Recommendation ITU-T G.872 describes the functional architecture of optical transport networks using the modelling methodology described in [ITU-T G.800] and [ITU-T G.805]. The optical transport network (OTN) functionality is described from a network level viewpoint, taking into account an optical network layered structure, client characteristic information, client/server layer associations, networking topology, and layer network functionality providing optical signal transmission, multiplexing, routing, supervision, performance assessment, and network survivability. The optical portion of the network is described in terms of spectrum management entities and maintenance entities.