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An optical transmission system is a method of transferring information in the shape of bits or symbols for the case of digital systems, and of analogue waves for the case of analogue systems, between fixed points located on a fiber optics cable. Today and in th near future there are numerous such transmission techniques available.The increase of demands for data transfer from phone subscribers can be met only by means of digital techniques applied in the local network, in addition to the use of digital telephone exchange and of the digital transmission systems in the trees network.In order to increase the quantity of information transferred through one fiber, optical multiplexing techniques have been conceived and tested. The optical multiplexing is additional to the electrical signal multiplexing.The requests for the access network will become more and more complex. a larger flexibility and a wider band being needed. For the purpose of complying with these requests, the coherent simultaneous or alternative transmission towards the optical amplifiers represents a factor oftechnical progress.The multiplexing with wave length division allows for more channels to be transported through the same fiber with different wave lengths, in one or both directions.