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<div class="div">
<h3>EGPRS impacts on TCP performance</h3>

<a name="feb2231723332" shape="rect"></a>
<div class="topic">
<h4>Data
link bandwidth of EGPRS</h4>

<p>The dynamic sharing of resources and variable radio conditions
introduce variation to available link level bandwidth. In addition, the signalling
procedures on the<a href="NED?action=retrieve&amp;identifier=general_glossary&amp;edition=13&amp;language=en&amp;coverage=global&amp;encoding=xhtml_1_0&amp;component=data&amp;item=data&amp;pointer=id06713#id06713" onclick="openSmallPopup(event,'NED?action=retrieve&amp;identifier=general_glossary&amp;edition=13&amp;language=en&amp;coverage=global&amp;encoding=xhtml_1_0&amp;component=data&amp;item=data&amp;pointer=id06713#id06713');return false" shape="rect" target="_blank"> MAC</a> layer generate interruptions in
the data transfer and thus increase the round-trip delay on the upper layers.
Several mechanisms like delayed <a href="NED?action=retrieve&amp;identifier=general_glossary&amp;edition=13&amp;language=en&amp;coverage=global&amp;encoding=xhtml_1_0&amp;component=data&amp;item=data&amp;pointer=id011257#id011257" onclick="openSmallPopup(event,'NED?action=retrieve&amp;identifier=general_glossary&amp;edition=13&amp;language=en&amp;coverage=global&amp;encoding=xhtml_1_0&amp;component=data&amp;item=data&amp;pointer=id011257#id011257');return false" shape="rect" target="_blank">TBF</a> release
have been implemented to reduce the latency introduced by the MAC layer.</p>


<p>The asymmetry ratio between uplink and downlink is quite
low for typical MS multi slot classes, with 8-PSK uplink capability.</p>


<p>EGPRS Protocol overhead consists of the following fields:</p>


<a name="mar631648276" shape="rect"></a>
<div>
<table border="0">
<tbody>
<tr align="left" char="" charoff="50" valign="top">
<td colspan="1" rowspan="1"><p><a href="NED?action=retrieve&amp;identifier=general_glossary&amp;edition=13&amp;language=en&amp;coverage=global&amp;encoding=xhtml_1_0&amp;component=data&amp;item=data&amp;pointer=id09003#id09003" onclick="openSmallPopup(event,'NED?action=retrieve&amp;identifier=general_glossary&amp;edition=13&amp;language=en&amp;coverage=global&amp;encoding=xhtml_1_0&amp;component=data&amp;item=data&amp;pointer=id09003#id09003');return false" shape="rect" target="_blank">SNDCP</a> header</p>
</td>
<td colspan="1" rowspan="1"><p>4 bytes per IP packet</p>
</td>
</tr>
<tr align="left" char="" charoff="50" valign="top">
<td colspan="1" rowspan="1"><p><a href="NED?action=retrieve&amp;identifier=general_glossary&amp;edition=13&amp;language=en&amp;coverage=global&amp;encoding=xhtml_1_0&amp;component=data&amp;item=data&amp;pointer=id08982#id08982" onclick="openSmallPopup(event,'NED?action=retrieve&amp;identifier=general_glossary&amp;edition=13&amp;language=en&amp;coverage=global&amp;encoding=xhtml_1_0&amp;component=data&amp;item=data&amp;pointer=id08982#id08982');return false" shape="rect" target="_blank">LLC</a> headers</p>
</td>
<td colspan="1" rowspan="1"><p>6 bytes per LLC block</p>
</td>
</tr>
<tr align="left" char="" charoff="50" valign="top">
<td colspan="1" rowspan="1"><p>LLC length indicator in RLC</p>
</td>
<td colspan="1" rowspan="1"><p>2 bytes per LLC block</p>
</td>
</tr>
</tbody>
</table>
</div>


<p>The BSS10.5 ED uses MCS-1 or CS-1 synchronisation frame
for every 18th block, which reduces the maximum TCP throughput by 4% for MCS-9.
The table below summarises the maximum link bandwidth limited <a href="NED?action=retrieve&amp;identifier=general_glossary&amp;edition=13&amp;language=en&amp;coverage=global&amp;encoding=xhtml_1_0&amp;component=data&amp;item=data&amp;pointer=id06079#id06079" onclick="openSmallPopup(event,'NED?action=retrieve&amp;identifier=general_glossary&amp;edition=13&amp;language=en&amp;coverage=global&amp;encoding=xhtml_1_0&amp;component=data&amp;item=data&amp;pointer=id06079#id06079');return false" shape="rect" target="_blank">TCP</a>
throughput [kB/s] for MTU=1500.</p>


<a name="feb223171742" shape="rect"></a>
<div>
<p>Table: Maximum TCP Throughput per MCS for single slot</p>


