orange 27 2 Wire orange white stripe.svg Orange/white
Wire white green stripe.svg White/green 28 3 Wire green white stripe.svg Green/white
Wire white brown stripe.svg White/brown 29 4 Wire brown white stripe.svg Brown/white
Wire white gray stripe.svg White/slate 30 5 Wire gray white stripe.svg Slate/white
Wire red blue stripe.svg Red/blue 31 6 Wire blue red stripe.svg Blue/red
Wire red orange stripe.svg Red/orange 32 7 Wire orange red stripe.svg Orange/red
Wire red green stripe.svg Red/green 33 8 Wire green red stripe.svg Green/red
Wire red brown stripe.svg Red/brown 34 9 Wire brown red stripe.svg Brown/red
Wire red gray stripe.svg Red/slate 35 10 Wire gray red stripe.svg Slate/red
Wire black blue stripe.svg Black/blue 36 11 Wire blue black stripe.svg Blue/black
Wire black orange stripe.svg Black/orange 37 12 Wire orange black stripe.svg Orange/black
Wire black green stripe.svg Black/green 38 13 Wire green black stripe.svg Green/black
Wire black brown stripe.svg Black/brown 39 14 Wire brown black stripe.svg Brown/black
Wire black gray stripe.svg Black/slate 40 15 Wire gray black stripe.svg Slate/black
Wire yellow blue stripe.svg Yellow/blue 41 16 Wire blue yellow stripe.svg Blue/yellow
Wire yellow orange stripe.svg Yellow/orange 42 17 Wire orange yellow stripe.svg Orange/yellow
Wire yellow green stripe.svg Yellow/green 43 18 Wire green yellow stripe.svg Green/yellow
Wire yellow brown stripe.svg Yellow/brown 44 19 Wire brown yellow stripe.svg Brown/yellow
Wire yellow gray stripe.svg Yellow/slate 45 20 Wire gray yellow stripe.svg Slate/yellow
Wire violet blue stripe.svg Violet/blue 46 21 Wire blue violet stripe.svg Blue/violet
Wire violet orange stripe.svg Violet/orange 47 22 Wire orange violet stripe.svg Orange/violet
Wire violet green stripe.svg Violet/green 48 23 Wire green violet stripe.svg Green/violet
Wire violet brown stripe.svg Violet/brown 49 24 Wire brown violet stripe.svg Brown/violet
Wire violet gray stripe.svg Violet/slate 50 25 Wire gray violet stripe.svg Slate/violet

RJ21 is a registered jack standard using a modular connector with contacts for up to fifty conductors. It is used to implement connections for up to 25 lines, or circuits that require many wire pairs, such as used in the 1A2 key telephone system. The miniature ribbon connector of this interface is also known as a 50-pin telco connector, CHAMP(AMP), or Amphenol connector, the latter being a genericized trademark, as Amphenol was a prominent manufacturer of these at one time.

A cable color scheme, known as even-count color code, is determined for 25 pairs of conductors as follows:[9] For each ring, the primary, more prominent color is chosen from the set blue, orange, green, brown, and slate, in that order, and the secondary, thinner stripe color from the set of white, red, black, yellow, and violet colors, in that order. The tip conductor color scheme uses the same colors as the matching ring but switches the thickness of the primary and secondary colored stripes. Since the sets are ordered, an orange (color 2 in its set) with a yellow (color 4) is the color scheme for the 4·5 + 2 − 5 = 17th pair of wires. If the yellow is the more prominent, thicker stripe, then the wire is a tip conductor connecting to the pin numbered 25 + the pair #, which is pin 42 in this case. Ring conductors connect to the same pin number as the pair number.

A conventional enumeration of wire color pairs then begins blue (and white), orange (and white), green (and white) and brown (and white), which subsumes a color-coding convention used in cables of 4 or fewer pairs (8 wires or less) with 8P and 6P connectors.

Dual 50-pin ribbon connectors are often used on punch blocks to create a breakout box for private branch exchange (PBX) and other key telephone systems.


8P8C keyed female connector (jack)

The RJ45S, a standard[10] jack once specified for modem or data interfaces, uses a mechanically-keyed variation of the 8P8C body with an extra tab that prevents it from mating with other connectors; the visual difference from the more-common 8P8C is subtle. The original RJ45S keyed 8P2C modular connector had pins 5 and 4 wired for tip and ring of a single telephone line, and pins 7 and 8 shorting a programming resistor,[11][12][13] but is obsolete today.


RJ48C and RJ48X wiring
Pin Pair Signal Color
1 R RX ring Wire orange white stripe.svg Orange/white
2 T RX tip Wire white orange stripe.svg White/orange
3 Reserved Wire white green stripe.svg White/green
4 R1 TX ring Wire blue white stripe.svg Blue/white
5 T1 TX tip Wire white blue stripe.svg White/blue
6 Reserved Wire green white stripe.svg Green/white
7 Shield Wire white brown stripe.svg White/brown
8 Shield Wire brown white stripe.svg Brown/white

RJ48 is a registered jack. It is used for T1 and ISDN termination and local area data channels or subrate digital services. It uses the eight-position modular connector (8P8C).

RJ48C is commonly used for T1 circuits and uses pin numbers 1, 2, 4 and 5.

