Synchronization
Synchronous is the first word in the term SDH for a very good reason.If synchronization is not guaranteed, considerable degradation in
network function, and even total failure
of the network can be the result. To avoid this worst case scenario, all network
elements are synchronized to a central clock. This central clock is generated by
a high precision primary reference clock (PRC) unit conforming to ITU-T Recommendation G.811.
This specifies an accuracy of 1X 10-11.
This Clock signal must be distributed through out the
entire network. A hierarchical structure is used for this; the signal is passed
on by the subordinate synchronization supply units(SSU) and synchronous equipment
clocks(SEC). The synchronization signal path scan be the same as those used for
SDH Communications.
The
clock signal is regenerated in the SSUs and SECs with the aid of phase-locked loops.
If the clock supply fails, the affected network element switches over to a clock
source with the same or lower quality, or if this is not possible, it switches to
hold-over mode. In this situation, the clock signal is kept relatively accurate by controlling the
oscillator by applying the stored frequency correction values for the previous hours
and taking the temperature of the oscillator into account.
Clock “islands” must
be avoided at all costs, as these would drift
out of synchronization with the passage of time and the total failure disaster would
be the result. Such islands are prevented by signaling the network elements with
the aid of synchronization status messages
(SSM, part of the S1byte). The SSM informs the neighboring network element about
the status of the clock supply and is part of the overhead.
Special
problems arise at gateways between networks with independent clock supplies.SDH
network elements can compensate for clock offsets with in certain limits by means
of pointer operations. Pointer activity is thus are liable indicator of problems
with the clock supply.
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