Dial backup is an important feature in a reliable WAN design. If the primary link to a remote site fails, dial backup links can ensure that you don’t lose all connectivity. Of course, in most cases the dial backup link will have significantly lower bandwidth than the primary link. However, the principle advantage of using a dialup connection for backup is that the link will connect only when required. The rest of the time the connection is down, which usually saves money because you only pay for the access and avoid the connection charges.
The examples in this chapter are also useful for WAN designs for which the dial links are used as the primary connections. There are two common examples of networks like this. The first are networks that only connect when there is data to send. For example, in many retail environments, the remote store-front sites only need to exchange data at the end of the day to update inventory and report the day’s sales.
The other common type of network that uses only dialup connections involve sites that are in separate buildings, but within the same local dialing area. In this case, if the telephone company doesn’t charge a usage fee, a pure dialup network can be a very cost effective way of delivering low-bandwidth WAN services.
Three technologies are commonly used for dialup links: standard analog telephone lines with asynchronous modems, switched 56 Kbps synchronous digital service (sometimes called Centrex), and ISDN.
The first ...