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 DigitalHierarchy. 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)