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

statistical time division multiplexing
StatMUX
STDM

    (STDM, StatMUX) A system developed to
   overcome some inefficiencies of standard time division
   multiplexing, where time slices are still allocated to
   channels, even if they have no information to transmit.

   STDM uses a variable time slot length and by allowing channels
   to vie for any free slot space.  It employs a buffer memory
   which temporarily stores the data during periods of peak
   traffic.  This scheme allows STDM to waste no high-speed line
   time with inactive channels.  STDM requires each transmission
   to carry identification information (i.e. a channel
   identifier).  To reduce the cost of this overhead, a number of
   characters for each channel are grouped together for
   transmission.

   ["Data Communications, Computer Networks and Open Systems",
   Halsall & Fred, Addison Wesley, p160-161, 1995].

   ["Digital, Analog, and Data Communication", Sinnema &
   McGovern, Prentice Hall, p245, 1986].

   (1997-03-05)