A multiplex is a piece of hardware or a function in software that is used to combine diferent input signals into a single signal. These input signals can be e.g. the telephone calls for different users that are combined at the base station to send them to the users or e.g. the audio- and videosignal that are combined into one single movie stream.
The different signals can be seperated in three different manners:
In analogue broadcasting each radio- and television channel has it's own transmitter. This relationship between a radio channel (such as BBC1) and it's own transmitter is abandoned in digital broadcasting. In digital video broadcasting (DVB) and digital audio broadcasting (DAB are different television- and/or radiochannels combined into a single transport stream. The transport stream also contains Service Information which gives details of the programmes being broadcast.
This is illustrated in the following figures.