Integrated Services Digital Network (ISDN) was originally conceived as an all-digital replacement for the existing phone network. ISDN is capable of moving digital voice, data, video, and more over existing telephone wires. Unfortunately, the deployment of ISDN hasn’t been nearly as comprehensive as service providers and various industry committees once thought it would be. Challenges in deploying ISDN have included different standards early in its development, the large investment required by service providers in new Central Office switching equipment, and the emergence of newer, faster technologies like DSL. Having said that, ISDN service certainly has a place in the world of Wide Area Networking, and is a popular choice for dial-on-demand and backup connections.
The world of ISDN is full of terms and acronyms. For example, different types of equipment are designated by codes, depending upon their function and whether or not they are ISDN-capable. There are still other codes used to reference the various ISDN interfaces between equipment. Finally, there are codes that reference the various ISDN standards and protocols.