15.11 Dial Backup Units

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Over the years, dial backup units (DBUs) have come into widespread use for temporarily rerouting modem and digital data set transmissions from failed facilities to the public-switched network. A DBU may be a standalone device that plugs into a modem or an optional add-in module that sits on top of the modem’s main circuit board.

On failure of the primary line, operation over the public-switched network can be manually or automatically initiated. At the remote site, the dial backup unit answers the two calls automatically. When the handshake sequence is completed and the dial backup connection is established, the flow of data resumes.

On restoration of the failed line, dial backup is terminated in one of two ways. The central site operator manually releases the backup switch on the dial backup unit. Alternatively, when in the automatic mode, the dial backup unit re-establishes the leased line on detection of acceptable signal quality. With either of these actions occurring at the central site, the remote site’s dial backup unit automatically disconnects, and all communications are transferred to the primary line.

There are two possible methods for initiating dial backup over the public switched network: manual-originate with automatic answer and auto-originate with automatic answer. In the manual-originate mode, the operator sets up a dial backup circuit for a failed data line, and the remote end answers. After a security check, the transfer of lines is implemented, which is accomplished by entering the backup mode, dialing a remote station over a regular business line, and switching to the backup circuit. No telephone set is necessary if the DBU does the dialing. In the auto-originate mode, the DBU recognizes a leased-line failure or signal-quality degradation and automatically establishes communications via the public-switched network to the remote site.

In both the manual-originate and auto-originate modes, the remote location is treated as unattended; the auto-answer capability provides the means to automatically answer a dial backup call. An auto-terminate capability disconnects the backup call when the originating site ceases to transmit due to the restoration of the primary link. An auto-abort capability disallows call transfer and disconnects the call if the handshake protocol (including the security check) is not properly completed. This prevents wrong-number calls from disturbing the network, thus minimizing its vulnerability to hackers.

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