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

SDH
       Synchronous Digital Hierarchy (FDDI, ATM, SDH)


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

Synchronous Digital Hierarchy
SDH

    (SDH) An international digital
   telecommunications network hierarchy which standardises
   transmission around the bit rate of 51.84 megabits per second,
   which is also called STS-1.  Multiples of this bit rate
   comprise higher bit rate streams.  Thus STS-3 is 3 times
   STS-1, STS-12 is 12 times STS-1, and so on.  STS-3 is the
   lowest bit rate expected to carry ATM traffic, and is also
   referred to as STM-1 (Synchronous Transport Module-Level 1).

   The SDH specifies how payload data is framed and transported
   synchronously across optical fibre transmission links without
   requiring all the links and nodes to have the same
   synchronized clock for data transmission and recovery
   (i.e. both the clock frequency and phase are allowed to have
   variations, or be plesiochronous).

   SDH offers several advantages over the current multiplexing
   technology, which is known as Plesiochronous Digital
   Hierarchy.  Where PDH lacks built-in facilities for automatic
   management and routing, and locks users into proprietary
   methods, SDH can improve network reliability and performance,
   offers much greater flexibility and lower operating and
   maintenance costs, and provides for a faster provision of new
   services.

   Under SDH, incoming traffic is synchronized and enhanced with
   network management bits before being multiplexed into the
   STM-1 fixed rate frame.

   The fundamental clock frequency around which the SDH or
   SONET framing is done is 8 KHz or 125 microseconds.

   SONET (Synchronous Optical Network) is the American version
   of SDH.

   (1995-03-02)