Source : Free On-Line Dictionary of Computing

Synchronous Digital Hierarchy
     
         (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)