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

Synchronous Optical NETwork
SONET

    (SONET) A broadband networking standard based
   on point-to-point optical fibre networks.  SONET will
   provide a high-bandwidth "pipe" to support ATM-based
   services.

   The SONET standard will establish a digital hierarchical
   network with a consistent worldwide transport scheme.  SONET
   has been designed to take advantage of fibre, in contrast to
   the plain old telephone system which was designed for copper
   wires.

   SONET carries circuit-switched data in frames at speeds in
   multiples of 51.84 megabits per second (Mbps) up to 48 * 51.84
   Mbps = 2.488 gigabits per second.  Since SONET uses multiple
   channels to transmit data, each SONET frame can be
   considered to be a two-dimensional table of bytes that is 9
   rows high and 90 columns deep.  For every OC-n level, SONET
   can transmit n number of frames at a given time.  Groups of
   frames are called superframes.

   SONET is the American version of SDH.

   [Wulf Losee; Corporate Computing 8.92; STACKS; LAN Magazine
   10.93].

   (1994-11-30)