Editing Non-Facility Associated Signalling

{{Short description|Primary rate interface configuration whereby multiple T1 carriers share a signaling channel}}
{{Unreferenced|date=September 2015}}
'''Non-Facility Associated Signaling''' or '''NFAS''' is a [[Primary Rate Interface]] configuration whereby multiple [[T-carrier|T1]] carriers share a [[signaling channel]] (or [[D channel]]).

A T1 circuit typically carries 24 individual [[timeslot]]s. 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.  A single Primary Rate [[ISDN]] circuit is thus sometimes described as 23B + D.  There are 23 [[bearer channel]]s carrying voice or data, and one D channel carrying the [[Common Channel Signaling]].

In an NFAS configuration, multiple T1 circuits share a single D channel, with an upper limit of 20 T1 circuits in a single NFAS configuration. A full NFAS configuration can then be described as 479B + D. There is one problem; 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 signaling.  So the final configuration is 478B + D + D-backup.

NFAS is a cost-cutting measure.  Customers ordering a Primary Rate [[ISDN]] service will be charged for each signaling channel.  Therefore an NFAS configuration can be cheaper than Facility Associated Signaling, due to historical reasons.  North American switches such as the Lucent [[5ESS switch|5ESS]], and the Nortel [[Digital Multiplex System|DMS-100]] did not handle common channel signaling such as [[ISDN]] on the same [[line card]] that terminated the T1 circuit.  So, the telephone company needs to buy and maintain a separate signaling card for every D channel.

The situation in Europe and the rest of the world is different.  A Primary Rate [[ISDN]] configurations uses [[E-carrier|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 signaling channel. The line cards in switches designed for the E1 system already include processing for the signaling timeslot.  As a result, Non-Facility Associated Signaling is rarely used with E-carrier.

[[Category:Integrated Services Digital Network]]

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