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SDH has advantages over PDH some of these are
Maintenance, Validity, easy access to tributary.

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Posted by HamidAliKhan

Improved quality,maintenance & validity.

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Posted by Hash007

Because of SDH advantages ove PDH.
Such as :
Maintanance, easy access to tributery, validation.

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Posted by waqqas1

The main reason for moving from PDH to SDH was to avail the following opportunity:

->Significantly improve the capability of monitoring the validity, integrity, and quality of transport by means of providing overheads in each network layer.

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Posted by sagitraz