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<div class="div">
<h2>Timeslot allocation between circuit switched and (E)GPRS
services</h2>

<p>The primary technique for dividing resources between the
circuit switched (CS) and packet ((E)GPRS) traffic in Nokia GSM is known as
the Territory Method. In this method, timeslots within a cell are dynamically
divided into the CS and (E)GPRS territories: </p>


<ul>
<li><p>a certain number of consecutive traffic timeslots are reserved
for CS GSM calls</p>
</li>
<li><p>the remaining timeslots are available for the (E)GPRS traffic.</p>
</li>
</ul>


<p>The dynamic variation of the territory boundary (and hence the number
of timeslots in each territory) are controlled by territory parameters. This
enables the system to adapt to different load levels and traffic proportions,
offering thus an optimised performance under a variety of load conditions.</p>


<p>The figure <em>Illustration of cell territories</em>
shows how a traffic resource within a cell (2 TRXs in this case) can be divided
into CS and (E)GPRS territories.</p>


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<img alt="urn:mars:dn0445374:2:none:global:cgm_fixed:data:data" border="0" src="NED?action=retrieve&amp;identifier=dn0445374&amp;edition=2&amp;language=none&amp;coverage=global&amp;encoding=gif&amp;component=data&amp;item=data" /><p class="figure-caption">Figure: Illustration of cell territories</p>
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<h3>Dedicated
(E)GPRS capacity </h3>

<p>It is possible to assign a dedicated (E)GPRS capacity,
which means that a number of timeslots are allocated to (E)GPRS on a permanent
basis. These timeslots are always configured for (E)GPRS and cannot be used
by the circuit switched traffic. This ensures that the (E)GPRS capacity is
always available in a cell. </p>


<p>The drawback with this approach is that, for a given cell configuration,
blocking levels for the CS traffic increases because the number of available
channels is reduced. Blocking can be decreased by increasing the cell capacity
(TRX count).</p>


<p>The decision on whether to assign the dedicated (E)GPRS
territory is a trade-off between providing a minimum level of (E)GPRS service
and increasing the blocking for CS services. This decision needs to take into
account operator priorities, network performance, and predicted (E)GPRS usage
levels. The change in blocking can be readily calculated using the Erlang-B
formula.</p>


<p>The dedicated capacity can be set anywhere between zero
and a full cell capacity.</p>
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<h3>Default
(E)GPRS capacity</h3>

<p>It is also possible to assign a default (E)GPRS capacity.
The default (E)GPRS territory is an area that always is included in the instantaneous
(E)GPRS territory, provided that the current CS traffic levels permit this.</p>


<p>With the exception of the dedicated (E)GPRS area, CSW services always
take priority over (E)GPRS services. Consequently, the circuit switched traffic
occupies as much of the (E)GPRS default territory as is needed. If the CSW
traffic level decreases, having previously occupied some of the (E)GPRS default
territory, these timeslots are automatically reallocated back to (E)GPRS irrespective
of the actual (E)GPRS load.</p>


<p>If the circuit switched traffic levels decrease outside
the (E)GPRS default territory, reallocation to (E)GPRS only occurs if the
(E)GPRS load reaches a pre-defined level (see Territory upgrade/downgrade
and Dynamic variation of timeslots).</p>


<p>Setting the default (E)GPRS territory level is a trade-off
between a high utilisation rate of the resources and improving the level of
service (data rate, delay) for (E)GPRS users. A large default (E)GPRS territory:</p>


<ul>
<li><p>tends to increase the level of service experienced by the (E)GPRS
users</p>
</li>
<li><p>may affect the overall (E)GPRS system capacity if a large number
of (E)GPRS timeslots are taking up PCU connections without actually carrying
(E)GPRS traffic</p>
</li>
<li><p>tends to increase the number of intra-cell handovers for CSW
users with the aim of keeping the (E)GPRS default territory free for (E)GPRS.</p>
</li>
</ul>


<p>It is recommended to set the default territory initially to a level
just below the anticipated mean load level. If TSLs are set to 3 as recommended
by Nokia, the default territory is a minimum of three TCHs, however, to accommodate,
where possible, 3-timeslot-capable mobiles at maximum available data rate
without having to negotiate resource allocation with the CS radio resource
management (RRM). </p>
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<h3>Additional
(E)GPRS capacity</h3>

<p>Where additional (E)GPRS capacity is assigned in response
to a load demand that is beyond the default capacity, this capacity is termed
the additional (E)GPRS capacity. The section <a href="NED?action=retrieve&amp;identifier=dn02256921&amp;edition=5&amp;language=en&amp;coverage=global&amp;encoding=xhtml_1_0&amp;component=data&amp;item=data&amp;pointer=mar2441424351#mar2441424351" onclick="sync(this);return true" shape="rect">Territory upgrade/downgrade – dynamic variation
of timeslots</a> describes when this capacity is used.</p>
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