The public switched telephone network (PSTN) represented a significant infrastructure investment based on the time division multiplexing (TDM) of DS0 channels over copper, fiber, and microwave. The investment began in the early 1960's and continued to be the primary infrastructure investment through the 1990's by carriers and many, perhaps most, corporate private networks.

The move towards IP based communications was only one of many protocols used in the 1980's. In the 1990's, it became the dominant data approach with the advent and acceptance of html, which made the Internet useful to the masses. It became the dominant communications protocol only within the past decade, I believe.

Your argument that the carriers would not consider a change to other protocols and approaches than IP ignores the historical precedent of IP.

Private Reply to Ken Hilving (new win)