The optical network layers, comprising the access, aggregation, and core layers, represent a holistic framework for efficient and robust data transmission. Moving upward, the. Recommendation ITU-T G. 872 describes the functional architecture of the optical transport network (OTN) using the modelling methodology described in Recommendations ITU-T G. However, for effectiveness and efficiency, optical networks are described in terms of functionality that is related to payload transport, client payload multiplex-ing, routing, service survivability and protection supervision, and network maintenance. ODUk (Optical Data Unit): Provides path-level monitoring and multiplexing of OPU payloads.