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68 Crocus SHDSL G703 Chapter 5
User manual
Step-by-step configuration
5.4.3 The Plesiochronous Digital Hierarchy (PDH) networks
Asynchronous tributaries
PDH systems allow multiplexing of asynchronous tributaries with substantial frequency offsets based on
bit justification. Therefore PDH networks do not need to be synchronised. Every equipment clock is independent
from the others, but their frequencies are kept close to the nominal values within a specified
tolerance (for example: +/- 50 ppm at 2 Mbit/s).
PDH systems are transparent to the timing content of the transported digital signals. This means that,
despite the fact that the multiplexer clocks are independent, the transported signal keeps its frequency,
only gathering some jitter due to transmission lines and justification. The bit justification technique allows
to transfer the timing content of a digital signal across a transmission chain where clocks are asynchronous,
as shown in the figure below.