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 endto-end path (e.g. HO/LO linear VC trail protection). Physical implementations of these protection architectures may include rings or linear chains of nodes.