Telephone Wiring Fundamentals: Tip & Ring and USOC
Before connecting telephones or related equipment, it's essential to understand the two foundational concepts of landline wiring: "Tip & Ring" nomenclature for the physical wires and the "USOC" system for identifying services. This guide covers both.
Tip & Ring Nomenclature
The names for the two wires of a telephone line were derived from the electrical contacts on the original 1/4 inch telephone plug, which is similar in appearance to a standard 6.5mm stereo phono plug.
One electrical contact is at the tip of the plug (used for Line +) and the other is a ring just behind it (used for Line -). A third contact is referred to as the sleeve.
Conventional Nomenclature:
| TIP (note) | A-wire | Line (+) positive | Earth | Red |
| RING | B-wire | Line (-) negative | Battery | Black |
Note: A useful memory aid is to remember "Red TAPE" : (Red = Tip, A-wire, Positive, Earth
Universal Service Ordering Code (USOC)
The Universal Service Ordering Code (USOC) system was introduced by AT&T in the 1970s as a means of identifying telecommunication services and equipment. Some of these codes were later adopted by the FCC and are now often used (sometimes incorrectly) to universally describe telephone jacks such as RJ11 and RJ45.
Our Guide to RJ Codes provides a more detailed description for each specific RJ code.
Common USOC Suffixes:
| Abbreviation | ||
| RJ | Registered Jack | (Prefix) |
| C | Flush or Surface Mount Jack | (Suffix) |
| W | Wall Phone Jack | (Suffix) |
| X | Complex Multi-line or Series type Jack | (Suffix) |
Common RJ Number Applications:
| R J Number | Specific Installation Application |
| RJ11C and RJ11W | Single line phones, accessories, answering machines, and modems |
| RJ14C and RJ14W | Two line phones, accessories and answering machines |
| RJ21X | 1 - 25 single or multiple line circuits bridged to network or customer equip |
| RJ22 | Used for Handsets or Headsets |
| RJ25C | Three-line-phones and accessories |
| RJ31X and RJ38X | Burglar and fire alarms circuits |
| RJ41S and RJ45S | Single line fixed loss loop data installations |
| RJ48C/J and RJ45S | Four wire data circuits |
| RJ61X | Four line phones and accessories |