The Free On-line Dictionary of Computing (30 December 2018):

V.42bis

    An extension of the ITU-T V.42
   standard modem protocol to included compression using a
   Lempel-Ziv related technique, which detects frequently
   occurring character strings and replaces them with tokens.
   This is similar to the Unix compress utility.  Typical
   compression for text is 50% or better; with nearly 20% gain
   from synchronous conversion this gives reduces transmission
   time by almost 60%.

   [Details?  On-line spec?]

   (2004-08-02)