This Recommendation defines the APS protocol and protection switching operation for the linear protection schemes for the Optical Transport Network at the Optical Channel Data Unit (ODUk) level. Protection schemes considered in this Recommendation are: - ODUk subnetwork connection protection with inherent monitoring (1+1, 1:n); - ODUk subnetwork connection protection with non-intrusive monitoring (1+1); - - ODUk subnetwork connection protection with sublayer monitoring (1+1, 1:n). - ODUk compound link subnetwork connection group protection with inherent monitoring (1+1, 1:1). In addition client related protection architectures are described.