scope:

This European Telecommunication Standard (ETS) specifies the Automatic Protection Switching (APS) requirements, switching initiation criteria, and the APS protocols of Synchronous Digital Hierarchy (SDH) multiplex section shared protection ring, multiplex section dedicated protection ring, multiplex section linear protection, and path trail and Sub-Network Connection (SNC) protection schemes. The APS protocols are specified in terms of their multiplex section or path overhead requirement, the signalling messages and their operations under various failure conditions.