(1) Protection granularity: a. At wavelength granularity. b. At sub-wavelength granularity
(2) Protection entity: a. Path protection. b. Sub-path protection. c. Segment protection.
(3) Routing: a. Single-path routing. b. Multi-path routing.
Tradeoffs between different objectives, e.g., resource efficiency vs. recovery time, are explored and practical approaches are proposed and analyzed.