RJ48X is a variation of RJ48C that contains shorting blocks in the jack so that a loopback is created for troubleshooting when unplugged by connecting pins 1 and 4, and 2 and 5. Sometimes this is referred to as a self-looping jack.

RJ48S is typically used for local area data channels and subrate digital services, and carries one or two lines. It uses a keyed variety of the 8P8C modular connector.

RJ48 connectors are fastened to shielded twisted pair (STP) cables, not the standard unshielded twisted pair (UTP) CAT-(1–5).


RJ61 wiring (USOC)
Pin Pair Signal Color
1 4 Tip Wire white brown stripe.svg White/brown
2 3 Tip Wire white green stripe.svg White/green
3 2 Tip Wire white orange stripe.svg White/orange
4 1 Ring Wire blue white stripe.svg Blue/white
5 1 Tip Wire white blue stripe.svg White/blue
6 2 Ring Wire orange white stripe.svg Orange/white
7 3 Ring Wire green white stripe.svg Green/white
8 4 Ring Wire brown white stripe.svg Brown/white

RJ61 is a physical interface often used for terminating twisted pair type cables. It uses an eight position, eight conductor (8P8C) modular connector.

This pinout is for multi-line telephone use only; RJ61 is unsuitable for use with high-speed data, because the pins for pairs 3 and 4 are too widely spaced for high signaling frequencies. T1 lines use another wiring for the same connector, designated RJ48. Ethernet over twisted pair (10BASE-T, 100BASE-TX and 1000BASE-T) also use a different wiring for the same connector, either T568A or T568B. RJ48, T568A, and T568B are all designed to keep pins close together for pairs 3 and 4.

The flat eight-conductor silver-satin cable traditionally used with four-line analog telephones and RJ61 jacks is also unsuitable for use with high-speed data. Twisted pair cabling is required for data applications. Twisted-pair data patch cable used with the three data standards is not a direct replacement for RJ61 cable, because RJ61 pairs 3 and 4 would be split among different patch cable twisted pairs, causing cross-talk between voice lines 3 and 4 that might be noticeable for long patch cables.

With the advent of structured wiring systems and TIA/EIA-568 (now ANSI/TIA-568) conventions, the RJ61 pinout is falling into disuse. The T568A and T568B standards are used in place of RJ61 so that a single wiring standard in a facility can be used for both voice and data.

Similar jacks and unofficial names

The following RJ-style names do not refer to official ACTA types.

The labels RJ9, RJ10, RJ22 are variously used for 4P4C and 4P2C modular connectors, most typically installed on telephone handsets and their cordage. Telephone handsets do not connect directly to the public network, and therefore have no registered jack designation.

RJ45 is often incorrectly used when referring to an 8P8C connector used for ANSI/TIA-568 T568A and T568B and Ethernet. It is distinct from the official RJ45S and ARJ45 interfaces.

RJ50 is often a 10P10C interface, often used for data applications.

The micro ribbon connector, first made by Amphenol, that is used in the RJ21 interface, has also been used to connect Ethernet ports in bulk from a switch with 50-pin ports to a Cat-5 rated patch panel, or between two patch panels. A cable with a 50-pin connector on one end can support six fully wired 8P8C connectors or Ethernet ports on a patch panel with one spare pair. Alternatively, only the necessary pairs for 10/100 Ethernet can be wired allowing twelve Ethernet ports with a single spare pair.

This connector is also used with spring bail locks for SCSI-1 connections. Some computer printers use a shorter 36-pin version known as a Centronics connector.

The 8P8C modular jack was chosen as a candidate for ISDN systems. In order to be considered, the connector system had to be defined by an international standard, leading to the creation of the ISO 8877 standard. Under the rules of the IEEE 802 standards project, international standards are to be preferred over national standards, so when the original 10BASE-T twisted-pair wiring version of Ethernet was developed, this modular connector was chosen as the basis for IEEE 802.3i-1990.

See also


  1. AT&T, Registration Interface—Selection and General Information, Bell System Practices, Section 463-400-100 Issue 1, May 1976
  2. 2.0 2.1 FCC 47 CFR Part 68 Connection of Terminal Equipment to the Telephone Network , Section 68.502 superseded by T1.TR5-1999
  3. 3.0 3.1 3.2 T1.TR5-1999 Network and Customer Installation Interface Connector Wiring Configuration Catalog
  4. 4.0 4.1 4.2 "ACTA Documents Main - Administrative Council for Terminal Attachments". 
  5. "RJ-11 Definition". 
  6. 6.0 6.1 TIA-968-A or tia-eia-is-968 documents of FCC specifications from the Administrative Council for Terminal Attachments , section 6.2 in particular
  7. Horn, F. W. (October 1958). "'Even-Count' Cable". Bell Laboratories Record 37 (5): 208–217. Retrieved October 13, 2022. 
  8. "(nl) Support document for the 'PTT norm 88'" (in nl). p. 8. 
  9. Ogletree, Terry William (2004). "Cables, Connectors, Concentrators, and Other Network Components". Upgrading and Repairing Networks (4 ed.). Que Publishing. p. 84. ISBN 0-7897-2817-6. 
  10. "Wiring Diagrams". 
  11. "RJ45S (Archived copy)". 
  12. "Modular Jack Wiring". Hamilton Video & Sound Limited. 
  13. "Modular Wiring Reference". Siemon. 

External links