The ITU's Optical Transport Network (OTN), as defined by recommendation G.709, provides a network-wide framework that adds SONET/SDH-like features to WDM equipment (also known as Wavelength Switched Optical Network equipment, or WSON equipment). It creates a transparent, hierarchical network designed for use on both WDM/WSON devices and TDM devices. Two switching layers are formed (TDM and WSON) and functions of transport, multiplexing, routing, management, supervision, and survivability are defined.
Carriers are continually looking for ways to reduce CAPEX/OPEX in their networks while simultaneously addressing dramatic shifts in traffic type. The promise of the Optical Transport Network (OTN) is designed to address these issues by delivering a transparent framework to efficiently carry diverse traffic types. Defined by recommendation G.709, the OTN creates a transparent, hierarchical network designed for use on both WDM/WSON and TDM devices. Two switching layers are formed (TDM and WSON) and functions of transport, multiplexing, routing, management, supervision, and survivability are defined.
The OTN control plane, which is based on GMPLS, automates many of these functions, with corresponding operational benefits relative to a statically provisioned OTN network.
How does G.709 OTN work?
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.
The G.709 OTN Control Plane
The G.709 OTN control plane extends GMPLS signaling to automate many of its functions, with corresponding operational benefits relative to a statically provisioned OTN network.
- The control plane automatically calculates an optimal path for each client connection when the connection is established.
- It can also automatically calculate a path for a diversely routed protecting connection, on which resources can either be held in reserve or pre-empted when required.
- When resources are added to or removed from the network, the control plane can automatically recalculate optimal routes for all existing connections to make best use of the newly available resources.