Version 5.2 387 September 2007
SIP User's Manual 9. Advanced PSTN Configuration
9.4 Using ISDN NFAS
In regular (non-NFAS) T1 ISDN trunks, a single 64 kbps channel carries signaling for the
other 23 B-channels of that particular T1 trunk. This channel is called the D-channel and
usually resides on timeslot # 24.
The ISDN Non-Facility Associated Signaling (NFAS) feature enables use of a single D-
channel to control multiple PRI interfaces.
With NFAS it is possible to define a group of T1 trunks, called an NFAS group, in which a
single D-channel carries ISDN signaling messages for the entire group. The NFAS group’s
B-channels are used to carry traffic, such as voice or data. The NFAS mechanism also
enables definition of a backup D-channel on a different T1 trunk, to be used if the primary
D-channel fails.
The NFAS group comprises several T1 trunks. Each T1 trunk is called an ‘NFAS member’.
The T1 trunk whose D-channel is used for signaling is called the ‘Primary NFAS Trunk’.
The T1 trunk whose D-channel is used for backup signaling is called the ‘Backup NFAS
Trunk’. The primary and backup trunks each carry 23 B-channels while all other NFAS
trunks each carry 24 B-channels.
The gateway supports multiple NFAS groups. Each group should contain different T1
trunks.
The NFAS group is identified by an NFAS GroupID number (possible values are 1, 2, 3 and
4). To assign a number of T1 trunks to the same NFAS group, use the ini file parameter
NFASGroupNumber_x = groupID (where x is the physical trunkID -- 0 to 7) or the
Embedded Web Server (refer to 'Trunk Settings' on page 178).
The parameter ‘DchConfig_x = Trunk_type’ is used to define the type of NFAS trunk.
Trunk_type is set to 0 for the primary trunk, to 1 for the backup trunk, and to 2 for an
ordinary NFAS trunk. ‘x’ depicts the physical trunkID (0 to 7). You can also use the
Embedded Web Server (refer to 'Trunk Settings' on page 178).
For example, to assign the first four gateway T1 trunks to NFAS group #1, in which trunk
#0 is the primary trunk and trunk #1 is the backup trunk, use the following configuration:
NFASGroupNumber_0 = 1
NFASGroupNumber_1 = 1
NFASGroupNumber_2 = 1
NFASGroupNumber_3 = 1
DchConfig_0 = 0 ;Primary T1 trunk
DchConfig_1 = 1 ;Backup T1 trunk
DchConfig_2 = 2 ;24 B-channel NFAS trunk
DchConfig_3 = 2 ;24 B-channel NFAS trunk
The NFAS parameters are described in 'PSTN Parameters' on page 307.
9.4.1 NFAS Interface ID
Several ISDN switches require an additional configuration parameter per T1 trunk that is
called ‘Interface Identifier’. In NFAS T1 trunks, the Interface Identifier is sent explicitly in
Q.931 Setup / Channel Identification IE for all NFAS trunks, except for the B-channels of
the Primary trunk (refer to note below).
The Interface ID can be defined per each member (T1 trunk) of the NFAS group, and must
be coordinated with the configuration of the Switch. The default value of the Interface ID is
identical to the number of the physical T1 trunk (0 for the first gateway trunk, 1 for the
second gateway T1 trunk, and so on, up to 7).
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