Registered Jack (RJ)

Techopedia explains Registered Jack (RJ)

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:

• RJ-11: This is the most popular modular form of registered jack. It is found in houses and offices where old telephone-wired systems are connected with the ISP's line.
• RJ-14 and RJ-61: These are similar to RJ-11 but designed for two lines and four lines respectively. RJ-61 is used for termination of twisted-pair cables and uses an eight-pin modular connector.
• RJ-25: This jack was introduced for three lines.
• RJ-11s: A suffix is added to indicate a slight addition in functionality. The suffix w, for example, means that this registered jack is used so the telephone set can be hung on the wall.
• RJ-21: This jack was designed with 50 conductors to implement 25 lines at a time. These connectors are used in area networks with multiple switches and devices.
• RJ-48: This is a modular jack using an eight-position modular connector for local area networks (LANs), T1 and ISDN termination.