RED ALARM indicates corruption, loss of signal or inability to frame reliably. Essentially this means that the local equipment cannot properly see the far end. In this case the local system will transmit a yellow alarm while in a red alarm state.
YELLOW ALARM indicates that the system has been instructed by the far end that there is an issue with communication. If the issue were local the alarm state would be RED but because a yellow alarm must be received. Also it is not possible to have both a red and yellow alarm state on the same piece of equipment in that you cannot receive the instruction to enter yellow state if you are in a red state.
BLUE ALARM indicates that there is no communication on the path so the equipment will transmit all 1s in the framing time slots to maintain the signal. When one end is in blue alarm the other end will be in a red alarm and as a result the end in a red alarm will transmit a request for the other end to enter a yellow alarm.
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