Synchronous Digital Hierarchy definition 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.