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<div class="div">
<h1>Common BCCH and channel allocation</h1>

<a name="nov26214492825" shape="rect"></a>
<div class="topic">
<h2>SDCCH
allocation procedure</h2>

<p>The general process of <a href="NED?action=retrieve&amp;identifier=general_glossary&amp;edition=13&amp;language=en&amp;coverage=global&amp;encoding=xhtml_1_0&amp;component=data&amp;item=data&amp;pointer=id011274#id011274" onclick="openSmallPopup(event,'NED?action=retrieve&amp;identifier=general_glossary&amp;edition=13&amp;language=en&amp;coverage=global&amp;encoding=xhtml_1_0&amp;component=data&amp;item=data&amp;pointer=id011274#id011274');return false" shape="rect" target="_blank">SDCCH</a>
selection (that applies in the case of an SDCCH-SDCCH intra-BSC handover)
is the following:</p>


<ul>
<li><p><a href="NED?action=retrieve&amp;identifier=general_glossary&amp;edition=13&amp;language=en&amp;coverage=global&amp;encoding=xhtml_1_0&amp;component=data&amp;item=data&amp;pointer=id03816#id03816" onclick="openSmallPopup(event,'NED?action=retrieve&amp;identifier=general_glossary&amp;edition=13&amp;language=en&amp;coverage=global&amp;encoding=xhtml_1_0&amp;component=data&amp;item=data&amp;pointer=id03816#id03816');return false" shape="rect" target="_blank">BTS</a> selection:</p>


<p>The BTSs where the SDCCH can be allocated are filtered
based on the information on the frequency capability of the accessing <a href="NED?action=retrieve&amp;identifier=general_glossary&amp;edition=13&amp;language=en&amp;coverage=global&amp;encoding=xhtml_1_0&amp;component=data&amp;item=data&amp;pointer=id03944#id03944" onclick="openSmallPopup(event,'NED?action=retrieve&amp;identifier=general_glossary&amp;edition=13&amp;language=en&amp;coverage=global&amp;encoding=xhtml_1_0&amp;component=data&amp;item=data&amp;pointer=id03944#id03944');return false" shape="rect" target="_blank">MS</a> and on the usability of radio resources in different
frequency bands.</p>


<p>The BTSs are divided into groups according to their frequency
band (BTSs using the <a href="NED?action=retrieve&amp;identifier=general_glossary&amp;edition=13&amp;language=en&amp;coverage=global&amp;encoding=xhtml_1_0&amp;component=data&amp;item=data&amp;pointer=id04445#id04445" onclick="openSmallPopup(event,'NED?action=retrieve&amp;identifier=general_glossary&amp;edition=13&amp;language=en&amp;coverage=global&amp;encoding=xhtml_1_0&amp;component=data&amp;item=data&amp;pointer=id04445#id04445');return false" shape="rect" target="_blank">BCCH</a> frequency band form one group
and BTSs using a frequency other than the BCCH frequency band form another
group).</p>


<p>The SDCCH load of each BTS group is calculated taking into
account only the static SDCCH resources. The channel is allocated from the
BTS group that has the lowest load.</p>
</li>
<li><p><a href="NED?action=retrieve&amp;identifier=general_glossary&amp;edition=13&amp;language=en&amp;coverage=global&amp;encoding=xhtml_1_0&amp;component=data&amp;item=data&amp;pointer=id04885#id04885" onclick="openSmallPopup(event,'NED?action=retrieve&amp;identifier=general_glossary&amp;edition=13&amp;language=en&amp;coverage=global&amp;encoding=xhtml_1_0&amp;component=data&amp;item=data&amp;pointer=id04885#id04885');return false" shape="rect" target="_blank">TRX</a> selection:</p>


