When a failure occurs in a PDH (low-order SONET/SDH) network and protection switching is unavailable, all the circuits that depend on the failed element lose connectivity. Since circuits are end-to-end entities that can span devices managed by multiple EMSs, they are not known about at the EMS level.
When the Optical Transport Manager diagnoses problems in a subnetwork connection that is part of a circuit, it creates a notification for the circuit.
“Impact correlation” on page 77 provides more information.
