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We consider multiwavelength wavelength-routing networks operating in circuit-switched mode. Wavelength utilization is poor in such networks if sessions require only a fraction of a wavelength's capacity. An all-optical approach to improve wavelength utilization is to use time division multiplexing (TDM) on each wavelength, and switch time slots and wavelengths. In this paper, we address the off-line multirate session scheduling problem, i.e., the problem of assigning time slots and wavelengths to a given static set of multirate sessions, in ring topologies. Given a set of sessions and their relative rates, our objective is to maximize network throughput. This objective translates to the problem of minimizing the maximum length of a TDM frame over all wavelengths. We first show that the off-line single-rate session scheduling problem is equivalent to the off-line wavelength assignment problem, and hence obtain bounds on frame length. We then present scheduling algorithms with provable worst-case bounds on frame length for multirate session scheduling.