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<h2>(E)GPRS occupancy levels and data throughput</h2>

<p>Since the number of available timeslots is limited, the
probability of a user gaining access to the desired number of timeslots decreases
as the number of users increases. With busy conditions, several users may
share a single timeslot, and therefore the net data rate per user is reduced
proportionally while the overall transmission time increases. Although the
system throughput may not be compromised under busy conditions, the level
of service experienced by the users decreases as a result of the reduced data
rates and increased transmission delays.</p>


<p>The figure <em>Offered load vs. throughput
and delay (1 slot)</em> illustrates the effects of increasing GPRS traffic
on user throughput and link-layer frame delay for single-slot users. The data
has been obtained from simulations run on a network consisting of 75 sectors
(1 TRX per sector, 3/9 reuse) used exclusively for GPRS. The figure shows
that the net throughput decreases rapidly when the offered load exceeds 4Mbit/s.
The effect is particularly severe above 6Mbit/s, and the delays occurring
for the same load levels increase in an almost exponential manner, with a
corresponding degradation in the level of service experienced by the users.
Similar behaviour can also be expected in a system containing a proportion
of circuit-switched traffic, even if the load levels were scaled appropriately.</p>


<a name="dec1321548235" shape="rect"></a>
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<img alt="urn:mars:dn02259639:1:en:global:cgm_fixed:data:data" border="0" src="NED?action=retrieve&amp;identifier=dn02259639&amp;edition=1&amp;language=en&amp;coverage=global&amp;encoding=gif&amp;component=data&amp;item=data" /><p class="figure-caption">Figure: Offered load vs. throughput and delay (1 slot)</p>
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<p>An efficient multi-slot operation is highly dependent upon
the number of available timeslots. Under high load conditions, it becomes
increasingly unlikely that a user has full access to, for example, three consecutive
timeslots. Therefore, the mean data rate for a 3-timeslot user rarely meets
the level theoretically available (that is, three times the single slot rate),
except on systems operating at low load levels. This is illustrated in figure <em>Offered load vs. throughput and delay (3 slot)</em>; roll-off
in the user throughput curve is steeper at lower offered load levels than
in the curve for the single slot user, figure <em>Offered load
vs. throughput and delay (1 slot)</em>.</p>


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<img alt="urn:mars:dn02259642:1:en:global:cgm_fixed:data:data" border="0" src="NED?action=retrieve&amp;identifier=dn02259642&amp;edition=1&amp;language=en&amp;coverage=global&amp;encoding=gif&amp;component=data&amp;item=data" /><p class="figure-caption">Figure: Offered load vs. throughput and delay (3 slot)</p>
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<p>As a result, it can be anticipated that the data rates
achieved with multi-slot operation often fall short of those possible in theory.
The high rates are available during low-load conditions, but often reduced
at medium to high load levels.</p>
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