<table border="1">
<thead>
<tr align="left" char="" charoff="50" valign="bottom">
<th colspan="1" rowspan="1" valign="top"><p>Modulation and Coding Scheme</p>
</th>
<th colspan="1" rowspan="1" valign="top"><p>Max. TCP Throughput for UL</p>


<p>(1 slot)</p>
</th>
<th colspan="1" rowspan="1" valign="top"><p>Max. TCP Throughput for DL</p>


<p>(2 slots)</p>
</th>
</tr>
</thead>
<tbody>
<tr align="left" char="" charoff="50" valign="top">
<td colspan="1" rowspan="1"><p>MCS-1 (8.8 kbit/s)</p>
</td>
<td colspan="1" rowspan="1"><p>1.0 kB/s</p>
</td>
<td colspan="1" rowspan="1"><p>2.0 kB/s</p>
</td>
</tr>
<tr align="left" char="" charoff="50" valign="top">
<td colspan="1" rowspan="1"><p>MCS-2 (11.2 kbit/s)</p>
</td>
<td colspan="1" rowspan="1"><p>1.3 kB/s</p>
</td>
<td colspan="1" rowspan="1"><p>2.6 kB/s</p>
</td>
</tr>
<tr align="left" char="" charoff="50" valign="top">
<td colspan="1" rowspan="1"><p>MCS-3 (14.8 kbit/s)</p>
</td>
<td colspan="1" rowspan="1"><p>1.7 kB/s</p>
</td>
<td colspan="1" rowspan="1"><p>3.4 kB/s</p>
</td>
</tr>
<tr align="left" char="" charoff="50" valign="top">
<td colspan="1" rowspan="1"><p>MCS-4 (17.6 kbit/s)</p>
</td>
<td colspan="1" rowspan="1"><p>2.0 kB/s</p>
</td>
<td colspan="1" rowspan="1"><p>4.0 kB/s</p>
</td>
</tr>
<tr align="left" char="" charoff="50" valign="top">
<td colspan="1" rowspan="1"><p>MCS-5 (22.4 kbit/s)</p>
</td>
<td colspan="1" rowspan="1"><p>2.5 kB/s</p>
</td>
<td colspan="1" rowspan="1"><p>5.1 kB/s</p>
</td>
</tr>
<tr align="left" char="" charoff="50" valign="top">
<td colspan="1" rowspan="1"><p>MCS-6 (29.6 kbit/s)</p>
</td>
<td colspan="1" rowspan="1"><p>3.3 kB/s</p>
</td>
<td colspan="1" rowspan="1"><p>6.7 kB/s</p>
</td>
</tr>
<tr align="left" char="" charoff="50" valign="top">
<td colspan="1" rowspan="1"><p>MCS-7 (44.8 kbit/s)</p>
</td>
<td colspan="1" rowspan="1"><p>5.0 kB/s</p>
</td>
<td colspan="1" rowspan="1"><p>10.0 kB/s</p>
</td>
</tr>
<tr align="left" char="" charoff="50" valign="top">
<td colspan="1" rowspan="1"><p>MCS-8 (54.4 kbit/s)</p>
</td>
<td colspan="1" rowspan="1"><p>6.1 kB/s</p>
</td>
<td colspan="1" rowspan="1"><p>12.0 kB/s</p>
</td>
</tr>
<tr align="left" char="" charoff="50" valign="top">
<td colspan="1" rowspan="1"><p>MCS-9 (59.2 kbit/s)</p>
</td>
<td colspan="1" rowspan="1"><p>6.7 kB/s</p>
</td>
<td colspan="1" rowspan="1"><p>13.0 kB/s</p>
</td>
</tr>
</tbody>
</table>
</div>
</div>


<a name="dec1441618301" shape="rect"></a>
<div class="topic">
<h4>GPRS and EGPRS TBF multiplexing</h4>

<p>If GPRS <a href="NED?action=retrieve&amp;identifier=general_glossary&amp;edition=13&amp;language=en&amp;coverage=global&amp;encoding=xhtml_1_0&amp;component=data&amp;item=data&amp;pointer=id05016#id05016" onclick="openSmallPopup(event,'NED?action=retrieve&amp;identifier=general_glossary&amp;edition=13&amp;language=en&amp;coverage=global&amp;encoding=xhtml_1_0&amp;component=data&amp;item=data&amp;pointer=id05016#id05016');return false" shape="rect" target="_blank">UL</a> TBF and EGPRS <a href="NED?action=retrieve&amp;identifier=general_glossary&amp;edition=13&amp;language=en&amp;coverage=global&amp;encoding=xhtml_1_0&amp;component=data&amp;item=data&amp;pointer=id04455#id04455" onclick="openSmallPopup(event,'NED?action=retrieve&amp;identifier=general_glossary&amp;edition=13&amp;language=en&amp;coverage=global&amp;encoding=xhtml_1_0&amp;component=data&amp;item=data&amp;pointer=id04455#id04455');return false" shape="rect" target="_blank">DL</a>
TBF have been allocated on the same timeslot, the USF signalling to the GPRS
TBF requires GMSK modulation on DL which may reduce the DL throughput for
the EGPRS TBF. However the S10.5 ED allocates the EGPRS and GPRS TBFs to different
territories whenever possible so this constraint is not valid if there are
separate territories for the GPRS and EGPRS TBFs. </p>


