The bits of the TS0 that does not contain the FAS in positions 3 to 8
make up what is known as the Non-Frame Alignment Signal or NFAS. This signal is sent in
alternate frames (Frame 1, Frame 3, Frame 5, etc.). The first bit of the NFAS (bit
nº 3 of the TS0) is used to indicate that an alarm has occured at the far end of the
communication. When operating normally it is set to "0", while a value of
"1" indicates an alarm.
The bits in positions four to eight are spare bits, that is, they do not have one single application, but can be used in a number of different ways as decided by the Telecommunications Carrier. In accordance with the ITU-T recommendation G.704, these bits can be used in specific point-to-point applications, or to establish a data link based on messages for operations management, maintenance or monitoring of the transmission quality, etc. If these spare bits in the NFAS are not used, they must be set to "1" in international links.
