By using optical fibre, very high data rates (gigabits per second and higher) can be transmitted over long distances (tens of kilometres) without amplifiers or regenerators. Optical fibre has completely superseded copper wires as the primary medium for cabled transmission over long distances. As demand for capacity has risen, fibre is now being used even in LANs and the access network. An understanding of optical-fibre communications is therefore very important for anyone with an interest in digital communications.