RJ

An RJ connector is a modular jack used to terminate twisted-pair or flat cable, most commonly in telecommunications and data networks. The term stands for registered jack, a name that dates to the 1970s when the Bell System had the FCC register the designs to ensure compatibility across network equipment. Today RJ refers to a family of physically similar connectors that differ chiefly in the number and arrangement of contact positions.

The most familiar variant is the RJ45, which carries eight contacts arranged in a single row inside a plastic housing roughly 12 mm wide. RJ45 terminates Category 5 (Cat5) and higher twisted-pair Ethernet cables; it is the standard connector for 10BaseT, 100BaseTX, and gigabit networks. The RJ11, smaller at six contacts, handles telephone lines and low-bandwidth applications. RJ12 and RJ25 variants exist with intermediate contact counts, though they are less common in modern installations.

The connector itself is the male component, and it plugs into a female jack mounted on a wall plate, patch panel, or device interface. The contact arrangement follows a color-coded standard: in the T568B scheme, the pairs run white-orange, orange, white-green, blue, white-blue, green, white-brown, brown from left to right. Deviating from this sequence when crimping wires into the connector's contact positions is the leading cause of failed connections, whether through corrosion, incorrect pair order, or damaged insulation inside the connector body.

Durability depends on proper installation and environmental conditions. The small contact springs inside corrode in damp settings, and the plastic housing becomes brittle in extreme cold or ultraviolet exposure. A loose crimp leaves air gaps that oxidize quickly; a too-tight crimp cuts through wire insulation. Connectors rated for outdoor or industrial use employ nickel-plated or gold-plated contacts and UV-resistant polymer bodies, adding cost but extending service life in harsh conditions.

In network design, RJ45 jack density on patch panels and switch backplanes has become a measure of infrastructure capacity. A 48-port switch occupies about 2 rack units and carries 48 RJ45 jacks on its rear face. The modular design allows field termination of cables using a crimping tool, avoiding the need for factory-terminated assemblies in many installations, though pre-made patch cables with factory-terminated RJ45 ends remain the standard for final drops to devices.