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

DS level

    (Digital Signal or Data Service level)
   Originally an AT&T classification of transmitting one or
   more voice conversations in one digital data stream.  The best
   known DS levels are DS0 (a single conversation), DS1 (24
   conversations multiplexed), DS1C, DS2, and DS3.

   By extension, the DS level can refer to the raw data rate
   necessary for transmission:

    DS0      64 Kb/s
    DS1   1.544 Mb/s
    DS1C  3.15  Mb/s
    DS2   6.31  Mb/s
    DS3  44.736 Mb/s
    DS4 274.1   Mb/s

   (where K and M signify multiplication by 1000 and 1000000,
   rather than powers of two).  In this sense it can be used to
   measure of data service rates classifying the user access
   rates for various point-to-point WAN technologies or
   standards (e.g. X.25, SMDS, ISDN, ATM, PDH).

   Japan uses the US standards for DS0 through DS2 but Japanese
   DS5 has roughly the circuit capacity of US DS4, while the
   European standards are rather different (see E1).  In
   the US all of the transmission rates are integral multiples of
   8000 bits per second but rates above DS1 are not necessarily
   integral multiples of 1,544 kb/s.

   (1998-05-18)