A registered jack (RJ) is a standardized physical network interface — both jack construction and wiring pattern — for connecting telecommunications or data equipment to a service provided by a local exchange carrier or long distance carrier. The standard designs for these connectors and their wiring are named RJ11, RJ14, RJ21, RJ45, RJ48, etc. Many of these interface standards are commonly used in North America, though some interfaces are used world-wide. The physical connectors that registered jacks use are mainly of the modular connector and 50-pin miniature ribbon connector types. For example, RJ11 uses a 6 position 2 conductor (6P2C) modular plug and jack, while RJ21 uses a 50-pin miniature ribbon connector. History
Registered jacks were created by industry and regulated by the FCC to be the standard interface between a telephone company and a customer. The wired communications provider (Telephone Company) is responsible for delivery of services to a minimum point of entry (MPOE) (physically a utility box) which connects the telephone/network wiring on the customer's property (customer-premises equipment/CPE) to the communication provider's network. The customer is responsible for jacks, wiring, and equipment on their side of the MPOE. The intent is to establish a universal standard for wiring and interfaces, and to separate ownership of in-home (or in-office) telephone wiring away from the wiring owned by the telephone company. Under the Bell System monopoly (following the Communications Act of 1934), the Bell System owned the phones and did not allow interconnection of separate phones or other terminal equipment. Phones were generally hardwired, or at times used proprietary Bell System connectors. This began to change with the case Hush-A-Phone v. United States  and the FCC's  decision, which required Bell to allow some interconnection, which culminated in registered jacks. Registered jacks were introduced by the Bell System under a 1976 FCC...
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