One of the key benefits of single-channel SONET/SDH systems is their extensive performance monitoring and error detection capabilities, provided by the SONET/SDH Section, Line, and Path overhead bytes. As networks grew from single-channel SONET systems to multi-wavelength DWDM networks, there was a need to provide a similar frame structure; performance monitoring, error correction, and management function at the optical (i.e. wavelength) layer so that carriers could manage their DWDM networks. The ITU standard G.709 “Interface for the Optical Transport Network” provides this industry-wide frame structure and overhead definition for the photonic layer; G.709 is sometimes referred to as the “digital wrapper.”
The G.709 OTN standards define:
1. An optical transport hierarchy among optical network elements
2. A frame structure for mapping client signals
3. A definition of the overhead bytes for optical layer performance management and FEC
In a similar manner to SONET/SDH definitions, the optical network hierarchy is divided into Optical Path, Optical Transport, and Optical Multiplex layers, as shown below.
A client signal is mapped into an OPU payload along with OPU overhead bytes, and this entire package gets mapped into an OTU frame along with additional ODU/OTU overhead information and FEC data.