This Recommendation describes the functional architecture of optical transport networks using the modelling methodology described in ITU-T Rec. G.805. The optical transport network functionality is described fiom a network level viewpoint, taking into account an optical network layered structure, client characteristic information, clienthewer layer associations, networking topology, and layer network functionality providing optical signal transmission, multiplexing, routing, supervision, performance assessment, and network survivability.

This Recommendation is restricted to the functional description of optical transport networks that support digital signals. The support of analogue or mixed digital/analogue signals is outside of the current scope.

It is recognized that the design of optical networks is subject to limitations imposed by the accumulation of degradations introduced by the number of network elements and their network topology. However, many of these degradations and the magnitude of their effects are associated with particular technological implementations of the architecture described in this Recommendation and are therefore subject to change as technology progresses. As such the description of these effects is outside the scope of this Recommendation.