Multiplexing covers a wide range of engineering techniques for combining multiple low bandwidth information channels into a high-bandwidth channel more suitable for actual electronic or optical transmissions. Demultiplexing takes place at the receiving end, where the low-bandwidth channels are broken out for use.
Variants of multiplexing include inverse multiplexing, in which a high-bandwidth information channel is split into subchannels that can be sent over slower transmission channels, Another variant is statistical multiplexing, in which the total bandwidth of the information channels is greater than the capacity of the transmission channel, but it can be safely assumed that all the information channels will not be active at the same time. Packet switching and routing are techniques of statistical multiplexing; the Internet is perhaps the greatest example of statistical multiplexing.
Multiplexing analog information
Continuously varying analog signals were the first multiplexed, as an extension of the idea of modulation. Ironically, one of the first applications was the sending of effectively digital information as analog tones: the "harmonic telegraph" invented by Alexander Graham Bell before the invention of the telephone. Manual telegraphy, because it used short and long pulses (dots and dashes), in modern terms is not strictly digital or analog, but an example of pulse duration modulation.
Multiplexing digital information
The most basic, non-statistical forms of digital multiplexing are bit- and byte-interleaving. In the first model, successive bits of the high-speed multiplexed trunk are assigned to different information channels. Byte interleaving assigns successive characters or other groups of bits to different information channels.