{{Unreferenced, date=September 2015
Non-Facility Associated Signaling or NFAS is a
Primary Rate Interface configuration whereby multiple
T1 carriers share a
signaling
In signal processing, a signal is a function that conveys information about a phenomenon. Any quantity that can vary over space or time can be used as a signal to share messages between observers. The '' IEEE Transactions on Signal Processing' ...
channel (or
D channel
D channel (delta channel) is a telecommunications term which refers to the ISDN channel in which the control and signalling information is carried.
The bit rate of the D channel of a basic rate interface is 16 kbit/s, whereas it amounts to 64 kb ...
).
A T1 circuit typically carries 24 individual
timeslots. Each timeslot in turn carries a single
telephone call
A telephone call is a connection over a telephone network between the called party and the calling party.
First telephone call
The first telephone call was made on March 10, 1876, by Alexander Graham Bell. Bell demonstrated his ability to "ta ...
. 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. Work ...
one of the timeslots is used to carry the D channel. A single 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. Work ...
circuit is thus sometimes described as 23B + D. There are 23
bearer channel
A bearer channel is a DS-0 that carries call content i.e. one that does not carry signaling.
In the common-channel signaling scheme for telecommunications, signaling is sent out-of-band, while all other traffic rides bearer channels. When a call ...
s carrying voice or data, and one D channel carrying the
Common Channel Signaling
In telecommunication, common-channel signaling (CCS), or common-channel interoffice signaling (CCIS), is the transmission of control information ''(signaling)'' via a separate channel than that used for the messages, The signaling channel usually c ...
.
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
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. Work ...
service will be charged for each
signaling
In signal processing, a signal is a function that conveys information about a phenomenon. Any quantity that can vary over space or time can be used as a signal to share messages between observers. The '' IEEE Transactions on Signal Processing' ...
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
The DMS-100 is a member of the Digital Multiplex System (DMS) product line of telephone exchange switches manufactured by Northern Telecom. Designed during the 1970s and released in 1979, it can control 100,000 telephone lines.
The purpose of t ...
did not handle common channel signaling such as
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. Work ...
on the same
line card
A line card or digital line card is a modular electronic circuit designed to fit on a separate printed circuit board (PCB) and interface with a telecommunications access network.
A line card typically interfaces the twisted pair cable of a plain ...
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
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. Work ...
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