Dial-on-demand routing
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In computing, Dial-on-demand routing (DDR) is a technique for whereby a host or router will automatically initiate a dial-up connection over an ISDN or regular public switched telephone network line when network access is required, and close the connection when no more data needs to be transmitted or received. The line will only be used when needed, which reduces the amount of money that must be spent to keep the ISDN/telephone curcuit-switched connection available.
This is commonly used by home PC users when the computer automatically dials out to an Internet service provider whenever a program requests a TCP/IP connection. More advanced setups may feature a router set up to provide a similar function for an entire computer network. In other situations, a router may be configured to use dial-out lines for backup connections if the primary routes of communication have somehow been severed. This can be useful for system administrators and network administrators.