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)
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