See
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for a good write-up. Essentially, it says that Bell Labs started muxing channels together, and just added more until the performance suffered, so they stopped at 24 channels.
Regards.
Charles G. Gray Senior Lecturer, Information Technology Oklahoma State University - Tulsa (918) 594-8433
***** Moderator's Note *****Sorry, that's not the real reason.
Bill Horne Moderator