Exploring the RJ45 ethernet standard and usage


By Jeremy Cook

Wired Ethernet, in its current iteration, is a truly amazing and often underappreciated standard. While top speeds have increased with each successive iteration (e.g. Cat-5a to Cat-6), the humble 8-position, 8-connector (8P8C) RJ45 jack standard that interfaces with everything from wall receptacles to routers to actual computers has stayed the same.

These RJ45 Ethernet connections can even supply power as well as data via Power over Ethernet (PoE), making installation even more of a literal "snap."

The RJ45 Plug

The RJ45 plug as we know it today is defined by the TIA-968 standard, as well as the IEC 6063-7 publication. These standards define both the RJ45 connector wiring sequence, as well as the physical dimensions, ensuring that each RJ45 8P8C connector plugs into a given receptacle without issue.

Once inserted, a retention feature pushes out of the plug, locking it into place. The 8 contacts on the bottom enable reliable transfer of bits and bytes. Many cables also feature an anti-snag device that keeps the retention feature from unintentionally hooking onto things and potentially breaking.

RJ45 Connector Wiring

In a world with so many connection standards — consider the number of screws in-use today, or even how many versions of “Universal” Serial Bus (USB) exist — it’s refreshing to see something that mostly just works. Pre-assembled plugs/cables can be purchased as patch cables, or RJ45 connectors can be crimped onto cut-to-length cables.

“Mostly” is noted here, in part because there are actually two RJ45 connector color code standards for Ethernet: T568A and T568B, as defined below:

  • T568A: white/green, green, white/orange, blue, white/blue, orange, white/brown, brown
  • T568B: white/orange, orange, white/green, blue, white/blue, green, white/brown, brown

The A version is wired in such a way that it is backwards-compatible with older tech like fax machines or traditional wired telephones, which was certainly more important when implemented decades ago.

The ANSI accredited Telecommunications Industry Association (ANSI/TIA) recommends the A wiring sequence, but B also appears to be in widespread use. There’s no performance difference between A and B, as long as they’re consistent between one end of a cable and the other.

Body Image 1 Exploring the RJ45 Jack

Data is transferred through bottom connectors

Terms: RJ45 vs. Ethernet Plug

I had some question about the technical validity of using the term RJ45 (RJ being short for “Registered Jack”) for such a connector, and upon a short email exchange with the TIA for this article, they noted that RJ45 is a “fine term, widely used.” They also pointed out that such plugs can be used for non-Ethernet purposes. Therefore, referring to an RJ45 connector as an “Ethernet plug” isn’t always correct, and is probably best avoided in formal technical discussions.

RJ45 ≠ Ethernet (Necessarily)

RJ45 jacks and Cat-X cable is a tempting medium for signal transfer, and perhaps even power in a custom (i.e. non-Ethernet or PoE) manner. However, if an RJ45 plug or receptacle is implemented in such a way, one has to entertain the possibility that the end user will make an assumption about where it goes, and it will end up connected to Ethernet equipment.

At best this could cause frustration, and potentially even equipment damage if circuitry is not designed to protect against such a possibility. On the other hand, if you are working with unknown equipment, be sure to observe and adhere to all plug/socket labeling and instructions.

To end on an encouraging note, the near-universal acceptance of RJ45-style connections for Ethernet communication is truly fantastic. One might even call the jumble RJ45 jack a beautiful piece of engineering. They’re inexpensive, easy to attach, stay secure and are largely unnoticed once in place. A vast army of these connectors in houses, businesses, and server farms work to funnel bits, bytes, and even power without complaint, hour after hour, day after day.


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