<p>On the single territory case the S10.5 ED allocates maximum
number of slots as per MS capability and try to allocate GPRS and EGPRS TBFs
to different timeslots whenever possible. So the larger the territory size
the less significant is the impact on performance. When size of territory
equals the sum of typical slot capabilities of GPRS and EGPRS MSs, the impact
is neglected.</p>
</div>


<a name="dec1441625462" shape="rect"></a>
<div class="topic">
<h4>Cell reselection</h4>

<p>The impact of Cell change depends on the type of cell change:</p>


<ul>
<li><p>RA+LAC update</p>
</li>
<li><p><a href="NED?action=retrieve&amp;identifier=general_glossary&amp;edition=13&amp;language=en&amp;coverage=global&amp;encoding=xhtml_1_0&amp;component=data&amp;item=data&amp;pointer=id08947#id08947" onclick="openSmallPopup(event,'NED?action=retrieve&amp;identifier=general_glossary&amp;edition=13&amp;language=en&amp;coverage=global&amp;encoding=xhtml_1_0&amp;component=data&amp;item=data&amp;pointer=id08947#id08947');return false" shape="rect" target="_blank">RA Update</a></p>
</li>
<li><p>Inter PCU Cell Reselection</p>
</li>
<li><p>Intra PCU Cell Reselection</p>
</li>
</ul>
</div>


<a name="feb2231726324" shape="rect"></a>
<div class="topic">
<h4>Round
Trip Delay</h4>

<p>TCP data transfer is very sensitive to the round-trip delay
of the end-to-end link. Therefore, one of the main targets in the end-to-end
optimisation is to minimise the overall delay. This concerns not only the <a href="NED?action=retrieve&amp;identifier=general_glossary&amp;edition=13&amp;language=en&amp;coverage=global&amp;encoding=xhtml_1_0&amp;component=data&amp;item=data&amp;pointer=id08942#id08942" onclick="openSmallPopup(event,'NED?action=retrieve&amp;identifier=general_glossary&amp;edition=13&amp;language=en&amp;coverage=global&amp;encoding=xhtml_1_0&amp;component=data&amp;item=data&amp;pointer=id08942#id08942');return false" shape="rect" target="_blank">RLC</a>/<a href="NED?action=retrieve&amp;identifier=general_glossary&amp;edition=13&amp;language=en&amp;coverage=global&amp;encoding=xhtml_1_0&amp;component=data&amp;item=data&amp;pointer=id06713#id06713" onclick="openSmallPopup(event,'NED?action=retrieve&amp;identifier=general_glossary&amp;edition=13&amp;language=en&amp;coverage=global&amp;encoding=xhtml_1_0&amp;component=data&amp;item=data&amp;pointer=id06713#id06713');return false" shape="rect" target="_blank">MAC</a> layer, which is probably the main
delay source in EGPRS data transfer, but also the terminal equipment on the
mobile side as well as the core network and the application server.</p>


<p>The data transmission time is only one part of the round-trip
delay. Other contributors include: </p>


<ul>
<li><p>MAC layer signalling delays</p>
</li>
<li><p>processing delays at the <a href="NED?action=retrieve&amp;identifier=general_glossary&amp;edition=13&amp;language=en&amp;coverage=global&amp;encoding=xhtml_1_0&amp;component=data&amp;item=data&amp;pointer=id014238#id014238" onclick="openSmallPopup(event,'NED?action=retrieve&amp;identifier=general_glossary&amp;edition=13&amp;language=en&amp;coverage=global&amp;encoding=xhtml_1_0&amp;component=data&amp;item=data&amp;pointer=id014238#id014238');return false" shape="rect" target="_blank">PCU</a> and MS.</p>
</li>
</ul>


<p>The Round Trip Time of EGPRS on TCP layer varies considerably
because of variation of link layer bandwidth. The obtainable <a href="NED?action=retrieve&amp;identifier=general_glossary&amp;edition=13&amp;language=en&amp;coverage=global&amp;encoding=xhtml_1_0&amp;component=data&amp;item=data&amp;pointer=id013460#id013460" onclick="openSmallPopup(event,'NED?action=retrieve&amp;identifier=general_glossary&amp;edition=13&amp;language=en&amp;coverage=global&amp;encoding=xhtml_1_0&amp;component=data&amp;item=data&amp;pointer=id013460#id013460');return false" shape="rect" target="_blank">RTT</a>
over the EGPRS bearer is approximately 1 second for a segment size of 1500
B.</p>


