   CCITT V.42bis - TECHNOLOGY UPDATE ON DATA COMPRESSION         
 
        Recent activities of CCITT (International Consultative
Committee on Telegraphy and Telephony) Study Group XVII have
enabled this organization to make significant strides toward
completing a recommendation for the next level of the V.42
standard, V.42bis.  The CCITT V.42 standard for modem
error-control was approved in November 1988.  Now, through the
use of a CCITT procedural option, the Group is accelerating the
approval process for V.42bis, the new international data
compression standard.
 
        The V.42 Rapporteur's Group (the V.42bis editor is Alan
Clark of the UK) presented a draft of a V.42bis specification at
Study Group XVII's meeting in Geneva, Switzerland, March 13-21.
 
The Study Group reviewed, edited and accepted the draft.   Final
acceptance will remain pending until the fall meeting, and
comments will be accepted, but there were no objections raised at
the Study Group meeting.  At the meeting in Geneva, the members
agreed to apply the accelerated approval procedures to V.42bis at
the September 25-29 meeting of Study Group XVII.  Under the new
CCITT rules, this will result in a final, accepted recommendation
(rather than a "provisional" recommendation) shortly after the
meeting.
 
        The specification of the new V.42bis standard for data
compression will be based upon the popular Lempel-Ziv compression
algorithm developed at Bell Laboratories in the United States.
British Telecom has enhanced Lempel-Ziv's performance, enabling
it to run faster.  Work performed by Hayes has resulted in a
technique that prevents the expansion of files which are not
compressible data.  At this time, the V.42bis specification
implements both the British Telecom and Hayes enhancements to
Lempel-Ziv and has been referred to as the HBTLZ method by
members of the study group.
 
        The Group evaluated MNP 5 and MNP 7 (both from Microcom),
CommPressor from ACT (Adaptive Computer Technologies), and Hayes
V-series Adaptive Data Compression (Hayes Microcomputer Products,
Inc.), but found the enhanced Lempel-Ziv algorithm best in
meeting the total needs of the new standard.
 
        The goals of the Group were to recommend an algorithm
that (1) did not require a high amount of microprocessor memory,
(2) performed the compression at a high speed, and (3) compressed
the data efficiently.  The current V.42bis specification enables
9600 bps modems to attain throughputs of 30,000 bps (text files)
and 38,400 bps (spreadsheet files) over dial-up connections.
 
        CCITT V.42bis data compression will only be available for
use with V.42's primary error-control protocol, LAPM (Link Access
Procedure for Modems).  When V.42 was approved in November 1988,
the CCITT made it clear that all future enhancements to the V.42
standard would be applicable only to LAPM.  The V.42bis standard
will not specify a way to use data compression with the
alternative error-control procedure in Annex A of the standard
even though products must implement both error-control protocols
to be compliant with the standard.
 
        By providing a well-defined, versatile error-control
standard through V.42's LAPM, the CCITT laid the foundation upon
which more advanced features can be built.  Providing data
compression through V.42bis validates the expandable nature of
V.42 and is a forerunner of more enhancements to come.  Those
possible future enhancements include features such as data
encryption, network management and statistical multiplexing.
 
        The process of developing standards through CCITT points
out the importance of basing a standard on technical merit so
that users can have confidence in the implementation of those
standards in their computer communication networks around the
world.  Hayes believes strongly that V.42 and V.42bis provide the
error-control and data compression standards for the present and
future.  They are consistent with existing standardized
techniques and independent of proprietary control, fostering a
healthy growth environment for a more homogeneous communications
environment.
 
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