Signal transmission in an optical system

A method of transmission by means of a transmission apparatus capable of transmitting a signal on a plurality of channels in a transmission band through optical transmission cable which exhibits a Raman effect of transferring power from shorter wavelength signals at one end of the transmission band towards longer wavelength signals at another end of the transmission band, comprising the steps of:

• changing a number of the channels during transmission of the signal,
• when half or fewer channels of the plurality of channels available for use are to be used for transmitting the signal, transmitting the signal in nonadjacent channels spaced apart from each other at a wider frequency spacing and at a higher power level,
• when a number of channels to be used to transmit the signal is increased to more than half of the plurality of channels available for use such that a set of adjacent channels are used, transmitting the signal in the set of adjacent channels spaced apart from each other at a normal frequency spacing and at a normal power level,
• wherein the higher power level is higher than the normal power level, and wherein the wider frequency spacing is greater than the normal frequency spacing,
• decreasing the frequency spacing between the channels, and simultaneously decreasing the power level of the signal, as the number of the channels increases during transmission of the signal, and
• amplifying the signal non-uniformly after travel through the transmission cable, by amplifying the signal at the longer-wavelength other end of the transmission band less than the signal at the shorter-wavelength one end of the transmission band, to compensate for the Raman effect.