RJ-45 Definition

RJ-45, short for registered jack-45, is an eight-wire connector that is used with Category 5 cable for connecting computers and other devices on local-area networks (LANs), especially Ethernets.

A LAN is a network that connects computers and other devices in a relatively small area, typically a part of a building or an entire building, usually for a business or other organization. Ethernet is the most widely used LAN architecture.

Category 5 cable, commonly known as Cat 5, is designed for use in high speed LANs. It consists of a thin plastic sheathing that contains four twisted pairs of copper wire, typically 24 gauge wire with three twists per inch. The twisting reduces electrical interference and crosstalk.

RJ-45 connectors look similar to the ubiquitous RJ-11 connectors, which are used primarily for connecting telephones, fax machines and modems, but they are somewhat wider and have eight contacts instead of four.

LANs originally used coaxial cable and BNC connectors. Coaxial cable typically consists of a single copper wire that is surrounded by a layer of insulation and then by a grounded shield of braided wire or an extruded metal tube. This, in turn, is usually wrapped in another layer of insulation and, finally, in an outer protective layer. BNC (Bayonet Neill Concelman) connectors were designed specifically for connecting coaxial cable. Cat 5 cable and RJ-45 connectors offer some important advantages over the coaxial alternative, including more flexibility, greater ease of use and lower cost.

RJ is an acronym for Registered Jack, because this interface is registered with the U.S. Federal Communications Commission (FCC). The RJ series of connectors (which ranges from RJ-10 through RJ-61) is primarily for use with telephone equipment, and the various models have a similar appearance but differ mainly with regard to the number of conductors and width.

Created September 24, 2005. Updated January 15, 2006.
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