The Free On-line Dictionary of Computing (19 January 2023):

E1

    A European framing specification for the
   transmission of 32 DS0 (64 kb/s) data streams.  By
   extension, it can also denote the transmission rate required
   (2.048 Mb/s = 2048 kb/s).  Unlike DS1 it is free of
   bit-robbing.

   (2002-03-22)