When a failure occurs in a 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 any element that is underlying a circuit, it creates a notification for the circuit.
Impact correlation on page 74 provides more information.
