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<h2>Free CS timeslots</h2>

<p>In addition to the circuit switched traffic load, the system
attempts to keep one or more timeslots free in the CS territory. The reason
for this is that reallocating resources from the (E)GPRS territory for CS
use when the CS territory is full would cause a delay as a result of the associated
signalling requirements. To avoid passing this delay onto the CS user, the
system attempts to keep a number of timeslots free (spare CS timeslots) for
such CS allocations. The number of spare CS timeslots depends on the cell
size and whether a downgrade or an upgrade has last occurred. The table <em>Free timeslots retained in CS territory (valid for GPRS rel.1)</em>
shows the values that have been implemented. For more information, see <em>(E)GPRS in BSC</em> in BSC/TCSM Product Documentation Set.</p>


<p>The values after a downgrade, shown in the table <em>Free timeslots retained in CS territory (valid for GPRS rel.1)</em>,
are based on a 95% probability that a further downgrade will not be required
while a downgrade is in process. The values after an upgrade have been chosen
with the aim of providing a 95% probability that there is no need for a subsequent
(E)GPRS downgrade within 4 seconds of the previous upgrade.</p>


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<p>Table: Free timeslots retained in CS territory (valid for GPRS rel. 1)</p>


<table border="1">
<thead>
<tr align="left" char="" charoff="50" valign="bottom">
<th colspan="1" rowspan="1" valign="top"><p>No. of TRXs</p>
</th>
<th colspan="1" rowspan="1" valign="top"><p>Free TSLs (after a downgrade)</p>
</th>
<th colspan="1" rowspan="1" valign="top"><p>Free TSLs (after an upgrade)</p>
</th>
<th colspan="1" rowspan="1" valign="top"><p>Mean free TSL in CS</p>
</th>
</tr>
</thead>
<tbody>
<tr align="left" char="" charoff="50" valign="top">
<td colspan="1" rowspan="1"><p>1</p>
</td>
<td colspan="1" rowspan="1"><p>1</p>
</td>
<td colspan="1" rowspan="1"><p>1</p>
</td>
<td colspan="1" rowspan="1"><p>1</p>
</td>
</tr>
<tr align="left" char="" charoff="50" valign="top">
<td colspan="1" rowspan="1"><p>2</p>
</td>
<td colspan="1" rowspan="1"><p>1</p>
</td>
<td colspan="1" rowspan="1"><p>2</p>
</td>
<td colspan="1" rowspan="1"><p>1.5</p>
</td>
</tr>
<tr align="left" char="" charoff="50" valign="top">
<td colspan="1" rowspan="1"><p>3</p>
</td>
<td colspan="1" rowspan="1"><p>1</p>
</td>
<td colspan="1" rowspan="1"><p>2</p>
</td>
<td colspan="1" rowspan="1"><p>1.5</p>
</td>
</tr>
<tr align="left" char="" charoff="50" valign="top">
<td colspan="1" rowspan="1"><p>4</p>
</td>
<td colspan="1" rowspan="1"><p>2</p>
</td>
<td colspan="1" rowspan="1"><p>3</p>
</td>
<td colspan="1" rowspan="1"><p>2.5</p>
</td>
</tr>
<tr align="left" char="" charoff="50" valign="top">
<td colspan="1" rowspan="1"><p>5</p>
</td>
<td colspan="1" rowspan="1"><p>2</p>
</td>
<td colspan="1" rowspan="1"><p>4</p>
</td>
<td colspan="1" rowspan="1"><p>3</p>
</td>
</tr>
<tr align="left" char="" charoff="50" valign="top">
<td colspan="1" rowspan="1"><p>6</p>
</td>
<td colspan="1" rowspan="1"><p>2</p>
</td>
<td colspan="1" rowspan="1"><p>4</p>
</td>
<td colspan="1" rowspan="1"><p>3</p>
</td>
</tr>
</tbody>
</table>
</div>


<p>The effect of the free timeslots is a decrease in the overall
cell capacity. It should be noted, however, that the free slots are occupied
when required by the circuit switched traffic load. This overhead must therefore
be taken into account when considering the overall cell (or TRX) capacity.</p>


<p>From GPRS rel.2 (S10.5) onwards the number of free TSLs
after a downgrade or an upgrade can be modified by the operator using parameters.
See section <em>Territory upgrade/downgrade – dynamic variation of timeslots</em>
for details.</p>
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