|  | 2.1 | General Model of Communication
| 2.1.3 | Media

During your study of networking, you will hear references to the word "medium". (Note: The plural form of medium is media.) In networking, a medium is a material through which data packets travel. It could be any of the following materials:

- telephone wires
- Category 5 UTP (used for 10BASE-T Ethernet)
- coaxial cables (used for cable TV)
- optical fibers (thin glass fibers that carry light)

There are two more types of media that are less obvious, but should nonetheless be taken into account in network communications. First, is the atmosphere (mostly oxygen, nitrogen, and water) that carries radio waves, microwaves, and light.

Communication without some type of wires or cables is called wireless or free-space communication. This is possible using electromagnetic (EM) waves. EM waves, which in a vacuum all travel at the speed of light, include power waves, radio waves, microwaves, infrared light, visible light, ultraviolet light, x-rays, and gamma rays. EM waves travel through the atmosphere (mostly oxygen, nitrogen, and water), but they also travel through the vacuum of outer space (where there is virtually no matter, no molecules, no atoms).

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