OTN defines a precise layered structure for transporting and managing data: Optical Payload Unit (OPU): Holds the client signal and ensures transparent mapping. An optical transport network (OTN) is a digital wrapper that encapsulates frames of data, to allow multiple data sources to be sent on the same channel. 709) that has become the foundation for modern optical communications. It encapsulates diverse client signals —. Optical Transport Network (OTN) The following table lists all of the known ITU-T Recommendations specifically related to the OTN Transport Plane. COMPARISON OF SDH AND OTN 1 Abbreviations 2 What is OTN/OTH 2.