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<div class="div">
<h1>Direct Access to Desired Layer/Band </h1>

<p>The purpose of the feature is to reduce the number of unnecessary
TCH reservations in macro cells and thus to provide more trunking efficiency
in the network.</p>


<p>The Direct Access to Desired Layer/Band (DADL/B) feature
directs traffic in call set-up phase from the SDCCH of a macro cell / GSM900
cell to the TCH of a micro cell / GSM1800 cell whenever possible. In addition,
the extended GSM900 band capability of the MS is taken into account. The functionality
of this feature is similar to that of Directed Retry. The difference is that
handover in DADL/B is triggered due to cell load, and the target cell selection
criteria are tighter.</p>


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<img alt="urn:mars:dn0468803:1:en:global:tiff:data:data" border="0" src="NED?action=retrieve&amp;identifier=dn0468803&amp;edition=1&amp;language=en&amp;coverage=global&amp;encoding=gif&amp;component=data&amp;item=data" /><p class="figure-caption">Figure: DADL/B is a DR-like procedure, triggered due to exceeded TCH load
in the accessed cell</p>
</div>


<p>The DADL/B handover can also be made in another direction,
from a micro cell to a macro cell or from a GSM1800 cell to a GSM900 cell.
The use of DADL/B is flexible with adjacent-cell-specific indications, and
it can be tailored to different environments and concepts. Note also that
this DADL/B feature is applicable to phase 1 mobiles, which do not support
C2 cell re-selection.</p>


<p>The DADL/B will be applied if the DADL/B is enabled in
the BSC and if the load of the accessed cell is higher than the BTSLoadThreshold
defined for the accessed cell, and there are adjacent cells defined as the
DADL/B handover target cells with the dadlbTargetCell(n) parameter. The adjacent
cells are verified according to the MS capabilities (single band, dual band,
and tri band).</p>


<p> If there is real TCH congestion in the accessed cell,
a DR due to congestion with or without queuing will be made. If there are
TCHs available in the accessed cell, the BSC will act according to the DADL/B
usage determination and cell load. If the DADL/B is applied and the TCH load
in the accessed cell is higher than BTSLoadThreshold, the DADL/B handover
procedure will be started. If the DADL/B is not applied or the TCH load is
lower than BTSLoadThreshold, a TCH will be allocated from the accessed cell.
Note that pre-emption always has higher priority, the TCH will be allocated
from the accessed cell, and thus no DADL/B handover will be attempted.</p>


<p>The target cells for the DADL/B handover are selected according
to the following criteria:</p>


<p>Adjacent cells handled by the BSC handling the accessed
cell, considered as DADL/B target cells with signal level exceeding HOLevelUmbrella,
will be sorted according to adjacent cell priority and load factor (HOPriorityLevel
or HOPriorityLevel(n) - LoadFactor(n) if adjacent cell is loaded) to be used
as target cells for the DADL/B handover.</p>


<p>The DADL/B handover attempt can fail due to following reasons:</p>


<ul>
<li><p>A TCH could not be allocated from any
cell included in the DADL/B list (TCH Congestion in DADL/B target cells)</p>
</li>
<li><p>The DADL/B handover fails, and the
MS returns to the old channel (SDCCH)</p>
</li>
<li><p>The timer monitoring the DADL/B target
cell list availability triggers. This timer is the same as the timer used
in Directed Retry (MinTimeLimitDR).</p>
</li>
</ul>


<p>In these cases, an attempt is made to allocate the TCH
from the accessed cell. DR and queuing are possible in the second TCH request
if no TCHs are available in the accessed cell. The BSC will evaluate the target
cells for DR as soon as possible. This means when at least one target cell
for DR is found.</p>


<p>Note that the DADL/B handovers are only possible as intra-BSC
handovers.</p>
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