Avoid WAN Outages: Three Leased-Line Failover Devices Keep You Connected

The IDB-64/2, like the OASIS200B, has two sets of DCE/DTE ports, which enable it to support up to two leased lines over a BRI circuit. It does this by allocating a single B channel to each of the ports. The IDB-64/2 supports 56-Kbps or 64-Kbps bandwidth to the remote failover device, but it doesn't have the ability to combine the remaining unused B channel for additional bandwidth. Because most leased-line connections are 56-Kbps DDS, the connection speed of a single BRI is the equivalent bandwidth, or better, of the original connection. The OASIS200B was the only unit tested to offer greater bandwidth through multiplexing B channels.

One of the most important features of a failover device is its ability to carefully initiate and manage backup calls. The features that allow intelligent cutovers enable the unit to judge when the primary circuit should be switched to the backup line and back again. If the unit switches over too quickly or easily, you run the risk of running up a significant ISDN charge.

We liked the IDB-64/2's ability to monitor line conditions, such as DTR (Data Terminal Ready), RTS (Ready to Send), DCD (Data Carrier Detect) and error byte sent from the DTE. We were able to program the IDB-64/2 (and the LineSaver) to monitor a combination of up to four of these types of conditions in a logical OR format; when any one condition occurred in the line, the unit switched to the backup circuit. The LineSaver and IDB-64/2 units were much more powerful than the OASIS200B for failover, and the IDB-64/2 offered the widest variety of line and equipment conditions.

In addition, we also liked the IDB-64/2's ability to set time-window restrictions, defining when a backup could take place. You can set restrictions based on days of the week, weekends or specific times. This is important because it lets sites back up a leased line only during normal operating ho urs and not waste money on ISDN charges after hours or over the weekend.

On the other hand, we found that the Controlware unit didn't monitor inactivity on the B channel as well as the LineSaver did. In no-traffic situations, the backup line could be temporarily brought down to reduce connection times during failover.

Like the LineSaver and OASIS200B, we were able to program delays in the failover and restore timings so that the unit would not fail over during minor glitches like electrical storms or power surges.

Another feature available to both the IDB-64/2 and the OASIS200B is the ability to automatically poll the destination failover device to ensure that the path and hardware are working correctly. This prevents unwanted surprises months after the leased line has been installed and a failover condition is attempted over a bad ISDN circuit or faulty receiving device.

The IDB-64/2 was the only unit tested that offered security for incoming backup calls. We were able to set call line ID (CLID) and in-band password protection against unwanted incoming calls. The OASIS200B offered a console password lockout, but no inbound access protection.

Unfortunately, we found the IDB-64/2's terminal user interface to be the least friendly of the bunch. The text-based interfaces offered by HTcomm and Promptus Communications were more intuitive, faster to configure and easier to navigate when monitoring status and line conditions. Controlware does, however, offer a Windows-based network management plug-in that provides real-time control, configuration, monitoring and testing of up to 255 locally attached units.

Promptus Communications OASIS200B SafeLink
Although it was the most expensive unit we tested, the OASIS200B does more than simply failover leased-line connections. It also offers bandwidth overflow. Bandwidth overflow is best-suited for organizations that want extra bandwidth during distributed backups or periods of peak traffic. Our testing showed that the OASIS200B has the strongest management capabilities with a built-in modem for easy NetWare Console-like remote management. Like the LineSaver, OASIS200B's failover parameters and security features weren't as strong as Controlware's, but its reliability was on par with the IDB-64/2.

For the Side Bar on
How We Tested ISDN Failover Devices

Making the diagnosis With Windows Protocol Analyzers
By Dan Backman

Updated November 10, 1997

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