Every link in every network on earth is copper, fiber, or radio. There is no fourth thing. That sentence is the organizing idea for a whole week of ITN 101, and it survives long after Category numbers fade.
Copper carries electricity. It is cheap, easy to terminate, and it picks up noise. Fiber carries light down a glass or plastic core. Because there is no electricity in the cable, motors, elevators, and power lines have nothing to grab; the trade is cost, kink sensitivity, and harder troubleshooting. Radio carries nothing at all. It just goes. Convenient, shared, and unpredictable (which is exactly why the wired links win in a classroom with historically bad Wi-Fi).
Transmission media is the physical stuff a signal travels through. Last week’s word was link, the connection between two nodes. This week we open the link and look inside it. The plural is media; the singular is medium. Hold those three families on the board and everything else hangs off them: twisted pair and coax under copper, single-mode and multimode under fiber, Wi-Fi and cellular under radio. If students leave with only one idea from the media day, make it this one. Copper, fiber, or radio. Pick the medium that matches the job.
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