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comp.compression Frequently Asked Questions (part 1/3)
Section - [11] What is the V.42bis standard?

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A description of the V.42bis standard is given in "The V.42bis
standard for data-compressing modems," by Clark Thomborson
<>, IEEE Micro, Oct 1992, pp. 41-53. 

If you are looking for freeware source of V.42bis, please read the note
below by Peter Gutman explaining why there is no such source code.

Short introduction, by Alejo Hausner <>:

  The V.42bis Compression Standard was proposed by the International
  Consultative Committee on Telephony and Telegraphy (CCITT, now ITU-T) as
  an addition to the v.42 error-correction protocol for modems. Its purpose
  is to increase data throughput, and uses a variant of the
  Lempel-Ziv-Welch (LZW) compression method.  It is meant to be
  implemented in the modem hardware, but can also be built into the
  software that interfaces to an ordinary non-compressing modem.

  V.42bis can send data compressed or not, depending on the
  data.  There are some types of data that cannot be
  compressed.  For example, if a file was compressed first,
  and then sent through a V.42bis modem, the modem would not
  likely reduce the number of bits sent.  Indeed it is likely
  that the amount of data would increase somewhat.

  To avoid this problem, the algorithm constantly monitors the
  compressibility of the data, and if it finds fewer bits
  would be necessary to send it uncompressed, it switches to
  transparent mode.  The sender informs the receiver of this
  transition through a reserved code word.  Henceforth the
  data is passed as plain bytes.

  While transmitting in transparent mode, the sender maintains
  the LZW trees of strings, and expects the receiver to do
  likewise.  If it finds an advantage in returning to
  compressed mode, it will do so, first informing the receiver
  by a special escape code.  Thus the method allows the
  hardware to adapt to the compressibility of the data.

  The choice of escape code is clever.  Initially, it is a
  zero byte.  Any occurrence of the escape code is replaced,
  as is customary, by two escape codes.  In order to prevent a
  string of escape codes from temporarily cutting throughput
  in half, the escape code is redefined by adding 51 mod 256
  each time it is used.

A note from Peter Gutman <> about V.42bis
  V.42bis is covered by patents, and the licensing terms are rather complex
  because you need to license it from multiple organisations.  At one point
  British Telecom were charging something like 30,000 pounds for a license
  (this was a few years ago, things may have changed since then). Because of
  this, noone has ever implemented a freely-available version of V.42bis as
  you'd find in a modem.  There is a Unix implementation (called "compact") of
  a V.42bis-like algorithm which comes with a great many disclaimers that it
  can only be used for research purposes. [Note from FAQ maintainer: "compact"
  is available in
  The 'shrink' method of zip 1.1 (see item 2 above) is also similar to V.42bis]

  If you've ever wondered why noone other than modem manufacturers ever use
  V.42bis for anything, this is it.  

Some CCITT (ITU-T) standards documents are available by ftp in

A mail server for CCITT (ITU-T) documents is available at
or A Gopher server is also available at gopher://

The V42bis standard is also in

For ISO documents, try

See also item 20 below for other sites with standards documents.

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Previous Document: [10] Fake compression programs (OWS, WIC)
Next Document: [12] I need source for the winners of the Dr Dobbs compression contest

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