In large scale telecommunication networks such as telephone network trunks, microwave relay networks, cable television systems, and communication satellite links, large bandwidth capacity links are shared by many individual communication channels by using heterodyning to move the frequency of the individual signals up to different frequencies, which share the channel. This is called frequency division multiplexing (FDM).
For example, a coaxial cable used by a cable television system can carry 500 television channels at the same time because each one is given a different frequency, so they don't interfere with one another. At the cable source or headend, electronic upconverters convert each incoming television channel to a new, higher frequency. They do this by mixing the television signal frequency, fCH with a local oscillator at a much higher frequency fLO, creating a heterodyne at the sum fCH + fLO, which is added to the cable. At the consumer's home, the cable set top box has a downconverter that mixes the incoming signal at frequency fCH + fLO with the same local oscillator frequency fLO creating the difference heterodyne frequency, converting the television channel back to its original frequency: (fCH + fLO) − fLO = fCH. Each channel is moved to a different higher frequency. The original lower basic frequency of the signal is called the baseband, while the higher channel it is moved to is called the passband.