Non-Facility Associated Signalling

Non-Facility Associated Signaling or NFAS is a Primary Rate Interface configuration whereby multiple T1 carriers share a signaling channel (or D channel). A T1 circuit typically carries 24 individual

timeslot Broadcast programming is the practice of organizing or ordering (scheduling) of broadcast media shows, typically radio and television, in a daily, weekly, monthly, quarterly, or season-long schedule. Modern broadcasters use broadcast automatio ...

s. Each timeslot in turn carries a single

telephone call A telephone call, phone call, voice call, or simply a call, is the effective use of a connection over a telephone network between the calling party and the called party. Telephone calls are the form of human communication that was first enabl ...

. When a T1 circuit is used to carry Primary Rate

ISDN Integrated Services Digital Network (ISDN) is a set of communication standards for simultaneous digital transmission of voice, video, data, and other network services over the digitalised circuits of the public switched telephone network. ...

one of the timeslots is used to carry the D channel. A single Primary Rate

circuit is thus sometimes described as 23B + D. There are 23 bearer channels 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

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, and the Nortel DMS-100 did not handle common channel signaling such as

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

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 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. Integrated Services Digital Network {{telecom-stub