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<h1>Overview of Priority Class Based Quality of Service (QoS)</h1>

<p>Initially, all temporary block flows (GPRS calls) have
the same priority: all users and applications receive the same service level.
However, the needs of different applications differ and therefore mechanisms
to create separate service levels are required. Priority Class Based Quality
of Service is introduced as the first step towards QoS.</p>


<p>The GSM specifications define a Quality of Service (QoS) functionality,
which gives a possibility to differentiate temporary block flows (TBFs) by
delay, throughput, and priority. With Priority Based Scheduling, an operator
can give users different priorities. Higher priority users will get better
service than lower priority users. There will be no extra blocking to any
user, only the experienced service quality changes.</p>


<p>The concept of ‘Priority Class’ is introduced at system
level. It is based on a combination of the GPRS Delay class and GPRS Precedence
class values. Packets will be evenly scattered within the (E)GPRS territory
between different time slots. After that packets with a higher priority are
sent before packets that have a lower priority.</p>


<p>Mobile-specific flow control is part of the QoS solution
in the PCU. It works together with the SGSN to provide a steady data flow
to the mobile from the network. It is also an effective countermeasure against
buffer overflows in the PCU. Mobile-specific flow control is performed for
every MS that has a downlink TBF. There is no uplink flow control.</p>


<p>The PCU receives the QoS information to be used in DL TBFs from the
SGSN in a DL unitdata PDU. It includes Precedence class information which
indicates the priority of the PDU.</p>


<p>In case of UL TBF, the MS informs its radio priority in a PACKET CHANNEL
REQUEST (PCR) or a PACKET RESOURCE REQUEST (PRR), and this is used for UL
QoS. In the UL direction, the PCU uses the radio priority received from the
MS. Exceptions to this rule are one phase access and single block requests;
in these cases the PCU always uses Best Effort priority.</p>


<p>Priority Class Based Quality of service is an operating
software in the BSC and is always active in an active PCU. The subscriber
priority must be defined in the HLR once Priority Class Based QoS is taken
into use.</p>


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<h2>Priority based scheduling algorithm</h2>

<p>The priority based scheduling algorithm hands out radio
resources according to the latest service time and scheduling step size of
the TBFs. Each TBF allocated to a timeslot has a timeslot-specific latest
service time, before which the TBF should get a chance to use the radio resource.
In each scheduling round, the TBF with the lowest service time is selected.
After the TBF has sent a radio block, its latest service time is incremented
by a predefined scheduling step size. The higher the scheduling step size,
the less often the TBF is selected and given a transmission turn.</p>


<p>In BSS9 (Nokia GPRS Release 1) the scheduling steps of all TBFs are
set to the same constant value. In the BSS10.5 release the step sizes depend
on the priority class of the TBF: each priority class has its own scheduling
step size that can be adjusted by the operator. There are 4 QoS classes for
uplink and 3 QoS classes for downlink. Each service class is given a fair
amount of radio time. The best effort customers are an exception to the rule
and are only given a small share of the radio interface.</p>


<p>The allocation process is designed to ensure that better priority TBFs
are not gathered into the same radio timeslot. TBFs in the same time slot
that have the same QoS get an equal share of air time. However, equal air
time does not provide equal data rates for the TBFs in the same time slot,
it only guarantees that inside a QoS group the air time is divided equally
and that a higher QoS class gets more air time.</p>


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<img alt="urn:mars:dn0469623:1:en:global:tiff:data:data" border="0" src="NED?action=retrieve&amp;identifier=dn0469623&amp;edition=1&amp;language=en&amp;coverage=global&amp;encoding=gif&amp;component=data&amp;item=data" /><p class="figure-caption">Figure: Example of transmission turns </p>
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<p>For information on planning Priority Class Based Quality of Service,
see <em>(E)GPRS TBF allocation</em> and <em>HLR and throughput optimisation
for EGPRS</em>.</p>
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