RPR and WDM

Hi All,
 
I am new to RPR world and probably have not good understanding of it
yet.
I was thinking how the network will work when there are WDM capabilities
working under RPR network. How does the RPR mechanism treat the WDM
case? Each wavelength is a resilient packet ring, and, therefore, each
node has several blocks of in-transit queues and trasmitting queues (one
block for each wavelength) or, according to some planning strategy,
several disjointed rings are virtual created where some nodes by-pass
some traffic, or ?? Is there any document available which talks about
this.

Thank you in advance.

Best regards,
Davide