ISDN allows multiple digital channels to operate simultaneously through the same regular phone wiring used for analog lines, but ISDN transmits a digital signal rather than analog. Latency is much lower on an ISDN line than on an analog line.

Dial-on-demand routing (DDR) is a technique developed by Cisco that allows the use of existing telephone lines to form a wide-area network (WAN), instead of using separate, dedicated lines. Public switched telephone networks (PSTNs) are involved in this process.

DDR is used when a constant connection is not needed, thus reducing costs. DDR defines the process of a router connecting using a dialup network when there is traffic to send, and then disconnecting when the transfer is complete.

Students completing this module should be able to:

• Define the ISDN standards used for addressing, concepts, and signaling
• Describe how ISDN uses the physical and data link layers
• List the interfaces and reference points for ISDN
• Configure the router ISDN interface
• Determine what traffic is allowed when configuring DDR
• Configure static routes for DDR
• Choose the correct encapsulation type for DDR
• Be able to determine and apply an access list affecting DDR traffic
• Configure dialer interfaces