The pros and cons of dial backup

Continuing in our discussion of making branch-office sites are highly available as possible without breaking the bank, we’ll explore the various options for link redundancy. “Last-mile” access links are notoriously the weakest link in the network chain.

Let’s start with dial/ISDN backup-the least expensive, albeit the slowest, option. This approach is most apropos for protecting low-speed branch-office traffic, such as point-of-sale information transfers from retail sites.

Dial backup, of course, generally provides a maximum capacity of 56K bit/sec, while most ISDN services are 128K bit/sec max. A 56K bit/sec link would be hard-pressed to adequately back up a primary T-1 circuit, for example, unless the T-1 were greatly underutilized.

When using dial backup, the WAN access router detects a last-mile link failure and automatically switches traffic over to a modem. The modem, in turn, dials around the WAN data network service to a modem bank or remote access server at headquarters across a public telephone line or an ISDN service. 

To implement this setup, a router needs intelligence to direct traffic away from the failed link to the dial network. This intelligence is usually provided by Layer 3 routing protocols, which automatically converge (exchange updated routing information) over the back-up dial circuit.

On the plus side, dial backup is inexpensive; the cost of a modem and a telephone line is minimal. An ISDN link could be more expensive and is not readily available in all areas, but still remains a comparatively low-cost option.  And failover requires no manual intervention.

On the other hand, dial backup is not transparent, in that users cannot work during the time it takes the router to dial the new connection and for Layer 3 protocols to converge. This could take a few seconds to a few minutes. In addition, if the modem bank or remote access server at headquarters has no available port to terminate the call, the dial back-up link will not be established.  This scenario could prolong the outage until a port becomes available.