Customers can have up to 479 B channels with one D channel. In North America a backup D channel is offered. The backup D channel is located on channel 24 of a second DS1 interface. While both DS1 interfaces are connected to the same Central Office, only one is used for signaling at a time.

In the event of a failover, if a different survivable core server controls the primary and the backup D channels, each survivable core server will think the D channel it does not control is out of service and will try to bring the D channel that it controls into service. The Service Provider will only use one of the D channels for signaling. When the D channel is not in service, the associated B channels of the DS1 will be out of service.