US PHONE SOCKETS DEMYSTIFIED
Curtis Wheeler - Pleasanton, CA
The foundation is from USOC codes (Uniform service ordering codes). The 'RJ' (as far as I understand) is Registered Jack. It is just a number that was assigned by the Bell System when the FCC started part 68 things. P.S. There are USOC's for just about everything. PBX's, instruments, cords, you name it!!
The USOC designation determines the configuration of a registered jack type. It determines what type of jack is used, how many pair are there and what pins the pairs go to.
An RJ-45 uses an 8 pin modular jack, but it only uses one pair. It is a common jack (or at least used to be) for two wire alarm circuits. It is common for people to refer to every 8 pin modular connection as an RJ-45. Take the same modular connecting hardware, install a four wire data circuit with the transmit tip and ring on pins 1 and 2, receive tip and ring on pins 7 and 8 and it's designated an RJ-48S.
The terms RJ-45, RJ-11, etc are often misused. Another example is the RJ-11. It is designated as a single circuit and only has two wires. It uses the center two pins (3 and 4) of a six position modular connection. If you add a second phone line and put T1 and R1 on pins 2 and 5, it is then designated a RJ-12. Add a third line to pins 1 and 6 and its now an RJ-14. Your single line RJ-11 instrument will still work in the RJ-14 interface, it just doesn't use those other two circuits.
RJ stands for "Registered Jack". The standards are called out in US Federal Communications Commission rules and regulations, 47CFR part 68. I believe that the entire text of CFR is online, but I'm not sure about the diagrams, of which there are several in this section.
The RJ number determines the usage of the jack, which includes the jack type and in some cases its mounting hardware. An RJ-11C is a single-pair jack in a six-position housing, when wired to a single telephone line. RJ-11W is the same, but designed for a wall-mounted telephone. The physical jack of an RJ-12 and RJ-14 are the same, but they are wired differently. One is for a single line behind a mechanical key system and the other is for two separate lines. An RJ-25 is a three-pair (six contact) jack used for three separate lines.
The same modular hardware originally developed for telephone use has proven valuable for other applications because it is readily available and inexpensive, and the more common RJ- numbers have been associated with the hardware as opposed to the usage by many who don't know any better. The common but incorrect usage among computer folks is that an RJ-11 is a jack of up to six positions, and an RJ-45 is an eight-position jack. Note that a six-contact jack when wired to a keyboard or any device other than a three-line telephone set is no longer an RJ-25, but merely a six-position modular connector. It is the usage that causes it to be an RJ-25.
This same kind of standards-drift has resulted in many people referring to any sub-D 25-pin connector as an "RS-232 connector", regardless of the actual wiring or function.
Note also that RJ-series connectors are often suffixed with the number of Positions and actual Connections. Thus a 6P4C connector would be wide enough for six ways but equipped with four connections only.
In Europe these connectors are generally called 'Western Electric' or 'Western' plugs and sockets. In North America they are called 'RJ' or 'Modular' connectors.