15.10 Intelligent Calling Systems

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While dial backup systems are adequate for rerouting communications to the public-switched network from a relatively small number of failed private lines, they can be quite cumbersome to implement when a large number of lines are knocked out of service at once. In such cases, the ability to reroute traffic rapidly over dial-up facilities can save organizations from financial disaster. For large private networks, an intelligent calling system (ICS) may provide the solution.

Via a network management system, multiple dial backup calls can be initiated to remote sites. The ICS provides an economical means of initiating four-wire dial backup for analog and digital point-to-point or multipoint circuits operating at data rates of up to 19.2 Kbps. The ICS is especially suited for use on multi-drop DDS lines because it reduces the number of modems and standalone dial backup units required.

On notification of a failure, the ICS restores the faulty line(s). The system initiates two calls (or multiple calls for as many drops) from a central or nodal site with two dial-up lines, one for transmitting and the other for receiving. Stored telephone numbers are passed from the NMS to the ICS, which performs the automatic dialing. The ICS then notifies the network operator of the successful or unsuccessful establishment of the dial backup link by monitoring call progress tones. The system continues to monitor the link and report its status to the operator.

Conventional dial backup configurations require two dial-up lines for each leased-line circuit being restored—one line to transmit (Tx) and the other to receive (Rx). Fault-tolerant dial backup systems are capable of single-call, full-duplex operation. These units provide the means to maintain nonstop communications while allowing users to save up to 50% in monthly dial backup charges.

From a central site, the ICS executes all restoration routines through commands issued from the NMS. Selected routines are monitored and the status reported back to the NMS. Under normal conditions, the fault-tolerant dial backup systems pass data between the leased line and the diagnostic modem. On failure of the leased line, the dial backup system automatically answers the phone call in response to ring signals carried over a switched network line, which initiates the dial backup process. When the connection is established, the data is transferred via the public network, thereby restoring communications. While in the dial backup mode, the diagnostic modem remains under constant management system surveillance. Termination of the dial backup call is under control of the network manager at the central site.

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