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Hospital builds life support for data network.

When a lost data link can mean life or death, care must be taken to ensure a dependable backup is in place. Allegheny General Hospital in Pittsburgh recently took the steps necessary to ensure its primary data link had an effective backup.

The data link connects an IBM mainframe in the Data Center to terminals and printers at the hospital. The Data Center is more than three-quarters of a mile from the main hospital. The primary link is provided over a fiber-optic cable, and the backup link over a microwave link. It is extremely unlikely these two vastly different transport media would be interrupted simultaneously.

Paradyne channel extenders are connected across two T1 circuits between the sites. The link had been made over Fibronics multiplexers which combine the two T1s onto a single pair of fiber strands.

These fibers are part of a 12-fiber cable between the two sites the hospital purchased from the local telco. The additional fiber strands will provide significant cant expansion capacity.

Installation of a back-up link between the sites was made easier because hospital management had the foresight to purchase channel extension equipment that offered a standard digital hierarchy transport interface (T1 or T3). This interface made it feasible to switch the primary link from fiber to a microwave system, making the system backhoe proof.

If channel extenders with a proprietary fiber interface had been purchased, there would have been no straightforward way to switch the transport from the fiber to the microwave system in the event of failure on the fiber link. It might have been necessary to replace the original channel extension equipment.

The disaster recovery link is provided by a 23-GHz Motorola Microwave System providing DS-2 service at 6 MB/s. The DS-2 is comprised of four T1 circuits combined through an M12 multiplexer.

Only two of the four T1 circuits currently are used, again allowing substantial expansion capacity.

For this short path, the reliability of the microwave system itself is calculated to be better than 99.9999%, taking into account all weather-related interruptions.

Of course, the fiber system has an even higher expected reliability, but is subject to catastrophic failure should the cable be cut.

If such a catastrophe occurred, it could take hours or even days to restore the cable, depending on the priority assigned to repairing the cable by the telco. Obviously, this type of outage cannot be tolerated in a hospital environment.

The actual switching between the transmission paths occurs at the T1 level.

Inexpensive T Switches from Digital Labs Inc. monitor the viability of each T1 circuit. In the event of failure of the primary path, the T Switches transfer the transmission from the primary to the secondary path.

Each circuit can switch independently. In this application, if the fiber is interrupted both circuits switch at once since both are transported over the same physical fiber strands.

The system has been in operation for a number of months. So far, the hospital has not had to rely on the disaster recovery system to back up the primary link.

Hospital personnel judiciously test the system regularly by manually interrupting the fiber link, forcing transfer from the primary fiber link to the backup microwave link. Surely, experience suggests, they will need it some day.

It could be tomorrow or years away. Either way, the management at Allegheny General Hospital has prepared for what would be a life-threatening emergency.
COPYRIGHT 1991 Nelson Publishing
No portion of this article can be reproduced without the express written permission from the copyright holder.
Copyright 1991 Gale, Cengage Learning. All rights reserved.

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Title Annotation:data communications
Publication:Communications News
Date:Aug 1, 1991
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