This Recommendation specifies multichannel optical line system interfaces for the purpose of providing future transverse compatibility among such systems. This Recommendation defines interface parameters for systems of four, eight and sixteen channels operating at bit rates of up to STM-16 on fibres, as described in Recommendations G.652, G.653 and G.655 with nominal span lengths of 80 km, 120 km and 160 km and target distances between regenerators of up to 640 km. A frequency grid anchored at 193.1 THz with interchannel spacings at integer multiples of 50 GHz and 100 GHz is specified as the basis for selecting channel central frequencies.