This is a book on wireless communication. That usually means communica-
tion by radio, though ultrasound and infrared light are also used occasion-
ally. The term “wireless” has come to mean nonbroadcast communication,
usually between individuals who very often use portable or mobile equip-
ment. The term is rather vague, of course, and there are certainly borderline
applications that are called wireless without falling exactly into the above
definition.
