The APS implementation also provides the revertive and non-revertive modes with non-revertive switching as the default option. In revertive switching, the activity is switched back to the working port after the working line has recovered from a failure (or the manual switch is cleared). In non-revertive switching, a switch to the protection line is maintained even after the working line has recovered from a failure (or if the manual switch is cleared).
A revert-time is defined for revertive switching so frequent automatic switches as a result of intermittent failures are prevented. A change in this value takes effect upon the next initiation of the wait to restore (WTR) timer. It does not modify the length of a WTR timer that has already been started. The WTR timer of a non-revertive switch can be assumed to be infinite.
In case of failure on both working and the protection line, the line that has less severe errors on the line is active at any point in time. If there is signal degrade on both ports, the active port that failed last stays active. When there is signal failure on both ports, the working port is always active. The reason is that the signal failure on the protection line is of a higher priority than on the working line.