A communication channel is a medium through which a signal can be transmitted. Electrical signals travel through two types of media: guided and unguided. A guided medium is one that is directed from transmitter to receiver via connecting cables. Examples include coaxial cables, telephone wires, twisted pairs, and other guided media. On the other hand, Unguided media refers to any communication channel that creates space between the transmitter and receiver. Air is the only medium between the transmitter and the receiver for radio communication. Everything from the vacuum of space to solid pieces of metal can be used as communication channels.