A method (10, 20, 40, 50) of transmitting communications traffic, the method comprising steps: a. receiving a first sequence of communications traffic bits for transmission during a first symbol time and receiving a second sequence of communications traffic bits for transmission during a second symbol time, after the first symbol time; and b. mapping each sequence of communications traffic bits onto a respective one of a plurality of transmission symbols for transmission during a symbol time (14, 26, 44, 54), wherein each transmission symbol is identified by a respective first symbol identifier indicative of a respective one or more of a plurality, M, of wavelengths for a transmission signal and a respective second symbol identifier indicative of a respective one or more of a plurality, N, of optical fibres on which to transmit the transmission signal, wherein the plurality of transmission symbols consists of M.N transmission symbols, said mapping comprising mapping the first sequence of communications traffic bits onto one of the plurality of transmission symbols and determining one of the plurality of transmission symbols that the second sequence of communications traffic bits is to be mapped onto thereby determining the one of the plurality of transmission symbols that the second sequence of communications traffic bits is to be mapped onto in advance of the second symbol time; and c. during the first symbol time, transmitting the transmission symbol for the first sequence of communications traffic bits and warming-up a laser for generation of a transmission signal at the respective wavelength of the transmission symbol for the second sequence of communications traffic bits. a. receiving a first sequence of communications traffic bits for transmission during a first symbol time and receiving a second sequence of communications traffic bits for transmission during a second symbol time, after the first symbol time; and b. mapping each sequence of communications traffic bits onto a respective one of a plurality of transmission symbols for transmission during a symbol time (14, 26, 44, 54), wherein each transmission symbol is identified by a respective first symbol identifier indicative of a respective one or more of a plurality, M, of wavelengths for a transmission signal and a respective second symbol identifier indicative of a respective one or more of a plurality, N, of optical fibres on which to transmit the transmission signal, wherein the plurality of transmission symbols consists of M.N transmission symbols, said mapping comprising mapping the first sequence of communications traffic bits onto one of the plurality of transmission symbols and determining one of the plurality of transmission symbols that the second sequence of communications traffic bits is to be mapped onto thereby determining the one of the plurality of transmission symbols that the second sequence of communications traffic bits is to be mapped onto in advance of the second symbol time; and c. during the first symbol time, transmitting the transmission symbol for the first sequence of communications traffic bits and warming-up a laser for generation of a transmission signal at the respective wavelength of the transmission symbol for the second sequence of communications traffic bits.