what is SDH and PDH relate the differences between them?

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what is SDH and PDH relate the differences between them?..

Answer / p jayakumar

PDH is Plesiochronous Digital Hierarchy and SDH is
Synchronous Digital Hierarchy.Both are used in PCM
transmission systems. Both use TDM principles for PCM
frames.
The difference is In PDH the clock used is not the
same throughout the transmission network. But in SDH a
synchronous clock is used in the entire Transmission system.
PDH is the old system used for only point to point
communication and interconnectivity between the sytem is
difficult.Whereas SDH is the lattest system used in any
kind of transmission configuration and interconnectivity
between different systems is very easy.PDH used in
2Mb,8Mb,34Mb and 140 Mb systems only.But SDH used in STM-1
(155.52Mbps,STM-4(622.08Mbps),STM-16(2.5Gbps) and STM-64
(10G BPS) transmission systems.
There are many merits in SDH

Easy for Add Drop multiplexing/Demultiplexing is easy.

Very useful for Ring configuration in optical
communications.

Easy growth to high bit rates in steps with evolution of
transmission technology.

Capable of transporting ATM(Asynchronous Transfer Mode)
channels,DLC(Digital Loop Carrier) is working on SDH.

SDH is the Indian sytem which is equivalent to American
SONET(Synchronous Optical Network)