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

DS1C

    A DS level and framing specification for
   digital signals in the North American digital transmission
   hierarchy.  A DS1C signal uses 48 PCM channels and has a
   transmission rate of 3.15 Megabits per second, twice that of
   DS1.

   DS1C uses two DS1 signals combined and sent on a 3.152
   megabit per second carrier which allows 64 kilobits per
   second for synchronisation and framing using "pulse
   stuffing".  The channel 2 signal is logically inverted, and a
   framing bit is stuffed in two out of three code words,
   resulting in 26-bit information units.  The channels are
   interleaved and then scrambled by the addition modulo 2 of
   the signal with the previous bit.  Finally the bit stream is
   combined with a control bit sequence that permits the
   demultiplexor to function by preceding each 52 bits with one
   DS1C framing bit.  A series of 24 such 53-bit frames forms a
   1272-bit "M-frame".

   (1995-02-07)