At a basic level, G.709 OTN defines a frame format that “wraps” data packets, in a format quite similar to that of a SONET frame. There are six distinct layers to this format.
- OPU: Optical Channel Payload Unit. This contains the encapsulated client data, and a header describing the type of that data. It is analogous to the ‘Path’ layer in SONET/SDH.
- ODU: Optical Data Unit. This level adds optical path-level monitoring, alarm indication signals and automatic protection switching. It performs similar functions to the ‘Line Overhead’ in SONET/SDH.
- OTU: Optical Transport Unit. This represents a physical optical port (such as OTU2, 10Gbps), and adds performance monitoring (for the optical layer) and the FEC (Forward Error Correction). It is similar to the ‘Section Overhead’ in SONET/SDH.
- OCh: Optical Channel. This represents an end-to-end optical path.
- OMS: Optical Multiplex Section. This deals with fixed wavelength DWDM (Dense Wavelength Division Multiplexing) between OADMs (Optical Add Drop Multiplexer).
- OTS: Optical Transport Section. This deals with fixed wavelength DWDM between relays.