When two or more interfaces connect a network and a user, a primary D-channel (labeled"one") is always present on one interface. On a different interface, a secondary D-channel (labeled"two") is present that can also send signalling packets. Figure F.3 shows the addition of a secondary (i.e. backup) D-channel to the arrangement shown in Figure F.2.
D-channel one is used to send signalling packets across the user-network interface for multiple interfaces including the interface containing D-channel two. D-channel two is in a standby role and is active at layer 2 only. All SAPI groups (e.g. 0, 16 and 63) are alive and can send packets. At periodic intervals determined by the appropriate layer 2 timer associated with SAPI 0, a link audit frame will be sent on the point-to-point signalling link with DLCI = 0 of D-channel two.
Since D-channel two is in a standby role,load sharing between D-channels one and two is not possible. Furthermore, D-channel two can not serve as a B-channel when it is in a standby role. Lastly, D-channel two can only back up the signalling functions provided by D-channel one and not some other D-channel on a different interface.
Figure F.3/Q.931 – Example of non-associated signalling with backup D-channel controlling three primary rate interfaces