Communication Media (Circuits/Channels)
Medium is the matter or substance that carries the voice or data transmission
copper, glass, plastic (fiber optic cable), or air (microwave or satellite)
Guided media are those media in which message flows through a physical media (e.g., twisted pair wire, coaxial cable)
Radiated media are those in which the message is broadcast through the air (e.g., infrared, microwave)
Communication services are circuits sold by common carriers