We present randomized approximation algorithms for the circular arc graph colouring problem and for the problem of bandwidth allocation in all-optical ring networks. We obtain a factor-of-(1 + 1/e + o(1)) randomized approximation algorithm for the arc colouring problem, an improvement over the best previously known performance ratio of 5/3. For the problem of allocating bandwidth in an all-optical WDM (wavelength division multiplexing) ring network, we present a factor-of