In a recent Introduction to Networking class, a participant asked whether the colors in the 10BaseT networking cable were always the same as the ones in the course notes. I replied that while “always” is a strong word, they should always be those colors. The participant asked why, and I followed up with a bit of the history of color coding cable pairs.
My summer job after graduating high school was as pulling cable for the installation of telephones and other communication systems. I learned early on to connect those cables to the systems. The first thing I had to learn was the telephone cable color code. This code is used for 25-pair cable and for cables in multiples of 25 pair including 100 and 250 pair cables.
Here is an image of the color code:
In the 25 pair cable each pair has a wire of the major (mate) color (White, Red, Black, Yellow, or Violet), and one of the minor (color) color (Blue, Orange, Green, Brown, or Slate). The major color is the first wire in the pair. In indoor wiring, the mate has a thin stripe of its corresponding color, and the color has a thin stripe of its corresponding mate.
The colors of the four pair in the networking cable are blue-white, orange-white, green-white, and brown-white. These are for connection to the 8P8C (8 position, 8 contact, commonly called RJ-45) connectors. Note that the order matches the first four pair of the 25-pair telephone cable color code. While there have been proprietary exceptions to this coding, these are generally the standards. There are two ways to wire the connectors, allowing for the seldom-needed “crossover cables.” used to let two devices talk to each other without an intervening hub or switch. Most networking hardware in use today can detect the proper data pair directions.
The networking world has borrowed a great deal from the telecom field. Part of the reason is that many networking installers (like me) started work in the telephone business. The two industries used similar connectors, cables, and connection blocks, leading to terminology transfer from telecom to datacom that has stuck with us.
If you are wondering about the 8P8C vs. RJ-45 comment I made earlier, the answer is simple. The RJ-45 (“Registered Jack”) was used for “high-speed” data. It was actually an “RJ-45S” and supported a single data line. It was keyed so it could not be used in other jacks. Because the unkeyed 8P8C looked virtually identical, installers referred to it as an RJ-45.
There are many similarities in the telecom and Datacom environments, and the Introduction to Networking course highlights many of them. I hope to see you in a future run of that class.