Optical Transport Hierarchy

Up until the advent of the OTN initiative, the optical layer had been used simply as a physical medium over which to transmit data. This was primarily because of the impossibility to process optical data without first handling it over to the electrical domain. The OTH principle was seen by many as the first step in rectifying this shortcoming and in attempting to deliver some 'intelligence' to a passive system. Now, the OTH does not rewrite the laws of physics and it is still impossible to process data optically. However not to be deterred by such detail they opted for reserving a specific wavelength in order to use as a management or supervisor channel on which they could transport management information between equipment. Designers deemed this out-of-band supervisory channel a good solution as it did not interfere or unduly affect 

The payload of the underlying system was in contrast to SDH's overhead bytes. The OTH out-of-band supervisor channel would carry the same management information as SDH frames carried as overhead, but without using up precious cargo space in the frames. 

The OTH however would have to face come challenges as WDM differed quite considerable from the SDH framed protocol. For instance, there was no defined frame, DWM was light waves, without any frame structure or protocol and that was one of its strengths. Secondly, SDH handled one signal trail per path whereas WDM would be carrying several client signals per path and each would have to have its own individual management information with regards its own client’s state and fault. 

However, the challenge to delivering the project was easily outweighed by the potential benefits. If the principles of the OTH initiative could be applied to a working WDM out-of-band optical supervisory channel (OSC) then the potential was massive. Optical networks would be able to take on new topologies, such as partial and complete mesh networks using optical add/drop multiplexors, cross connects and optical switches in practical real world deployments. This would be due to the underlying management information being carried across the supervisory channel providing insight into the state of the network at every optical termination point, just like SDH. 

Furthermore, it would be possible to even implement protection and link fail-over as a network reliability measure, again due to having insight as to the link state between each optic termination link. However, the comparisons with SDH don't end there the OSC could also make provisioning of services possible, as it provides the ability to create, delete, and manage a service remotely, as can be done with SDH. 

In short, the OSC would do away with the WDM black-box approach to service management and provide all the SDH style visibility, manageability, flexibility and those precious OAM&P features for which everyone was clamoring.