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Once you've determined the most vital links to be protected, you must decide which network components will provide the circuit redirection. First, failover links could be enabled by a router. It's especially appropriate to do this when using frame relay, X.25 or SMDS, or when special protocol handling is required. However, this approach may dictate the need for more expensive, higher density routers.

Failover may also be incorporated into the CSU/DSU. However, most internal CSU/ DSUs don't provide this capability, forcing a complete replacement at a higher cost. In many cases, it makes sense to use an external device that provides failover service that's completely transparent to other network access devices.

ISDN Failover to the Rescue In Network Computing's lab in San Mateo, Calif., we tested three leased-line failover devices--Controlware IDB-64/2, HTcomm LineSaver and Promptus Communications OASIS200B SafeLink. These units can be placed on existing networks between the router and CSU/DSU for fast and reliable ISDN backup of leased-line connections to corporate sites.

We found the best combination of failover responsiveness, reliability and price in Controlware's IDB-64/2. The Promptus Communications OASIS200B SafeLink let us inversely multiplex multiple "B" channels across up to four BRI (Basic Rate Interface) circuits and reliably back up failed circuits. But its price was a bit steep and its backup dial management was weak when compared to the other units. Judging from its features, price and functionality, it was obvious that the OASIS200B was built for larger scale networks.

HTcomm's LineSaver--the most inexpensive of the bunch we tested--had many of the desired failover features of Controlware's IDB-64/2, but it lacked the scale of management, security and failover functionality offered by the others. However, for the price, LineSaver could hardly be beat for many low-budget sites.

Overall, all of the units performed well for basic failover, but we were surprised by their lack of SNMP support. We would have expected better integration with network management systems. Even in situations where a service provider owns and manages the WAN interconnection equipment, these units provided little to no notification that the failover device successfully took over. The router was unaware that the primary connection was down, and CSU/DSUs were not always able to report problems.

Controlware IDB-64/2
Of the three products tested, the IDB-64/2 offered the best security and the most flexibility when programming the conditions that must be met before a circuit would fail over to ISDN--and when moving back to the primary circuit once it has been fixed. Although it didn't match the low price of the LineSaver, the IDB-64/2 surpassed many of the failover features found in the much more expensive OASIS200B, thus offering the best combination of services for its price.

To download an Adobe Acrobat .pdf format version of the ISDN Failover Device features charts, click here.