V.E.R.A. -- Virtual Entity of Relevant Acronyms (February 2016):

DS1
       Digital Signal level 1 (ISDN, T1), "DS-1"


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

DS1

    A DS level and framing specification for
   synchronous digital streams, over circuits in the North
   American digital transmission hierarchy, at the T1
   transmission rate of 1,544,000 bits per second (baud).

   DS1 is commonly used to multiplex 24 DS0 channels.  Each DS0
   channel, originally a digitised voice-grade telephone signal,
   carries 8000 bytes per second (64,000 bits per second).  A DS1
   frame includes one byte from each of the 24 DS0 channels and
   adds one framing bit, making a total of 193 bits per frame
   at 8000 frames per second.  The result is 193*8000 = 1,544,000
   bits per second.

   In the original standard, the successive framing bits
   continuously repeated the 12-bit sequence 110111001000, and
   such a 12-frame unit is called a super-frame.  In voice
   telephony, errors are acceptable (early standards allowed as
   much as one frame in six to be missing entirely), so the least
   significant bit in two of the 24 streams was used for
   signaling between network equipments.  This is called
   robbed-bit signaling.

   To promote error-free transmission, an alternative called the
   extended super-frame (ESF) of 24 frames was developed.  In
   this standard, six of the 24 framing bits provide a six bit
   cyclic redundancy check (CRC-6), and six provide the actual
   framing.  The other 12 form a virtual circuit of 4000 bits per
   second for use by the transmission equipment, for call
   progress signals such as busy, idle and ringing.  DS1 signals
   using ESF equipment are nearly error-free, because the CRC
   detects errors and allows automatic re-routing of connections.

   Compare T-carrier systems.

   [Kenneth Sherman, "Data Communications : a user's guide",
   third edition (1990), Reston/Prentice-Hall/Simon & Schuster].

   (1996-03-30)