Both PDH and SDH are terminologies associated with digital multiplexers used in exchanges.
The different hierarchies having different bit rates are combined. These hierarchies will comprised of
many such bit streams/channels (either 24,30 or multiples) often called as tributaries.
SDH
In SDH, digital multiplexer's inputs are of same bit rate and are derived from common clock, hence are in phase.
Hence it is synchronous.Word(group of bits) Interleaving is used in SDH to combine digital signals.
SDH uses basic rate of 155.52 Mbps and four times multiples of this i.e. 622.08 Mbps and 2488.32Mbps.
Basic SDH rate of 155.52 is also as STM-1. STM stands for Synchronous Transport Module.
SDH is referred as SONET(Synchronous Optical Network) in USA due to optical interfaces used.
As per CCITT, any of PDH rates upto 140 Mbps can be integrated with SDH rates up to 155.52 Mbps.
PDH
In PDH, digital multiplexer's inputs(bit streams) are of same bit rate
and are derived from different clocks from different oscillators.
Each will differ within tolerance of few clock periods. Hence it is called plesiochronous. Bit Interleaving is used in PDH to combine digital signals.
In PDH there are two main standards i.e. 30 channel one used in Europe
and 24 channel one used in North America/Japan.
Rates derived from 2.048 Mbps basic rate including bit stuffing in 30 channel case are mentioned below.
2.048 x 4 gives 8.448 Mbps (120 channels)
8.448 x 4 gives 34.368 Mbps (480 channels)
34.368 x 4 gives 139.264 Mbps (1920 channels)
139.264 x 4 gives 564.992 Mbps (7680 channels)
Rates derived from 1.544 Mbps basic rate including bit stuffing in 24 channel case are mentioned below.
1.544 x 4 gives 6.312 Mbps (96 channels)
6.312 x 7 gives 44.736 Mbps (672 channels)
44.736 x 6 gives 274.16 Mbps (4032 channels)
44.736 x 3 gives 139.264 Mbps
44.736 x 12 gives 564.992 Mbps
