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<div class="div">
<h1>High Speed Circuit Switched Data (HSCSD)</h1>

<p>The high speed circuit switched data (HSCSD) feature provides
accelerated data rates for end-user applications. A current trend is for an
increased demand for high data rate applications like the World Wide Web (www),
file transfer, and facsimile.</p>


<p>The BSS implementation is to reserve multiple sets of basic
(current) resources for one high speed data call. The data rate and number
of reserved time slots vary  between one and user application defined maximum.
The variable rate is needed, for example, for handovers to a new cell if the
requested data rate cannot be given right away. The BSS implementation of
HSCSD supports a simultaneous usage of 4 time slots per HSCSD call.</p>


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<p>Table: Corresponding maximum data rates with different channel codings</p>


<table border="1">
<thead>
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<th colspan="1" rowspan="1" valign="top"><p>RTSLs</p>
</th>
<th colspan="1" rowspan="1" valign="top"><p>9.6 kbit/s</p>
</th>
<th colspan="1" rowspan="1" valign="top"><p>14.4 kbit/s</p>
</th>
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<td colspan="1" rowspan="1"><p>1</p>
</td>
<td colspan="1" rowspan="1"><p>9.6 kbit/s</p>
</td>
<td colspan="1" rowspan="1"><p>14.4 kbit/s </p>
</td>
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<td colspan="1" rowspan="1"><p>2</p>
</td>
<td colspan="1" rowspan="1"><p>19.2 kbit/s</p>
</td>
<td colspan="1" rowspan="1"><p> 28.8 kbit/s</p>
</td>
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<td colspan="1" rowspan="1"><p>3</p>
</td>
<td colspan="1" rowspan="1"><p>28.8 kbit/s</p>
</td>
<td colspan="1" rowspan="1"><p>43.2 kbit/s</p>
</td>
</tr>
<tr align="left" char="" charoff="50" valign="top">
<td colspan="1" rowspan="1"><p>4</p>
</td>
<td colspan="1" rowspan="1"><p>38.4 kbit/s</p>
</td>
<td colspan="1" rowspan="1"><p>57.6 kbit/s</p>
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<p>Both asynchronous and synchronous bearer services and transparent
and non-transparent data services are supported. The transparent HSCSD uses
a fixed data rate through the call but non-transparent is flexible, and the
data rate can be changed during the call, for example, due to a traffic situation.
The radio interface is either symmetric or asymmetric according to MS capability.
However, Abis- and A interfaces are always symmetric.</p>


<p>Support for new A interface circuit pool(s) is required
at the MSC. New BSC supported HSCSD pools can handle all existing connection
types and either maximum of two or four channel HSCSD connections. HSCSD pools
are supported with and without 14.4 kbit/s channel coding capability.</p>


<p>In a basic channel allocation, consecutive timeslots are
kept free for multichannel HSCSD connection, if possible. If there are not
enough appropriate free channels to fulfil the requested data rate, the non-transparent
HSCSD connection is started with fewer channels than needed. By the resource
upgrade procedure, the data rate of the HSCSD connection has increased when
appropriate channel is freed from other connection. If a transparent connection
cannot be established in a cell, the DR to another cell can be tried.</p>


<p>At least one channel is allocated for a non-transparent
HSCSD call request if there is not a total congestion in the cell. In a congested
cell, the HSCSD load can be adjusted by parameterisation. The resource downgrade
procedure is used to lower the HSCSD connection data rate to give radio channels
for other connections. Due to the nature of the transparent service, the blocking
for the transparent service is higher compared to the non-transparent service.</p>


<p>This feature provides a fast way of transferring a huge
amount of data within a short time.</p>


<p>The pre-emption procedure is applicable for transparent
HSCSD connections only when it is a target for a pre-emption release. If the
target of the pre-emption is a non-transparent HSCSD connection with more
than one channel, the handover or release is changed to a resource downgrade.</p>
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