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A registered jack (RJ) is a standardized network interface used for network cabling, wiring and jack construction. The primary function of registered jacks is to connect different data equipment and telecommunication devices with services normally provided by telephone exchanges or long-distance carriers. Different standard designs for RJ connectors and wiring are RJ-11, RJ-45, RJ-21, RJ-28 and many more.
The term registered jack refers to the physical connector and often its wiring as well. A registered jack is a female physical connector. Initially the registered jack was regulated by the Federal Communications Commission (FCC) as a standard interface for customers and telephone companies. Telephone companies are only responsible for delivery of their services for minimum port of entry. The customer is responsible for managing all of the physical details, including jacks and wiring.
The standard modular jack was only designed for integrated services digital network (ISDN) systems. However, modular jacks were internationally standardized in IEEE 802.3i in 1990.
There are a variety of types of registered jacks: