The system interchanges data from time slots in its input to time slots in its output. Carried by single-mode optical fibers and switched by lithium-niobate directional couplers, the data remains in photonic form from input to output. The data format is a series of frames, each with a start-bit and time slots with bits each. An electronic controller reads the arbitrary one-to-one assignment of input time slots to output time slots (time-slot permutation) and orchestrates photonic directional couplers to steer the data among input, output, and recirculating fiber-optic delay lines. While a prototype with , and a pedestrian 90-MHz bit rate is a 3 × 3 switch, where each channel carries 28.8 Mbit/s, the basic design is not limited to these numbers.