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SONET/SDH Automatic Protection Switching (APS)
The simplest class of mechanism for network survivability in the event of failure on a network element or link is automatic protection switching (APS). APS schemes involve reserving a protection channel (dedicated or shared) with the same capacity as the channel or facility to be protected. This white paper provides an overview of the different types of protection switching available with SONET/SDH networks, focusing on technologies that automatically work at the level of either the SONET (SDH) Line (Multiplex Section) or Path.
While SONET/SDH protection began with the relatively simple point-to-point line protection scheme, it has proved to be very extensible in terms of topologies (e.g., rings) and flexibility (e.g., Link Capacity Adjustment Scheme (LCAS) service restoration). The 1+1, 1:1, Unidirectional Path Switched Ring (UPSR), and Bi-Directional Line Switched Ring (BLSR) technologies each have an important role in the network. LCAS-based restoration shows promise for increasing the cost effectiveness of many packet-based services. As readers of this white paper become familiar with the processing requirements for APS and LCAS service restoration, they will appreciate the enhanced functionality in the PMC-Sierra CHESS III, Arrow, and ADM devices which greatly ease APS implementations.

About PMC
PMC-Sierra™ is a leading provider of high speed broadband communications and storage semiconductors and MIPS-Powered™ processors for Enterprise, Access, Metro Optical Transport, Storage Area Networking and Wireless network equipment. The company offers worldwide technical and sales support, including a network of offices throughout North America, Europe and Asia. The company is publicly traded on the NASDAQ Stock Market under the PMCS symbol and is included in the S&P 500 Index.

About the Author
Steve Gorshe, Ph.D. is a Principal Engineer in the Product Research Group and oversees ICs for SONET, optical transmission and access systems.
Currently Steve is a senior member of the IEEE and co-editor for the IEEE Communications magazine's Broadband Access Series. He is the chief editor for the ANSI T1X1 Subcommittee, which is responsible for SONET and optical network interface standards. He is a recent recipient of the Committee T1 Alvin Lai Outstanding Achievement Award for his standards work and has been a technical editor for T1.105, T1.105.01, T1.105.02, and T1.105.07 within the SONET standard series as well as the ITU-T G.7041 (GFP) G.7043 (Virtual concatenation of PDH signals), G.8040 (GFP mapping into PDH signals), and G.8011.1 (Ethernet Private Line Service) recommendations. He has 26 patents issued or pending and several published papers.

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