You can use ISDN as a backup service
for a leased-line connection between remote offices and the central
office. If the primary connectivity goes down, an ISDN
circuit-switched connection is established, and traffic is rerouted
over ISDN. When the primary link is restored, traffic is redirected to
the leased line, and the ISDN call is released.
ISDN dial backup can also be configured
based on traffic thresholds or limits as a dedicated primary link. If
traffic load exceeds a user-defined value on the primary link, the
ISDN link is activated to increase bandwidth between the two sites, as
shown in the figure.