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.
