15.9 DDS Dial Backup

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Despite carrier claims of 99.5% availability on DDSs, this still leaves room for 44 hours of downtime annually. This amount of downtime can be very costly to brokerage houses, airline reservation systems, and other firms that depend heavily on time-sensitive transactions in their daily business operations. To offset this potential amount of downtime, several dial backup solutions have become available for digital data services.

Digital data sets—the front-end devices used on the DDS lines—have the ability to “heal” interruptions in transmission. Should the primary digital network facility fail, communication is reestablished over the public-switched network via the data set’s built-in modem and integral single-call dial backup unit.

Sensing loss of energy on the line, the local data set automatically dials the remote data set, which sets up a connection through the public-switched network. Data is then rerouted from the leased facility to the dial-up circuit. If the normal DDS operating rate is 19.2 Kbps, dial restoration entails a fallback to 9.6 Kbps. For all other DDS rates (i.e., 2.4, 4.8, and 9.6 Kbps), the transmission speed remains the same in the dial backup mode, so the capability of adjusting the speed downward is not necessary.

While in the dial backup mode, the data set continues to monitor the failed facility for a return of normal energy levels, which indicates an active line. When the data set senses that the primary digital network service has been restored, it reestablishes that communications link and drops the temporary switched connection.

In addition to supporting automatic transfer between primary (digital) and dial-up (analog) facilities, these units can also be used to perform less complex tasks such as simply notifying support staff of a service disruption so that an operator can manually implement dial restoration. For larger networks, the data set can be used in conjunction with a network management system for control and management reporting at the console. The unit can store telephone numbers to facilitate restoration in multi-tiered networks, which may have several potential points of restoration.

For DDS at 56 Kbps, organizations can use data service units (DSUs) on leased lines to initiate restoration over switched 56-Kbps services such as AT&T’s Switched 56 Service for dial-up, point-to-point 56-Kbps operation. For standalone applications, transmission over dial-up 56-Kbps facilities translates into lower costs because the user pays only for connect time to the network. In addition to supporting full-duplex synchronous data transmission at 56 Kbps over four-wire facilities, DSUs also provide line equalization, data format conversion, and diagnostic capabilities.

After a problem with the DDS link has been identified, the connection to switched 56-Kbps service is set up through available dialing procedures. When the backup link has been verified as operational, operators can switch the terminal’s business equipment interface from the dedicated DSU to the spare DSU.

Depending on the applications supported and the number of dedicated DDS lines within the network, patch panels or intelligent data switches can be used for “1 × N” switching between dedicated and switched lines. In this way, a single DSU at the central site may be used to back up multiple DSUs.

When the primary link is restored, the process of reverting to the normal line configuration takes place. The primary link is first verified as operational, and then the terminal business equipment interface is switched from the spare DSU to the dedicated DSU.

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