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<div class="div">
<h3>Traffic Analysis for the Gb interface</h3>

<p>Different applications and protocols generate packets of
variable lengths. For example, the maximum size of an IP packet is 65535 octets.
The LLC protocol limits the packet length that can be sent over the Gb interface: </p>


<ul>
<li><p>minimum packet length is 140 octets</p>
</li>
<li><p>maximum packet length is 1560 octets.</p>
</li>
</ul>


<p>The overhead size is strongly influenced by the configuration
and the packet size. For every fixed configuration, the overhead varies between
a minimum value and a maximum one. </p>


<p>The packet size (PS) also varies, and the possible values must be fixed
by the operator, and it is important to underline that this only includes
the IP layer. Packet size for user data can be calculated as:</p>


<p>USR<sub>PACKET</sub> = IP<sub>HEADER</sub> + USR<sub>DATA</sub></p>


<p>The overheads that can be added on the Gb are reported
in the table <em>Overhead minimum and maximum for the different
protocols layers</em>.</p>


<a name="dec17215334812" shape="rect"></a>
<div>
<p>Table: Overhead minimum and maximum for the different protocols layers</p>


<table border="1">
<thead>
<tr align="left" char="" charoff="50" valign="bottom">
<th colspan="1" rowspan="1" valign="top"><p>Configuration/layer</p>
</th>
<th colspan="1" rowspan="1" valign="top"><p>Overhead min (octets)</p>
</th>
<th colspan="1" rowspan="1" valign="top"><p>Overhead max (octets)</p>
</th>
</tr>
</thead>
<tbody>
<tr align="left" char="" charoff="50" valign="top">
<td colspan="1" rowspan="1"><p>(GSM 04.65) SNDCP</p>
</td>
<td colspan="1" rowspan="1"><p>3</p>
</td>
<td colspan="1" rowspan="1"><p>4</p>
</td>
</tr>
<tr align="left" char="" charoff="50" valign="top">
<td colspan="1" rowspan="1"><p>(GSM 04.64) LLC</p>
</td>
<td colspan="1" rowspan="1"><p>5</p>
</td>
<td colspan="1" rowspan="1"><p>40</p>
</td>
</tr>
<tr align="left" char="" charoff="50" valign="top">
<td colspan="1" rowspan="1"><p>(GSM 08.18) BSSGP</p>
</td>
<td colspan="1" rowspan="1"><p>12</p>
</td>
<td colspan="1" rowspan="1"><p>54</p>
</td>
</tr>
<tr align="left" char="" charoff="50" valign="top">
<td colspan="1" rowspan="1"><p>GSM 08.16) NS</p>
</td>
<td colspan="1" rowspan="1"><p>4</p>
</td>
<td colspan="1" rowspan="1"><p>4</p>
</td>
</tr>
<tr align="left" char="" charoff="50" valign="top">
<td colspan="1" rowspan="1"><p>FR</p>
</td>
<td colspan="1" rowspan="1"><p>6</p>
</td>
<td colspan="1" rowspan="1"><p>6</p>
</td>
</tr>
<tr align="left" char="" charoff="50" valign="top">
<td colspan="1" rowspan="1"><p>GbOH</p>
</td>
<td colspan="1" rowspan="1"><p>30</p>
</td>
<td colspan="1" rowspan="1"><p>108</p>
</td>
</tr>
</tbody>
</table>
</div>


<p>The overhead factor can be defined as:</p>


<p>Gb<sub>OHFACTOR</sub> = 1+ Gb<sub>OH</sub>/USR<sub>PACKETSIZE</sub></p>


<p>Figure <em>Percentage of the overhead in GPRS traffic
at the Gb interface</em> shows a curve of the percentage overhead as a function
of the packet size. Two curves are used for the values of OHmin and OHmax
respectively.</p>


<p>The distance between the two curves narrows down as the
packet size grows and the overhead percentage varies quite significantly according
to changes in the packet length. </p>


<a name="dec18218414330" shape="rect"></a>
<div>
<img alt="urn:mars:dn02263341:1:en:global:cgm_fixed:data:data" border="0" src="NED?action=retrieve&amp;identifier=dn02263341&amp;edition=1&amp;language=en&amp;coverage=global&amp;encoding=gif&amp;component=data&amp;item=data" /><p class="figure-caption">Figure: Percentage of the overhead in GPRS traffic at the Gb interface</p>
</div>


<p>The following figure can be used to calculate the overheads
involved with a given packet size.</p>


<a name="dec202811212" shape="rect"></a>
<div>
<img alt="urn:mars:dn02263353:1:en:global:tiff:data:data" border="0" src="NED?action=retrieve&amp;identifier=dn02263353&amp;edition=1&amp;language=en&amp;coverage=global&amp;encoding=gif&amp;component=data&amp;item=data" /><p class="figure-caption">Figure: Link between packet size and overhead values</p>
</div>


<div>
<h6>Example</h6>



<p>Consider the output of the following ping command with
32 octets of data:</p>


<ul>
<li><p>Data: 32 octets</p>
</li>
<li><p>ICMP: 8 octets</p>
</li>
<li><p>IP: 20 octets</p>
</li>
</ul>



<p>With this data, the packet size to be considered is 60
octets. The overhead percentage can vary between 60% (OH % min) and 216% (OH
% max).</p>
</div>


<a name="nov304119111" shape="rect"></a>
<div>
<h6>Example</h6>



<p>Consider the output of the following ping command with
484 octets of data:</p>


<ul>
<li><p>Data : 484 octets</p>
</li>
<li><p>ICMP : 8 octets	</p>
</li>
<li><p>IP : 20 octets</p>
</li>
</ul>



<p>With this data, the packet size to consider is 512 octets.
The overhead percentage can vary between 5.86% (OH % min) and 21.09% (OH %
max).</p>
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