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A recovery time of 30 minutes may seem tolerable for voice traffic, in which the public switched network itself is a backup vehicle, but data subscribers may need to implement alternate routing more quickly. Therefore, AT&T’s DACS and customer-controlled reconfiguration service, and the similar offerings of other carriers, are typically used to remedy a long-term failure rather than to rapidly restore service on failed lines.

ISDN Facilities

T1 multiplexers offer many more functions than does DACS with customer-controlled reconfiguration. In fact, the instantaneous restoration of high-capacity facilities on today’s global networks calls for a T1 networking multiplexer with an advanced transport management system.

An ISDN-equipped T1 networking multiplexer offers yet another efficient and economical means to back up T1 and fractional T1 facilities. With ISDN, the typical time required for call setup is from 3 to 10 seconds. An appropriately equipped T1 multiplexer permits traffic to be rerouted from a failing T1 line to an ISDN facility in a matter of seconds rather than hours or days, as is required by other recovery methods.

Using ISDN facilities is more economical than other methods, including ACCUNET T1.5 reserved service. With the reserved service, the user pays a flat fee for a dedicated interoffice circuit over a certain length of time, including the time when the circuit remains unused. With ISDN, the user pays for the primary rate local access channels and pays for the interoffice channels only when used because these charges are time and distance dependent — just like ordinary phone calls.

With AT&T’s high-capacity HO (384K bps) and H11 (i.544M bps) ISDN channels, users can avail themselves of the ISDN for backing up fractional or full T1 lines rather than pay for idle lines that may only be used occasionally during recovery. This is accomplished through a T1 multiplexer’s capability to implement intelligent automatic rerouting, which ensures the connectivity of critical applications in an information-intensive business environment.

When confronted with congestion or impending circuit failure, the intelligent automatic rerouting system calculates rerouting on the basis of each likely failure. During a failure, the system automatically recalculates optimal routing, based on current network conditions. After restoration, the system again automatically calculates the most effective rerouting, should a second failure occur on the network. In this way, the system is always ready to handle the next emergency.

Because applications require different grades of service to continue operating efficiently during line failures, circuits must be routed to the best path for each application, not just switched to available bandwidth. This ensures that the network continues to support all applications with the best response times.

To avoid service denial during rerouting, voice transmissions can be automatically compressed to use less bandwidth. This can free up enough bandwidth to support all applications, both voice and data.

DDS Dial Backup

Despite carrier claims of 99.5% availability on digital data services (DDS), this seemingly impressive figure still leaves room for 44 hours of annual downtime. This amount of downtime can be very costly, especially to financial institutions, whose daily operations depend heavily on the proper operation of their networks. A large financial services firm, for example, can lose as much as $200 million if its network becomes inoperative for only an hour.

An organization that cannot afford the 44 hours of annual downtime might consider a digital data set with the ability to “heal” interruptions in transmission. Should the primary facility fail, communication can be quickly reestablished over the public switched network by the data set’s built-in modem and integral single-call dial back unit.

Sensing loss of energy on the line, the dial-backup unit automatically dials the remote unit, 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 19.2K bps, dial restoration entails a fallback to 9.6K bps. For all other DDS rates — 2.4K, 4.8K, and 9.6K bps — the transmission speed remains the same in the dial-backup mode. Downspeeding is not necessary.

While in the dial backup mode, the unit continues to monitor the failed facility for the return of energy, which indicates an active line. Sensing that service has been restored, the unit reestablishes communication over it. The dial-up connection is then dropped.

RECOVERY OPTIONS FOR LANS

The LAN is a data-intensive environment requiring special precautions to safeguard one of the organization’s most valuable assets — information.

The procedural aspect of minimizing data loss entails the implementation of manual or automated methods for backing up all data on the LAN to avoid the tedious and costly process of recreating vast amounts of information. The equipment aspect of minimizing data loss entails the use of redundant circuitry as well as components and subsystems that are activated automatically upon the failure of various LAN devices to prevent data loss and maintain network availability.

Recovery and Reconfiguration

In addition to the ability to respond to errors in transmissions by detection and correction, other important aspects of LAN operation are recovery and reconfiguration. Recovery deals with bringing the LAN back to a stable condition after an error, and reconfiguration is the mechanism by which the network is restored to its previous condition after a failure.

LAN reconfigurations involve mechanisms to restore service upon loss of a link or network interface unit. To recover or reconfigure the network after failures or faults requires that the network possess mechanisms to detect that an error or fault has occurred and to determine how to minimize the effect on the system’s performance. Generally, these mechanisms provide:

  Performance monitoring.
  Fault location.
  Network management.
  System availability management.
  Configuration management.

These mechanisms work in concert to detect and isolate errors, determine errors’ effects on the system, and remedy these errors to bring the network to a stable state with minimal impact on network availability.

Reconfiguration. Reconfiguration is an error-management scheme used to bypass major failures of network components. This process entails detection that an error condition has occurred that cannot be corrected by the usual means. Once it is determined that an error has occurred, its impact on the network is assessed so an appropriate reconfiguration can be formulated and implemented. In this way, normal operations can continue under a new configuration.


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