This Recommendation describes the functional architecture of optical transport networks (OTNs) using the modelling methodology described in [ITU-T G.800], [ITU-T G.805] for the digital layer networks and [ITU-T G.807] for the optical media network. The OTN functionality is described from a network-level viewpoint. This takes into account the characteristic information (CI) of the clients of the OTN, the client/server layer associations, network topology, the optical media network structure, and the layer network functionalities that provide multiplexing, routing, supervision, performance assessment and network survivability for digital clients. The media portion of the network is described in terms of media constructs, media elements and optical signal maintenance entities described in [ITU-T G.807]. This Recommendation provides the functional description of OTN that support digital clients. The support of analogue signals as clients is outside the scope of this Recommendation.