The authors demonstrate bidirectional transmission of a 40Gbit/s (4 x 10Gbit/s) WDM signal over 100 km of dispersion shifted fibre at ITU=T standard wavelengths. The cross-talk penalties induced by Rayleigh back-scattering and four-wave mixing are suppressed by using an arrayed-waveguide grating demultiplexer and channel allocation, respectively.