Narrowband ISDN (N-ISDN) supports the convergence of voice and data communications via transport based on circuit switching and common channel signaling. N-ISDN does not support complete convergence. In particular, its support for video is very limited. Although N-ISDN can support video teleconferencing, it does not support full motion video. N-ISDN could have been enhanced to support full motion video and other applications.
Specifically, the following enhancements would have enabled N-ISDN to be the basis for full convergence:
� Wider bandwidth - More DS0 (64 Kb/s) channels for each subscriber;
� Faster switching - Specifically, dynamically varying the number of
DS0 channels assigned to a connection;
� Asymmetric connections - Usually, more DS0 channels
assigned in the downstream direction than in the upstream direction.
Enhancing N-ISDN to support full convergence is described in detail in the paper "ISDN and the Internet," which was originally published in Computer Networks - The International Journal of Computer and Telecommunications Networking, Elsevier Science B.V. (copyright 1999).
Convergence Based On B-ISDN and ATM
Convergence based on enhanced N-ISDN didn't catch on. Instead, convergence based on Broadband ISDN (B-ISDN) with Asynchronous Transfer Mode (ATM) as the means for transport was promoted. B-ISDN moved away from the circuit switching/common channel signaling approach of N-ISDN and instead adopted an approach based on small fixed size packets (ATM cells).
The small size of ATM cells (53 bytes) reduces delays and makes ATM better than others packet switching techniques for transporting voice. For other applications, ATM is not the best approach. ATM has achieved a certain degree of success in enabling the convergence of voice and data communications. However, the spectacular success of the Internet eclipsed ATM and made convergence based on the Internet Protocol (IP) more attractive.