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<h1>Synchronisation</h1>

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<h2>General</h2>

<p>In normal network conditions, synchronisation information
is carried by selected 2.048 Mbit paths from upper to lower hierarchical levels
according to the synchronisation plan, from the MSC down to the BTSs.</p>


<p>The synchronisation units are used in master-slave network
hierarchy, or they can run plesiochronously if synchronisation information
has been lost.</p>
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<h2>BSC</h2>

<p>The BSC synchronisation unit selects the signal, which
has the highest priority from a group of pre-selected digital paths as the
active synchronisation signal. Changeover of the synchronisation signal can
be done using MML commands. It is automatic in case of input failure.</p>


<p>The synchronisation unit is duplicated. Changeover between
an active and a spare unit can be done by using MML commands and is automatic
in case of failure. The changeover functions are all controlled and cause
no disturbance in traffic or signal quality.</p>
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<h2>TC</h2>

<p>The transcoder is able to use one PCM of a predefined set
of PCM inputs as the synchronisation sources. Selection method is automatic.</p>
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<h2>BTS</h2>

<p>Each BTS synchronises to the incoming 2MBit signal via
its PCM interface. This way it is ensured that the whole transmission bit
stream path TC -&gt; BSC -&gt; BTS is fully synchronised.</p>
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