If the signaling link fails on the active D-channel, D1, or the hardware that carries the D1 channel fails, the system sends a message over the standby D-channel, D2. D2 then becomes the active D-channel and carries all subsequent signaling messages. When the signaling link or the hardware on D1 recovers from the failure, D1 becomes the standby D-channel.
System Technician
If a system technician sends a command to a switch over a D-channel, the system tears down the signaling link on D1. Then, the system sends a message on D2 to request that D2 become the active D-channel. D2 then becomes the active D-channel, and the switchover is complete.
