Synchronous Digital Hierarchy

(communications, standard)(SDH) An international digitaltelecommunications network hierarchy which standardisestransmission around the bit rate of 51.84 megabits per second,which is also called STS-1. Multiples of this bit ratecomprise higher bit rate streams. Thus STS-3 is 3 timesSTS-1, STS-12 is 12 times STS-1, and so on. STS-3 is thelowest bit rate expected to carry ATM traffic, and is alsoreferred to as STM-1 (Synchronous Transport Module-Level 1).

The SDH specifies how payload data is framed and transportedsynchronously across optical fibre transmission links withoutrequiring all the links and nodes to have the samesynchronized clock for data transmission and recovery(i.e. both the clock frequency and phase are allowed to havevariations, or be plesiochronous).

SDH offers several advantages over the current multiplexingtechnology, which is known as Plesiochronous Digital Hierarchy. Where PDH lacks built-in facilities for automaticmanagement and routing, and locks users into proprietarymethods, SDH can improve network reliability and performance,offers much greater flexibility and lower operating andmaintenance costs, and provides for a faster provision of newservices.

Under SDH, incoming traffic is synchronized and enhanced withnetwork management bits before being multiplexed into theSTM-1 fixed rate frame.

The fundamental clock frequency around which the SDH orSONET framing is done is 8 KHz or 125 microseconds.

SONET (Synchronous Optical Network) is the American versionof SDH.

单词	synchronous digital hierarchy
释义	Synchronous Digital Hierarchy Synchronous Digital Hierarchy (communications, standard)(SDH) An international digitaltelecommunications network hierarchy which standardisestransmission around the bit rate of 51.84 megabits per second,which is also called STS-1. Multiples of this bit ratecomprise higher bit rate streams. Thus STS-3 is 3 timesSTS-1, STS-12 is 12 times STS-1, and so on. STS-3 is thelowest bit rate expected to carry ATM traffic, and is alsoreferred to as STM-1 (Synchronous Transport Module-Level 1). The SDH specifies how payload data is framed and transportedsynchronously across optical fibre transmission links withoutrequiring all the links and nodes to have the samesynchronized clock for data transmission and recovery(i.e. both the clock frequency and phase are allowed to havevariations, or be plesiochronous). SDH offers several advantages over the current multiplexingtechnology, which is known as Plesiochronous Digital Hierarchy. Where PDH lacks built-in facilities for automaticmanagement and routing, and locks users into proprietarymethods, SDH can improve network reliability and performance,offers much greater flexibility and lower operating andmaintenance costs, and provides for a faster provision of newservices. Under SDH, incoming traffic is synchronized and enhanced withnetwork management bits before being multiplexed into theSTM-1 fixed rate frame. The fundamental clock frequency around which the SDH orSONET framing is done is 8 KHz or 125 microseconds. SONET (Synchronous Optical Network) is the American versionof SDH. AcronymsSeeSDH
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