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Topic Title: Plesiochronous Digital Hierarchy
Topic Summary: PDH
Created On: 19 October 2012 11:42 AM
Status: Read Only
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 19 October 2012 04:36 PM
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GeoffChop

Posts: 1
Joined: 04 December 2010

PDH works with the tributaries (2048 kbit/s) deliberately running at a rate relative to the higher order multiplex so that a justification bit is needed on approximately every other opportunity. So if the tributary runs a fraction faster then a justification bit is needed more often. There are 3 majority voted justification control bits to say if justification is necessary in which case a bit is added. If it is not necessary, the opportunity for an extra bit is not used.

A 2048 kbit/s stream carries a synchronisation pattern in timeslot zero. If a bit is corrupted then the sync pattern still occurs in the correct place, so provided several successive sync patterns are not corrupted then there is no loss of synchronisation. If you lose a bit or add a bit when you should not then, the sync pattern will appear to be corrupted on several occasions and loss of sync will be declared and then a search for the synch pattern has to start again from scratch in order to regain synchronisation. Loss of synchronisation is not good!
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