This is the
component of
signal that is
modulated. It is used to
separate signals so that more than one
message can be
transmitted in a
medium.
For example: The
air (our medium) has many signals in it at all times. They all
travel on different
carrier frequencies like 900 MHz for your
cordless phone or any
frequency that you listen to on your
FM radio in an
automobile. Each station has its own carrier frequency. Another medium could be copper wires, this is how DSL works.
Voice comes in at the base frequency and the
DSL comes over the wire with a higher carrier frequency.