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<h2>Recovery from loss of synchronisation</h2>

<p>Handling of exceptional situations where the LMU loses the GPS signal
is fully automated. If a BCF reset is needed to recover from an exceptional
situation, the BSC performs forced handovers whenever possible to avoid interruption
to the traffic. Also, the synchronisation status of each BTS is available
in the BSC.</p>


<p>If a GPS signal is lost, the LMU GPS alarm <span class="alarm-sem">8048 LOSS
OF INCOMING SIGNAL</span> triggers. The BTSs stay in BSS synchronisation for
up to 4 minutes after this alarm. If the LMU reacquires the GPS signal within
this period, the synchronised operation continues uninterrupted.</p>


<p>If the GPS signal is not reacquired within the four-minute period, the
clock out alarm is set (UltraSite/MetroSite: <span class="alarm-sem">7601/7602</span>,
Talk-family: <span class="alarm-sem">7876</span>). The BSC starts recovery actions
and forced handovers are executed for ongoing calls. The master BCF is reset
and starts to act as a clock master and feed the clock signal to the slave
cabinets. Slave BCFs are also automatically restarted after a master BCF reset.</p>


<p>A scenario of what happens when the LMU loses the GPS signal is described
below.</p>


<p><em>1. The system is in BSS synchronisation</em></p>


<ul>
<li><p>The LMU generates the frame clock signal from GPS reference and
feeds it to the BTS cabinets.</p>
</li>
<li><p>The BSS Synchronisation mode indicator in the BSC shows 'SYNCH'
for these BCFs.</p>
</li>
</ul>


<a name="aug1241747142" shape="rect"></a>
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<img alt="urn:mars:dn04130534:1:en:global:cgm_fixed:data:data" border="0" src="NED?action=retrieve&amp;identifier=dn04130534&amp;edition=1&amp;language=en&amp;coverage=global&amp;encoding=gif&amp;component=data&amp;item=data" /><p class="figure-caption">Figure: BSS synchronisation - LMU is the clock source</p>
</div>


<p><em>2. LMU loses the GPS fix</em></p>


<ul>
<li><p>Alarm <span class="alarm-sem">8048 ‘LOSS OF INCOMING SIGNAL’</span>
is set.</p>
</li>
<li><p>The LMU starts the timer 'Time from fix lost to clock out alarm'.</p>
</li>
<li><p>The BTS cabinets continue synchronised operation uninterrupted
by relying on the internal LMU oscillator.</p>
</li>
<li><p>The LMU oscillator slowly drifts away from the global BSS Synchronisation
timing.</p>
</li>
<li><p>The BCF chain is still considered to be in BSS Synchronisation.</p>
</li>
</ul>


<a name="aug1241753203" shape="rect"></a>
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<img alt="urn:mars:dn04130546:1:en:global:cgm_fixed:data:data" border="0" src="NED?action=retrieve&amp;identifier=dn04130546&amp;edition=1&amp;language=en&amp;coverage=global&amp;encoding=gif&amp;component=data&amp;item=data" /><p class="figure-caption">Figure: The LMU loses the GPS signal but continues to feed the clock signal</p>
</div>


<p><em>3a. LMU reacquires the GPS fix before the timer 'Time from fix
lost to clock out alarm' in LMU has expired</em></p>


<ul>
<li><p>Alarm <span class="alarm-sem">8048 ‘LOSS OF INCOMING SIGNAL’</span>
is cancelled.</p>
</li>
<li><p>The LMU oscillator slowly drifts back to the global BSS Synchronisation
timing.</p>
</li>
<li><p>The synchronised operation of the BTS cabinets continues uninterrupted.</p>
</li>
</ul>


<p><em>3b. Timer 'Time from fix lost to clock out alarm' in the LMU expires
before it has regained the GPS fix – the system performs synchronisation recovery</em></p>


<ul>
<li><p>Clock out alarm (<span class="alarm-sem">7601/7602</span>in UltraSite/MetroSite
and <span class="alarm-sem">7876</span> in Talk-family) is set.</p>
</li>
<li><p>The BSC performs forced handovers in all the BCFs of the chain
to move the traffic to the surrounding cells.</p>
</li>
<li><p>'Time from clocks out to clock disable' timer in the LMU expires
and the LMU disables the clock feed.</p>
</li>
<li><p>The BSC resets all the BCFs in the chain.</p>
</li>
<li><p>The first BCF in the chain is configured to be the clock master
for the chain.</p>
</li>
<li><p>Alarm <span class="alarm-sem">7787 SYNCHRONIZATION IS MISSING</span>
is set to indicate that the chain has lost BSS synchronisation.</p>
</li>
<li><p>The chain continues to operate in BTS site synchronisation.</p>
</li>
<li><p>The synchronisation mode indicator in the BSC shows 'UNSYNCH'
for these BCFs.</p>
</li>
</ul>


<a name="aug1241757324" shape="rect"></a>
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<img alt="urn:mars:dn04130558:1:en:global:cgm_fixed:data:data" border="0" src="NED?action=retrieve&amp;identifier=dn04130558&amp;edition=1&amp;language=en&amp;coverage=global&amp;encoding=gif&amp;component=data&amp;item=data" /><p class="figure-caption">Figure: Behaviour after the timer 'Time from fix lost to clock out alarm'
has expired</p>
</div>


<p><em>4. The LMU reacquires the GPS fix after synchronisation reconfiguration
(3b)</em></p>


<ul>
<li><p>Alarm <span class="alarm-sem">8048 LOSS OF INCOMING SIGNAL</span> is
cancelled.</p>
</li>
<li><p>The BSC performs forced handovers in all the BCFs of the chain
to move the traffic to the surrounding cells.</p>
</li>
<li><p>The BSC resets the master clock BCFs.</p>
</li>
<li><p>The LMU frame clock output is re-started by the BSC and the LMU
starts to supply the clock.</p>
</li>
<li><p>The first BTS is configured back to slave clock mode.</p>
</li>
<li><p>The BSC resets the the other BCFs.</p>
</li>
<li><p>Alarm <span class="alarm-sem">7787 SYNCHRONIZATION IS MISSING</span>
is cancelled.</p>
</li>
<li><p>The BTS site is back in BSS synchronisation.</p>
</li>
<li><p>The synchronisation mode indicator in the BSC shows again 'SYNCH'
for these BCFs.</p>
</li>
</ul>


<a name="aug124180375" shape="rect"></a>
<div>
<img alt="urn:mars:dn04130573:1:en:global:cgm_fixed:data:data" border="0" src="NED?action=retrieve&amp;identifier=dn04130573&amp;edition=1&amp;language=en&amp;coverage=global&amp;encoding=gif&amp;component=data&amp;item=data" /><p class="figure-caption">Figure: Behaviour after the LMU reacquires the GPS fix. The system reconfigures
back to BSS synchronisation</p>
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