The transmission network enables carrying all kinds of traffic (voice, video, data). It consists
of nodes called multiplexers and links among multiplexers. The goal of the multiplexer is to
multiplex/demultiplex traffic onto/from the link. There exists three multiplexing technologies :
PDH, SDH and D-WDM. The link technology is generally optic fiber but may also be coax,
radio, etc. A transmission network generally consists of hundreds of multiplexers and tens of
thousands of kilometers of optic fiber.