So far I know, the OTN networks based on wavelength division is asynchronous and thats why no clock synchronization is required between nodes.

Can anybody explain in more better way?

My Friend,

The reason why clock synchronization is not needed in OTN networks it?s because since network synchronization can be transported within the payload, mainly by SDH/SONET client tributaries. In order to meet these requirements the OTN frame has been designed so that the client mapping/demapping and the transfer through OTN network equipments and 3R regenerators, do not prevent SDH tributaries to meet theG.825 jitter and wander requirements.

You can find more details in the attached document.

Best Regards!

                        Felipe.