The method used to transmit the alarm makes use of the fact that in telephone systems
transmission is always two-way. Multiplexing/demultiplexing devices (known generically as
multiplex devices) are installed at both ends of the communication for the transmission and
reception of frames. When a device detects either of the following in its multiplexer or
demultiplexer:
- a power failure
- a failure of the coder/decoder
or any of the following in its demultiplexer:
- loss of the 2 Mbit/s signal received
- loss of frame alignment (synchronization)
- bit error rate (BER) greater than or equal to 10-3
an alarm must be sent to the transmitter.
This Remote Alarm Indication (RAI) is sent in the
NFAS of the return frames, with bit 3 being set to "1". The transmitter then considers
how serious the alarm is and goes on to generate a series of operations depending on the type
of alarm condition detected.
