In an unbounded, or wireless, medium, the signals are not confined to the medium. Wireless media propagate signals through the atmosphere, the ocean, and outer space. Wireless communication makes a mobile office easier to implement: airline passengers can keep in touch with their offices through the use of wireless phone, fax, and E-mail services and can even access online information services from the air- or from anywhere. They are microwave radio, communication satellites, cellular phones, and high-frequency radios.
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