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.
