The A bit of the FAS in 8, 34 and 140 Mbit/s frames allows the multiplexers that correspond to these hierarchies to transmit alarms to the far ends of the communication when a multiplexer detects the following in its demultiplexer:
- loss of signal received
- loss of frame alignment (synchronization).
In addition, 140 Mbit/s multiplexers also transmit an alarm indication when faced with the loss of frame alignment of the 34 Mbit/s signals received inside the 140 Mbit/s signals, but in the NFAS of the 34 Mbit/s signal that has lost its alignment (bit 11 of group I changes from "0" to "1") in the return channel.
