Hi Ramiro. I’m posting this link that you might find useful.
http://www.comsoc.org/pubs/surveys/4q98issue/kawamura.html
Designing the FO rings to make the network fail-proof or ‘non-stop’ has an important penalty in overhead, that many managers can’t justify until everything crashes. What I can suggest is to design the FO net with the needed capacity for 4-5 years, making it WDM compatible and no redundancy. The you could develop several ‘Contingency Plans’ that contain the security of parts of the network, sorted from most to least important protected links with the associated budget. Is important that you don’t allow to exist a SPF or Single Point of Failure. You could plan the use of a DXC/DACS for automatic restoration or a manual procedure, with a list of pros and cons. As everything besides the basic network is ‘unplanned cost’ and is hard to sell, I’d suggest that you rank the solutions regarding the network availability in terms of protected traffic. No single solution exists, given the economy of network planning, even with fully redundant paths. Still it can fail. As the approach to 100% availability skyrocket the costs, start protecting the first most important arc, calculate the costs and protential availability and recursively go on. Hope it can help. rarcadi@hotmail.com