Editing Synchronous optical networking (section)

===Unidirectional path-switched ring===
In unidirectional path-switched rings (UPSRs), two redundant (path-level) copies of protected traffic are sent in either direction around a ring. A selector at the egress node determines which copy has the highest quality, and uses that copy, thus coping if one copy deteriorates due to a broken fiber or other failure. 

UPSRs tend to sit nearer to the edge of a network, and as such are sometimes called ''collector rings''. Because the same data is sent around the ring in both directions, the total capacity of a UPSR is equal to the line rate ''N'' of the OC-''N'' ring.<ref name="UPSR" /> For example, in an OC-3 ring with 3 STS-1s used to transport 3 DS-3s from ingress node ''A'' to the egress node ''D'', 100 percent of the ring bandwidth (''N''=3) would be consumed by nodes ''A'' and ''D''. Any other nodes on the ring could only act as pass-through nodes. The SDH equivalent of UPSR is ''[[subnetwork connection protection]]'' (SNCP); SNCP does not impose a ring topology, but may also be used in mesh topologies.