<p>Within the selected BTS(s), the TRX that has the lowest
channel load (busy traffic and signalling channels) is selected. In <a href="NED?action=retrieve&amp;identifier=general_glossary&amp;edition=13&amp;language=en&amp;coverage=global&amp;encoding=xhtml_1_0&amp;component=data&amp;item=data&amp;pointer=id08877#id08877" onclick="openSmallPopup(event,'NED?action=retrieve&amp;identifier=general_glossary&amp;edition=13&amp;language=en&amp;coverage=global&amp;encoding=xhtml_1_0&amp;component=data&amp;item=data&amp;pointer=id08877#id08877');return false" shape="rect" target="_blank">RF
hopping</a> , the BTSs with an RF hopping TRX prioritisation, the priority
is given to non-BCCH TRXs. The SDCCH channel from the BCCH TRX is allocated
only if there are no idle SDCCHs in other TRXs at all.</p>
</li>
<li><p><a href="NED?action=retrieve&amp;identifier=general_glossary&amp;edition=13&amp;language=en&amp;coverage=global&amp;encoding=xhtml_1_0&amp;component=data&amp;item=data&amp;pointer=id04980#id04980" onclick="openSmallPopup(event,'NED?action=retrieve&amp;identifier=general_glossary&amp;edition=13&amp;language=en&amp;coverage=global&amp;encoding=xhtml_1_0&amp;component=data&amp;item=data&amp;pointer=id04980#id04980');return false" shape="rect" target="_blank">RTSL</a> selection:</p>


<p>The RTSL that has the highest number of idle SDCCH channels
left is selected. However, if a signalling channel was last allocated from
the same TRX, another RTSL than last time is allocated, when possible.</p>


<p>If there are no idle static SDCCH resources in the BTSs,
dynamic SDCCH resources are searched for in every BTS group. From all TRXs,
the RTSL which has the least idle dynamic SDCCH channels is selected.</p>
</li>
</ul>
</div>


<a name="nov2621457026" shape="rect"></a>
<div class="topic">
<h2>SDCCH
assignment (Immediate Assignment)</h2>
</div>


<p>In a multi-band Common BCCH segment, the initial SDCCH
channel for a call setup is always allocated in the layer where the segment's
BCCH resides.</p>


<p>This is because the capabilities of an accessing MS are
not known when the MS sends the establish indication message. It is not possible
to define the usability of the non-BCCH frequencies of the segment, as the
MS only starts sending measurement reports after it has been moved to a dedicated
channel.</p>


<p>Within the BCCH frequency band, the SDCCH to be allocated
is selected according to the algorithm described above.</p>


<p>If there are no idle static or dynamic SDCCH resources
in the BTSs, an idle TCH timeslot is configured as a new temporary SDCCH resource.
Dynamic SDCCH reconfiguration is only applied in the Immediate Assignment
phase, not in handovers.</p>


<a name="nov26216273427" shape="rect"></a>
<div class="topic">
<h2>SDCCH
external handover</h2>

<p>During inter-BSC <a href="NED?action=retrieve&amp;identifier=general_glossary&amp;edition=13&amp;language=en&amp;coverage=global&amp;encoding=xhtml_1_0&amp;component=data&amp;item=data&amp;pointer=id03626#id03626" onclick="openSmallPopup(event,'NED?action=retrieve&amp;identifier=general_glossary&amp;edition=13&amp;language=en&amp;coverage=global&amp;encoding=xhtml_1_0&amp;component=data&amp;item=data&amp;pointer=id03626#id03626');return false" shape="rect" target="_blank">handovers</a>, the
usability of radio resources in different frequency bands than the BCCH cannot
be defined in the target BSC. If the non-BCCH layer is regarded as a layer
with less coverage (as indicated by the positive value of the <span class="parameter-sem">Non
BCCH Layer Offset</span> parameter), only BCCH frequency band resources are
used in the SDCCH allocation for an external handover. (If the BCCH is on
GSM 900/800, the EGSM 900 band can also be utilised according to the MS capabilities.)
If the non-BCCH layer is regarded as a layer with more coverage (as indicated
by the negative value of the <span class="parameter-sem">Non BCCH Layer Offset</span>
parameter), the non-BCCH layer resources can also be used in the SDCCH allocation
for an external handover according to the mobile capabilities.</p>
</div>


