For those channels that have a D-channel backup, D-channel pairs must be allocated.

A Signaling Group is a group of B-channels for which a given D-channel, or D-channel pair, carries the signaling information. Each Signaling Group must be designated as either a FAS or an NFAS Signaling Group.

• A FAS Signaling Group must contain all the ISDN B-channels on the T1/E1 interface that are associated with the D-channel of the group. An FAS signaling group cannot contain B-channels from any other DS1 media module. For 24-channel DS1 boards, some DS1 ports can use in-band, robbed-bit, signaling and be members in a tie trunk group instead of an ISDN trunk group. These tie trunks cannot be members of a Signaling Group.

• No restriction exists on which T1/E1 ports can belong to an NFAS Signaling Group. Normally, an NFAS Signaling Group consists of one or two D-channels, and several complete T1/E1 interfaces.

• If a Signaling Group contains only a subset of the B-channels of a T1/E1 interface (ports 1 to 12, for example), the group is considered to be an NFAS Signaling Group, not an FAS Signaling Group. The remaining B-channels on the T1/E1 interface are then assigned as members of another NFAS Signaling Group.

For example, if the B-channels in a Signaling Group span three T1/E1 facilities, you must assign a unique Interface ID to each of the three facilities. This designation is required to uniquely identify the same B-channel (port) number on each of the T1/E1 facilities in the Signaling Group. Therefore, this interface must be agreed upon by both sides of the interface, and administered before initialization.

If the IDs do not match, the signaling group comes up, but calls fail.