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COURIER HIGH SPEED MODEMS
J-8 Glossary
HDLC
High Level Data Link Control. A standard protocol developed
by the International Standards Organization for software appli-
cations and communicating devices operating in synchronous
environments. The protocol defines operations at the link level
of communications, for example, the format of data frames
exchanged between modems over a phone line. See Bisync,
Protocol, SDLC.
HST
High Speed Technology, U.S. Robotics' proprietary signaling
scheme, design and error control protocol for high-speed
modems. HST incorporates trellis-coded modulation, for
greater immunity from variable phone line conditions, and
asymmetrical modulation for more efficient use of the phone
channel at speeds of 4800 bps and above. HST also incorporates
MNP-compatible error control procedures adapted to
asymmetrical modulation.
Hz
Hertz, a frequency measurement unit used internationally to
indicate one cycle per second.
ITU-T
International Telecommunication Union-Telecommunication
sector. Formerly referred to as CCITT. An international
organization that defines standards for telegraphic and
telephone equipment. For example, the Bell 212A standard for
1200 bps communication in North America is observed
internationally as ITU-T V.22. For 2400 bps communication,
most U.S. manufacturers observe V.22 bis.
LAPM
Link Access Procedure for Modems, an error control protocol
incorporated in ITU-T Recommendation V.42. Like the MNP
and HST protocols, LAPM uses cyclic redundancy checking
(CRC) and retransmission of corrupted data (ARQ) to ensure
data reliability.