SDH and SONET are transmission equipment. To understand their working, we need to discuss the PDH structure first.

PDH Plesiochronous Digital Hierarchy.

Plesio = Near                    Chronous  = Time.

This system is nearly synchronized. Different Stages are almost synchronized. It was very famous Multiplexing technology.The basic data rate is 2.048Mbps for European standard E1.  For North American standard T1 is 1.544Mbps. 

Explanation in Urdu and Hindi




Pulse code modulation is the basic block for PDH SDH and SONET. Sampling frequency must be twice the higher frequency component of the signal.
Voice frequency sampling rate 8KHz (0.3 to 3.4 KHz = 4KHz).
Each sample encoded into 8 bits (256 level quantization).
Bit rate = 8000 * 8 = 64 Kbps.

PDH Hierarchy

•30 Channel PCM = 2 Mbps 30 Voice channel
•2 Mbps x 4 = 8 Mbps 120 Voice channel
•8 Mbps x 4 = 34 Mbps 480 Voice channel
•34 Mbps x 4 = 140 Mbps 1920 Voice channel
•140 Mbps x 4 = 565 Mbps 7680 Voice channel

PDH disadvantages

Plesiochronous Structure.
Every apparatus has its own clock. Input tributary clock rate is slow than multiplexing equipments. No network level synchronisation.bit stuffing is used.

Restricted management.
Low level tributary access require full system de-multiplex.
No monitoring overhead.
No management overhead.
No NMS provisioning.
No common standard among vendors.
Only point to point communication.
A complex cabling infrastructure between multiplexers. Big size.
No standard optical interfaces.



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