An optical transmission network comprises nodes which support a plurality of different wavelength channels and support at least a first bitrate traffic type and the second bitrate traffic type on respective wavelength channels. A connection of the second bitrate traffic type is established on an available wavelength, if the wavelength offers an acceptable quality of transmission using a first quality of transmission calculation. Alternatively, a connection of the second bitrate traffic type is established on a wavelength which is spaced, by a guard band, from wavelengths used for connections of the first bitrate traffic type, if the wavelength offers an acceptable quality of transmission using a second quality of transmission calculation. The second quality of transmission calculation is less stringent than the first quality of transmission calculation, and can ignore the effects of interference due to cross-phase modulation. The guard band is a wavelength spacing at which the interference between a connection of the first bitrate traffic type and a connection of the second bitrate traffic type is less than a predetermined amount.