Internet Speed

ISDN technologies were originally designed in the 1980’s, succeeding in allowing a video conferencing system to work effectively where earlier attempts had generally failed. An ISDN-based video conferencing system allows users to send two or more types of data (such as audio, video, text, or fax transmissions) simultaneously through a standard telephone connection in a digital format.
A minimum transfer rate is maintained throughout, so as to help prevent choppy video, glitches in audio, and so many other problems that were common with early attempts at creating a video conferencing system.

ISDN was originally envisioned as a very fast service, but this was a long time ago when it was hoped to have fiber all the way to your house. It turned out that running all that fiber would be too expensive, so they designed ISDN to run on the copper wiring that you already have. Unfortunately, that slowed things down considerably - too slow for quality video, for instance.
ISDN is available now in many places, but it is not widely used. Further most of the products and services that people have forecast for ISDN still aren’t available.

B-ISDN

B-ISDN is Broadband ISDN. This is not simply faster ISDN, or ISDN with the copper to your home finally upgraded to fiber. B-ISDN is a complete redesign. It is still capable of providing all the integrated services (voice, data, video, etc.) through a single interface just like ISDN was supposed to. But it will do it a lot faster than ISDN could. Of course, that copper to your house will still have to be replaced with fiber. But B-ISDN is still in development - it seems to be moving faster than ISDN, but it is still quite a ways off.

ISDN is now used mostly as an alternative to analog connections, most commonly for Internet access. Some of the services envisioned as being delivered over ISDN are now delivered over the Internet instead.