International Telecommunications Union (UN)

Digital communications link - voice, data, and video at a 1.544 Mbps rate (24 64 Kbps channels)

American National Standards Institute. ANSI administers and coordinates the U.S. voluntary standardization and conformity assessment system for the
telecommunications industry. More details on these standards may be
viewed at www.ansi.org.

ANSI Publication on Functional Description of MTP