<a name="nov26216284728" shape="rect"></a>
<div class="topic">
<h2>SDCCH-SDCCH
intra-BTS handover</h2>

<p>In the case of an intra-BTS handover, the SDCCH allocation
differs from the basic procedure. The SDCCH is trying to be allocated in a
TRX other than the call serving TRX. The channel in the call-serving RTSL
is never selected (therefore the search procedure is started only if a SDCCH
RTSL other than the serving one is defined in the BTS). If the call-serving
TRX is blocked, the basic search procedure is used.</p>
</div>


<a name="nov26216294729" shape="rect"></a>
<div class="topic">
<h2>TCH
assignment</h2>

<p>The basic difference between <a href="NED?action=retrieve&amp;identifier=general_glossary&amp;edition=13&amp;language=en&amp;coverage=global&amp;encoding=xhtml_1_0&amp;component=data&amp;item=data&amp;pointer=id04070#id04070" onclick="openSmallPopup(event,'NED?action=retrieve&amp;identifier=general_glossary&amp;edition=13&amp;language=en&amp;coverage=global&amp;encoding=xhtml_1_0&amp;component=data&amp;item=data&amp;pointer=id04070#id04070');return false" shape="rect" target="_blank">TCH</a>
allocation in a Common BCCH controlled segment and a single BTS cell is that
the target of a TCH request in a segment is a set of BTSs instead a single
BTS. Basically, all existing rules for selecting a TCH in a single BTS cell
are also valid between BTSs in a segment cell. The general process for TCH
selection is as follows:</p>


<ul>
<li><p>BTS(s) selection:</p>


<p>The BTSs where a TCH can be allocated are filtered on the
basis of the information on the frequency capability of the accessing MS and
on the usability of radio resources in different frequency bands.</p>


<p>The BTSs are filtered according to their load. The load
calculation is based on the BTS-specific parameter <span class="parameter-sem">BTS
Load In SEG</span>. TCHs are assigned from the BTSs whose load is less than
the <span class="parameter-sem">BTS Load In SEG</span> value. For more information,
see<a href="NED?action=retrieve&amp;identifier=dn02228416&amp;edition=5&amp;language=en&amp;coverage=global&amp;encoding=xhtml_1_0&amp;component=data&amp;item=data&amp;pointer=nov26217115331#nov26217115331" onclick="sync(this);return true" shape="rect">Handover causes</a>.</p>


<p>When each BTS has reached its load limit the allocation
continues in those BTSs where the load is less than the highest load threshold
value among the BTSs.</p>


<p>When the load in all the BTSs has reached the level of
the highest load threshold value among the BTSs, the GSM 1800 band, the EGSM
900, and PGSM 900 bands are respectively preferred. Therefore the prioritisation
between the frequency layers is applied only in high-load conditions, where
the better GSM 900 resources are saved for the MSs with limited frequency
capability or in the cell border area.</p>
</li>
<li><p>RTSL(s) selection:</p>


<p>After the primary target group of BTSs for TCH allocation
has been selected, all the idle RTSLs are ranked according to their interference
level and to the interference level recommendation defined in the BTS they
belong to.</p>


<p>If there are several candidate RTLSs with the same interference
level, the TRX prioritisation in TCH allocation is applied: the RTSL is allocated
from the BCCH TRX or from a non-BCCH TRX according to the defined prioritisation.</p>


<p>If several candidate RTLSs exist after applying the TRX
prioritisation in TCH allocation, the RTSL is allocated from the BTS with
the lowest Circuit Switched load by using the round robin method so that the
BTS allocated the previous time is the last choice.</p>
</li>
</ul>
</div>


<p><a href="NED?action=retrieve&amp;identifier=dn02222031&amp;edition=5&amp;language=en&amp;coverage=global&amp;encoding=xhtml_1_0&amp;component=data&amp;item=data&amp;pointer=ned_top#ned_top" onclick="sync(this);return true" shape="rect">Overview of Common BCCH</a></p>
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