This Recommendation provides the necessary equipment-level specifications to implement different choices of protection architectures for Synchronous Digital Hierarchy (SDH) networks. Protected entities may range from a single SDH multiplex section (e.g. linear multiplex section protection), to a portion of an SDH end-to-end path (e.g. subnetwork connection protection), or to an entire SDH end-to-end path (e.g. HO/LO linear VC trail protection). Physical implementations of these protection architectures may include rings or linear chains of nodes. Each protection classification includes guidelines on network objectives, architecture, application functionality, switching criteria, protocols, and algorithms.