Optical Network Design and Implementation

SDH Protection Architectures

This section discusses various SDH protection architectures and mechanisms. This includes APS, linear, and ring protection architectures. SDH rings use path-switching or line-switching, self-healing techniques. SDH defines a maximum switch time of 50 ms for an MS-SPRing ring with no extra traffic and less than 1200 km of fiber. The specification actually states 60 ms, with 10 ms for discovery of the problem and 50 ms to perform the switch, but this is commonly referred to as just 50 ms. However, most SDH networks are much smaller and simpler than this example and switch even faster. Y-cable switchover times are much higher and are typically in the order of 250 ms. SDH protection switching is invoked if there is an ...