This Recommendation covers the functional requirements of optical transport network functionality within equipment.

This Recommendation uses the specification methodology defined in [ITU-T G.806], in general for transport network equipment, and is based on the architecture of optical transport networks defined in [ITU-T G.872] and the interfaces for optical transport networks defined in [ITU-T G.709]. The description is generic and no particular physical partitioning of functions is implied. The input/output information flows associated with the functional blocks serve for defining the functions of the blocks and are considered to be conceptual, not physical.

The functionality defined in this Recommendation can be applied at user-to-network interfaces (UNIs) and network node interfaces (NNIs) of the optical transport network. It is recognized that for interfaces used within optical subnetworks, aspects of the interface are optical technology dependent and subject to change as technology progresses. Therefore, optical-technology dependent aspects (for transverse compatibility) are not defined for functional blocks used for these interfaces to allow for technology changes. The overhead processing functionality necessary for operations and management of optical subnetworks is defined.

Not every functional block defined in this Recommendation is required for every application. Different subsets of functional blocks from this Recommendation and others (e.g., [ITU-T G.783]) may be assembled in different ways according to the combination rules given in these Recommendations to provide a variety of different capabilities. Network operators and equipment suppliers may choose which functions must be implemented for each application.

The internal structure of the implementation of this functionality (equipment design) need not be identical to the structure of the functional model, as long as all the details of the externally observable behaviour comply with the equipment functional specification (EFS).

Equipment developed prior to the production of this Recommendation may not comply in all details with this Recommendation.

Equipment which is normally stated to be compliant with this Recommendation may not fulfil all the requirements in the case that it is interworking with old equipment that is not compliant with this Recommendation.

The functions for the processing of communication channels (COMMS) are not shown in these figures in order to reduce the complexity of the figures. For the COMMS functions, refer to the specific layer network descriptions.