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<h2>SNDCP/LLC and throughput optimisation for EGPRS</h2>

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<h3>LLC
Un-ACK mode</h3>

<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> un-ACK mode was typically used
for GPRS for Best Effort. In some cases LLC-ACK is recommended instead of
LLC-unACK for Best Effort.</p>


<p>If LLC ACK mode is used, the sufficient LLC window size
needs to be ensured. The relation between LLC window size, <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>
and throughput is as with TCP window size. It is recommended to have a large
LLC Transmit Window. The optimal value in case of MSS=1460 is 10kB.</p>
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<h3>LLC
frame size</h3>

<p>A large LLC frame size (N201-U = 1520 B) is recommend to
be used with EGPRS.</p>


<p>LLC frame size is negotiated between the MS and the SGSN.</p>
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<h3>Header
Compression</h3>

<p>TCP/IP header compression is not recommended with LLC un-ACK
mode. This is because a packet header can be decompressed only if the previous
packet has been received successfully. Therefore, after a single packet loss,
all the following packets are discarded until an un-compressed packet is received
which initialises the compression algorithm.</p>
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<h3>Lifetime
Value for LLC</h3>

<p>A large lifetime value is needed for the LLC frames in
order to avoid packet discarding. 20 sec. is an optimal value.</p>
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<h3>Payload
Compression (V.42bis)</h3>

<p>The payload compression may provide 3-folded throughput
for html or text formatted content.</p>
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