There is not a simple definition of the spectrum. From the technical viewpoint the spectrum is simply the range of electromagnetic waves that can be used to transmit information, but from the practical viewpoint the economic and political aspects, as well as the technology actually used to convey the information by means of these waves, play pivotal roles. As an example, when Marconi in 1902 first spanned the Atlantic with his “wireless telegraph message”, he used the whole spectrum available at the time to send a few bits/s over an area of thousands of square kilometres. With the spark transmitter used for this achievement that occupied all the frequencies that the existing receivers were able to understand, nobody else could use radio for communications on a radius of some 3500 km from the transmitting station in England. So, if other users wanted to send messages in the same area, they would need to coordinate their transmissions in different “time slots” in order to share the medium. This technique is called “TDMA”, Time Division Multiple Access.