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<h1>Reversed Hunting</h1>

<p>In the current GSM system, the MSC always allocates A interface
circuits. The MSC also must know TC-configuration (pools of which circuits
are FR, and of which EFR...). With the Reversed Hunting feature, the BSC allocates
A interface circuits. All circuits are similar in the MSC, and pools are not
needed in the MSC, but they are still needed in the BSC.</p>


<a name="apr154182061" shape="rect"></a>
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<img alt="urn:mars:dn0469011:1:en:global:tiff:data:data" border="0" src="NED?action=retrieve&amp;identifier=dn0469011&amp;edition=1&amp;language=en&amp;coverage=global&amp;encoding=gif&amp;component=data&amp;item=data" /><p class="figure-caption">Figure: Current Configuration: MSC gives the circuit assignment order to
BSC</p>
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<img alt="urn:mars:dn0468987:1:en:global:tiff:data:data" border="0" src="NED?action=retrieve&amp;identifier=dn0468987&amp;edition=1&amp;language=en&amp;coverage=global&amp;encoding=gif&amp;component=data&amp;item=data" /><p class="figure-caption">Figure: Reversed Hunting: BSC will give Circuit Assignment order</p>
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<p>When using Reversed Circuit Allocation, the MSC configuration
maintenance is easier since there is no need for circuit pools. The circuit
pools are, however, needed in the BSS. It will still be possible to use the
Remote TC.</p>


<p>The feature Reversed Hunting has impact on the following
connection-oriented and connectionless procedures on the A interface:</p>


<p><span class="menupath-sem">Connection-oriented:</span></p>


<ul>
<li><p>Assignment</p>
</li>
<li><p>Handover</p>
</li>
<li><p>Circuit Re-selection (a new procedure)</p>
</li>
<li><p>Call Clearing procedure.</p>
</li>
</ul>


<p><span class="menupath-sem">Connectionless:</span></p>


<ul>
<li><p>Blocking and Unblocking</p>
</li>
<li><p>Circuit Group Blocking and Circuit
Group Unblocking</p>
</li>
<li><p>Reset</p>
</li>
<li><p>Reset Circuit</p>
</li>
<li><p>Unequipped Circuit.</p>
</li>
</ul>


<p>New messages and information elements are also introduced
to fulfil the requirements for this functionality.</p>


<p>When Reversed Hunting is enabled in the GSM system, it
needs to be taken into account already when creating the A interface connection
between the MSC and BSS as both of them have to support the feature. Circuits
must be created as outgoing in the BSS and also routes for every pool must
be created towards the MSC, and only one route per pool is defined. It is
also recommended that the route number should be the same as the pool number.</p>


<p>A Conversion program in the BSC will be provided for making
the changes for the current configuration to reversed hunting.</p>


<p>In the current network, all A interface external circuits
are of incoming type. They need to be changed to outgoing by the conversion
program. Also creating routes according to existing circuit groups must be
done by the conversion program. All existing external circuit pools (for example
FR, HS4, and EFR) need their own route.</p>


<p>Reference: GSM 08.08 ver 5.6.3: MSC - BSS interface; Layer
3</p>
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