For many years, lots of enterprises have run frame relay networks in conjunction with direct site-to-site ISDN links as a back-up strategy. In other words, in the case of a primary circuit failure, an ISDN call is triggered to dial around the frame network for a direct connection to the remote site.This alternative doesn't give you much bandwidth but is usually priced affordably. A strong application for ISDN backup is protecting low-speed point-of-sale information transfers from retail sites. This is because retailers usually support lots of remote locations, each needing network access services and equipment but not requiring a whole lot of capacity. So the ISDN price tag has generally been budget-friendly to these locations.However, if your primary frame relay connection has grown to a T-1 or higher speed, 128K bit\/sec ISDN backups will likely not be sufficient during an outage. The ISDN links might end up so congested that it doesn't feel like anything is really being backed up at all.The inadequacy of ISDN in these situations could provide a motive for you to get your feet wet with the reliability of Internet-based VPNs (the most affordable type) without making a wholesale transition. You might consider replacing an ISDN frame relay backup link with an IPSec-encrypted Internet link appropriately sized to handle the loads that would fail over during a glitch.This way, you could kill two birds with one stone: you could gain greater capacity on your back-up path while testing out how well the connection works for your mix of application traffic.Should you discover that the Internet-based VPN works reliably, over time, you might gain the courage to replace your primary frame relay link with one, as well. Once both primary and backup links run IP, you can use hot failover protocols in routers for Layer 3 redundancy, as well as route optimization and load balancing appliances that choose the best Internet connection at any point in time based on performance and cost.