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.800, ITU T G.805 and ITU T G.807. The OTN functionality is described from a network level viewpoint, taking into account, client characteristic information, client/server layer associations, networking topology, layer network functionality and optical media network structure, that provide multiplexing, routing and supervision of digital clients. The media portion of the network is described in terms of media constructs, media elements and optical signal maintenance entities.