Failures are the highest-priority events because they indicate drastic problems. Most failures are simply extended presences of the associated defect. The only exception is that so-called path failures (frame loss and alarm indication), which are associated with the logical data path between endpoints, have mutually exclusive signaling and only one can be declared at any point. Here is a list of failures, the conditions of which are detailed in Table C-2:
Signal loss is the most basic failure. Signal loss initially triggers the LOS defect. If the defect persists, failure is declared. LOS failure is cleared when a signal with an appropriate ones density is received at the network interface. The time to clear the failure depends on the equipment. Generally, it is only a few seconds, though it is permitted to be as many as 20.
Signal loss corresponds to the most basic transmission on a T1. Performing the T1 framing appropriately is one step up from moving electrons from one end to the other. If either aspect breaks down, the line will fail.
LOF failure is declared when the out of frame (OOF) condition persists. (OOF is described later in this appendix.) Like the signal loss alarm, it may take up to 20 seconds to clear the LOF failure.
LOF is not declared when the alarm indication signal (AIS) is present. AIS is a higher-priority alarm, and the signaling bits must be used for only one ...