Search the FAQ Archives

3 - A - B - C - D - E - F - G - H - I - J - K - L - M
N - O - P - Q - R - S - T - U - V - W - X - Y - Z
faqs.org - Internet FAQ Archives

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

( Part1 - Part2 - Part3 - Single Page )
[ Usenet FAQs | Web FAQs | Documents | RFC Index | Forum ]


Top Document: comp.compression Frequently Asked Questions (part 1/3)
Previous Document: [10] Fake compression programs (OWS, WIC)
Next Document: [12] I need source for the winners of the Dr Dobbs compression contest
See reader questions & answers on this topic! - Help others by sharing your knowledge

A description of the V.42bis standard is given in "The V.42bis
standard for data-compressing modems," by Clark Thomborson
<cthombor@theory.lcs.mit.edu>, 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 <hausner@qucis.queensu.ca>:

  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 <pgut01@cs.auckland.ac.nz> about V.42bis
implementations:
 
  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
ftp://ftp.sunet.se/pub/usenet/ftp.uu.net/comp.sources.misc/volume15/compact_sv/part01.gz
  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
ftp://ftp.fdn.org/pub/Library/Ccitt-standards/ccitt/

A mail server for CCITT (ITU-T) documents is available at teledoc@itu.arcom.ch
or itudoc@itu.ch. A Gopher server is also available at gopher://info.itu.ch

The V42bis standard is also in
ftp://ftp.std.com/obi/Standards/Telcom/CCITT/

For ISO documents, try http://www.iso.ch/

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

User Contributions:

Report this comment as inappropriate
May 23, 2019 @ 4:04 am
Hello, my name is Jim and I was just looking your website faqs.org
over and thought I would message you on your contact form and offer
some help. I really like your site but I noticed you weren_t getting a
lot of traffic and your Alexa ranking isn_t as strong as it could be.

https://besttrafficpros.com

Fortunately, I may have an answer for you. I can get you 1,000_s of
visitors looking at faqs.org ready to buy your product, service or
sign up for an offer and fast. Our advertising network of over 9000
websites provides a low cost and effective online marketing solutions
that actually works. I can help your business get more online quality
traffic by advertising your business on websites that are targeted to
your specific market. The Internet is vast but you don_t have to spend
huge amounts of cash to jump start your business. I can get you 5,000
highly targeted visitors directly to your website for as little as
$29.00 for a 30 day trial run.

https://besttrafficpros.com

If you would like to talk personally and have specific questions, call
me @ 480-331-6775 from 9am to 5pm MST. Also check out the short video
here and see how everything works.

Best Regards,
James Anthony
support@besttrafficpros.com
BestTrafficpros.com
https://besttrafficpros.com

Comment about this article, ask questions, or add new information about this topic:

CAPTCHA




Top Document: comp.compression Frequently Asked Questions (part 1/3)
Previous Document: [10] Fake compression programs (OWS, WIC)
Next Document: [12] I need source for the winners of the Dr Dobbs compression contest

Part1 - Part2 - Part3 - Single Page

[ Usenet FAQs | Web FAQs | Documents | RFC Index ]

Send corrections/additions to the FAQ Maintainer:
jloup@gzip.OmitThis.org





Last Update March 27 2014 @ 02:11 PM