Non-Facility Associated Signalling
Encyclopedia
Non-Facility Associated Signalling 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 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. 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 signalling. 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 Signalling, due to 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 and maintain a separate signalling card for every D channel.
The situation in Europe and the rest of the world is 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 rarely used with E-carrier.
Primary rate interface
The Primary Rate Interface is a standardized telecommunications service level within the Integrated Services Digital Network specification for carrying multiple DS0 voice and data transmissions between a network and a user....
configuration whereby multiple T1
T-carrier
In telecommunications, T-carrier, sometimes abbreviated as T-CXR, is the generic designator for any of several digitally multiplexed telecommunications carrier systems originally developed by Bell Labs and used in North America, Japan, and South Korea....
carriers share a signaling channel (or D channel
D channel
D 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 kbit/s on a primary rate interface.For DSS1 signalling, the D channel...
).
A T1 circuit typically carries 24 individual timeslots. Each timeslot in turn carries a single telephone call
Telephone call
A telephone call is a connection over a telephone network between the calling party and the called party.-Information transmission:A telephone call may carry ordinary voice transmission using a telephone, data transmission when the calling party and called party are using modems, or facsimile...
. 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
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...
s carrying voice or data, and one D channel carrying the Common Channel Signaling
Common Channel Signaling
In telephony, Common Channel Signaling , in the US also Common Channel Interoffice Signaling , is the transmission of signaling information on a separate channel from the data, and, more specifically, where that signaling channel controls multiple data channels.For example, in the public switched...
.
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 signalling. 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 Signalling, due to historical reasons. North American switches such as the Lucent 5ESS
5ESS Switch
The 5ESS Switch is a Class 5 telephone electronic switching system sold by Alcatel-Lucent. This digital central office telephone circuit switching system is used by many telecommunications service providers.-History:...
, and the Nortel DMS-100
Digital Multiplex System
Digital Multiplex System is the name shared among several different telephony product lines from Nortel Networks for wireline and wireless operators...
did not handle common channel signalling such as ISDN on the same line card
Line card
A line card or Digital Line Card is a modular electronic circuit on a printed circuit board, the electronic circuits on the card interfacing the telecommunication lines coming from the subscribers to the rest of the telecommunications access network.A line card commonly interfaces the twisted pair...
that terminated the T1 circuit. So, the telephone company needs to buy and maintain a separate signalling card for every D channel.
The situation in Europe and the rest of the world is different. A Primary Rate ISDN configurations uses E1
E-carrier
In digital telecommunications, where a single physical wire pair can be used to carry many simultaneous voice conversations by time-division multiplexing, worldwide standards have been created and deployed...
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 rarely used with E-carrier.