Synchronous Digital Hierarchy
<communications, standard> (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)
Nearby terms:
Synchronized Multimedia Integration Language «
synchronous « Synchronous Data Link Control «
Synchronous Digital Hierarchy » Synchronous DRAM
» Synchronous Dynamic Random Access Memory »
Synchronous Graphics RAM
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