Editing Synchronous optical networking (section)

==Network architectures==
SONET and SDH have a limited number of architectures defined. These architectures allow for efficient bandwidth usage as well as protection, i.e. the ability to transmit traffic even when part of the network has failed, and are fundamental to the worldwide deployment of SONET and SDH for moving digital traffic. Every SDH/SONET connection on the optical physical layer uses two optical fibers, regardless of the transmission speed.

===Linear Automatic Protection Switching===
Linear Automatic Protection Switching (APS), also known as ''1+1'', involves four fibers: two working fibers (one in each direction), and two protection fibers. Switching is based on the line state, and may be unidirectional (with each direction switching independently), or bidirectional (where the network elements at each end negotiate so that both directions are generally carried on the same pair of fibers).

===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.

===Bidirectional line-switched ring===
Bidirectional line-switched ring (BLSR) comes in two varieties: two-fiber BLSR and four-fiber BLSR. BLSRs switch at the line layer. Unlike UPSR, BLSR does not send redundant copies from ingress to egress. Rather, the ring nodes adjacent to the failure reroute the traffic "the long way" around the ring on the protection fibers. BLSRs trade cost and complexity for bandwidth efficiency, as well as the ability to support "extra traffic" that can be pre-empted when a protection switching event occurs. In four-fiber ring, either single node failures, or multiple line failures can be supported, since a failure or maintenance action on one line causes the protection fiber connecting two nodes to be used rather than looping it around the ring.

BLSRs can operate within a metropolitan region or, often, will move traffic between municipalities. Because a BLSR does not send redundant copies from ingress to egress, the total bandwidth that a BLSR can support is not limited to the line rate ''N'' of the OC-''N'' ring, and can actually be larger than ''N'' depending upon the traffic pattern on the ring.<ref name="BLSR"/> 

In the best case, all traffic is between adjacent nodes. The worst case is when all traffic on the ring egresses from a single node, i.e., the BLSR is serving as a collector ring. In this case, the bandwidth that the ring can support is equal to the line rate ''N'' of the OC-''N'' ring. This is why BLSRs are seldom, if ever, deployed in collector rings, but often deployed in inter-office rings. The SDH equivalent of BLSR is called ''Multiplex Section-Shared Protection Ring'' (MS-SPRING).