Recommendation ITU‑T G.872 describes the functional architecture of optical transport networks (OTNs) using the modelling methodology described in Recommendations ITU‑T G.800 and ITU‑T G.805. The 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 and supervision. The optical portion of the network is described in terms of media constructs, media elements and optical signal maintenance entities. The use of the black link approach defined in Recommendations ITU‑T G.698.1 and ITU‑T G.698.2 within the context of an OTN network is also described.