<p>S10.5ED supports neither NCCR or NACC, so a cell change
may cause a peak delay of several seconds. The figure <em>Simulated
Mean RTT versus C/I (TU3iFH)</em> illustrates TCP RTT versus C/I for MTU=1500
at TU3iFH. The used congestion window was 2 for MCS-1 and 9 for MCS-9.</p>


<a name="mar631656198" shape="rect"></a>
<div>
<img alt="urn:mars:dn03316865:1:en:global:cgm_fixed:data:data" border="0" src="NED?action=retrieve&amp;identifier=dn03316865&amp;edition=1&amp;language=en&amp;coverage=global&amp;encoding=gif&amp;component=data&amp;item=data" /><p class="figure-caption">Figure: Simulated Mean RTT versus C/I (TU3iFH)</p>
</div>
</div>


<a name="feb2231729215" shape="rect"></a>
<div class="topic">
<h4>Bandwidth
delay product</h4>

<p>The bandwidth-delay product of the EGPRS link is rather
high, around ten kilobytes. EGPRS increases the data transmission capability,
that is, bandwidth of the radio link, but the round-trip delay of the link
is still rather long.</p>


<p>Large bandwidth-delay product requires large window sizes
on the TCP layer, which in turn sets high buffering requirements at the <a href="NED?action=retrieve&amp;identifier=general_glossary&amp;edition=13&amp;language=en&amp;coverage=global&amp;encoding=xhtml_1_0&amp;component=data&amp;item=data&amp;pointer=id08949#id08949" onclick="openSmallPopup(event,'NED?action=retrieve&amp;identifier=general_glossary&amp;edition=13&amp;language=en&amp;coverage=global&amp;encoding=xhtml_1_0&amp;component=data&amp;item=data&amp;pointer=id08949#id08949');return false" shape="rect" target="_blank">SGSN</a>, PCU and MS.</p>
</div>


<a name="feb2231729456" shape="rect"></a>
<div class="topic">
<h4>Packet
(SDU) Loss</h4>
</div>


<p>TCP protocol slows down the transmission rate each time
a packet loss is detected by the TCP transmitter. Therefore, it is crucial
to avoid bit errors in the radio transmission as well as packet discarding
in the network elements. This requires RLC ACK mode and adequate buffering
capability at the SGSN, PCU and MS. </p>


<p>The <a href="NED?action=retrieve&amp;identifier=general_glossary&amp;edition=13&amp;language=en&amp;coverage=global&amp;encoding=xhtml_1_0&amp;component=data&amp;item=data&amp;pointer=id011623#id011623" onclick="openSmallPopup(event,'NED?action=retrieve&amp;identifier=general_glossary&amp;edition=13&amp;language=en&amp;coverage=global&amp;encoding=xhtml_1_0&amp;component=data&amp;item=data&amp;pointer=id011623#id011623');return false" shape="rect" target="_blank">PDP</a> Context related Rel99 QoS attributes
can be used to select different operation modes for RLC and LLC protocol layers.</p>


<p>The 12bit <a href="NED?action=retrieve&amp;identifier=general_glossary&amp;edition=13&amp;language=en&amp;coverage=global&amp;encoding=xhtml_1_0&amp;component=data&amp;item=data&amp;pointer=id02660#id02660" onclick="openSmallPopup(event,'NED?action=retrieve&amp;identifier=general_glossary&amp;edition=13&amp;language=en&amp;coverage=global&amp;encoding=xhtml_1_0&amp;component=data&amp;item=data&amp;pointer=id02660#id02660');return false" shape="rect" target="_blank">CRC</a> error detection scheme for RLC
may pass some errors to LLC layer at low C/I or Es/No ratios. In unACK mode
the LLC can detect the errors with 24bit CRS but not correct them.</p>


<p>The resulting TCP/IP packet loss in EGPRS non ACK LLC mode
is typically of the order 10<sup>-3</sup> … 10<sup>-5</sup>. </p>


<a name="mar631657159" shape="rect"></a>
<div>
<img alt="urn:mars:dn03316877:1:en:global:cgm_fixed:data:data" border="0" src="NED?action=retrieve&amp;identifier=dn03316877&amp;edition=1&amp;language=en&amp;coverage=global&amp;encoding=gif&amp;component=data&amp;item=data" /><p class="figure-caption">Figure: Simulated SDU error probability because of bit errors passed by RLC
(TU3iFH)</p>
</div>
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