Non-Facility Associated Signalling
NFAS Describes a Primary Rate ISDN configuration where multiple T1 carriers share a signaling channel (or D Channel). Usually referred to as NFAS.
What is NFAS?
In the North American telecommunications system, a T1 circuit typically carries 24 individual timeslots. Each timeslot in turn carries a single telephone call. When a T1 circuit is used to carry Primary Rate ISDN one of the timeslots is used to carry the D channel. In an NFAS configuration, multiple T1 circuits share a single D channel. There is an upper limit of 20 T1 circuits in a single NFAS configuration. A single Primary Rate ISDN circuit is sometimes described as 23B + D. There are 23 bearer channels carrying voice or data, and one D channel carrying the signaling. A full NFAS configuration can then be described as 479B + D. There is one problem, now a failure on the T1 trunk carrying the D channel will also affect all 19 other trunks. The solution is D channel backup where a second D channel is configured on another trunk. In the event of failure the backup D channel takes over the signalling. So the final configuration is 478B + D + D-backup.
Why NFAS?
Cost. When you as a customer order a Primary Rate ISDN service, you will be charged for each signaling channel. An NFAS configuration reduces the cost to you. Why is it more expensive? For historical reasons, North American switches such as the Lucent 5ESS, and the Nortel DMS-100 did not handle common channel signalling such as ISDN on the same line card that terminated the T1 circuit. So, the telephone company needs to buy a separate signalling card for every D channel.
Outside North America
Note that the situation in Europe and the rest of the world, is rather different. A Primary Rate ISDN configurations uses E1 carriers, where each carrier has 32 timeslots. 30 of the timeslots are used to carry calls, one timeslot is used for synchronization, and one timeslot is used to carry the signalling channel. The line cards in switches designed for the E1 system already include processing for the signalling timeslot,. As a result Non-Facility Associated Signalling is very uncommon in Europe.
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