G709
The ITU standard G.709 is called "Network Node Interface for the Optical Transport Network" and forms the framework structure of the Optical Transport Module( OTM) for an asynchronous transport medium. In G.709, the interface as well as the transport service for the user data are defined via the standardized Synchronous Digital Hierarchy( SDH) and Optical Transport Hierarchy( OTH).
The International Telecommunication Union (ITU) has defined interfaces for the optical transport network( OTN). These include the Network Node Interface( NNI), the interface to the Optical Transport Network (OTN), which is part of Recommendation G.709.
The three layers of the optical transport network are mapped in this interface: The physical one representing the sections between two optical system components, the multiplex layer containing the transport and payload data such as the Optical Payload Unit( OPU), Optical Data Unit( ODU) and Optical Transport Unit( OTU), and the top layer containing the various payload data for Ethernet, ATM, Synchronous Digital Hierarchy (SDH), etc. Issue 3 of G.709, released in 2009, publishes additional OTN aspects. These include the client signals of the Optical Data Units, ODU-0, and the Timing Transparent Transcoding( TTT). Furthermore, the Generic Mapping Procedure (GMP), a multi- level multiplexing and some other aspects.
In G.709, the transmission rates for the Optical Transport Units (OTU) are defined, as well as the G.709 header, which consists of the three data fields for the overhead, the payload and the forward error correction( FEC). Furthermore, additional signaling performance characteristics for optical channels are specified in G.709.