March 05, 2024

The Evolution of RJ45 Connectors: A Historical Perspective

The Evolution of RJ45 Connectors: A Historical Perspective

Origins of the RJ45 Connector

The RJ45 connector, a ubiquitous component in networking, has a fascinating history that intertwines telephony, data communication, and standardization. Let’s delve into its journey from humble beginnings to its current status as the de facto interface for Ethernet over copper twisted-pair networks.

Registered Jack (RJ): The term “RJ” originally referred to the Registered Jack standard, established by the Universal Service Ordering Code (USOC) scheme of the Bell System in the 1970s. These jacks were designed for telephone interfaces. The familiar RJ11 connector, used for one-line telephone services, is a prime example. However, the RJ11’s 6-position, 2-contact (6P2C) interface is different from the RJ45 we know today1.

RJ45’s Telephony Roots: The early use of the 8P8C wiring (8-position, 8-contact) was in the telephony world. A keyed RJ45 connector connected dual trunk lines. When data transmission entered the picture, the name “RJ45” stuck. However, true keyed RJ45 connectors are not entirely compatible with our current 8P8C Ethernet connectors. While 8P8C plugs fit into a true RJ45 outlet, the reverse is not true without modifying the connector1.

4P4C Connector: Before the widespread adoption of Ethernet, the 4P4C connector (4-position, 4-contact) played a crucial role. Commonly known as the modular connector, it was used for telephone handsets and early data devices. The 4P4C connector facilitated voice communication and low-speed data transfer. Its compact design allowed for easy insertion and removal, making it a staple in homes and offices.

Two Color Code Standards: Ethernet cables follow either the T568A or T568B color code standards. These standards dictate the order of wire connections within the RJ45 connector. The T568A sequence (white/green, green, white/orange, blue, white/blue, orange, white/brown, brown) is backward-compatible with older technologies like fax machines and traditional wired telephones. The T568B sequence (white/orange, orange, white/green, blue, white/blue, green, white/brown, brown) is also widely used. Both sequences perform equally well, as long as consistency is maintained across both ends of a cable2.


The Rise of RJ45 as an Ethernet Standard

Standardization: Bell Laboratories introduced RJ connectors in the 1970s, and the Federal Communications Commission (FCC) standardized them shortly afterward. The RJ45 connector became the go-to-choice for connecting devices in local area networks (LANs) and wide area networks (WANs)3.

Interoperability and Backward Compatibility: The RJ45 interface (technically the modular 8P8C interface) gained prominence due to its universal recognition. It allowed for interoperability and backward compatibility. Whether connecting wall receptacles, routers, or computers, the RJ45 remained consistent, making installations straightforward. Moreover, RJ45 connectors can supply both data and power through Power over Ethernet (PoE)2.

Staying Power: Despite advancements in network speeds (from Cat-5a to Cat-6), the RJ45 jack has remained unchanged. Its eight contacts ensure reliable data transfer, and anti-snag features prevent accidental breakage. The RJ45 plug adheres to defined standards, ensuring compatibility across various devices and systems2.


The RJ45 connector, born from telephony roots, has become synonymous with Ethernet connectivity. Its enduring legacy lies in its simplicity, reliability, and widespread adoption. So next time you plug in an RJ45 connector, remember its rich history and the role it plays in keeping our networks connected