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+<?php
+# dvdisaster: English homepage translation
+# Copyright (C) 2004-2012 Carsten Gnörlich
+#
+# UTF-8 trigger: äöüß 
+#
+# Include our PHP sub routines, then call begin_page()
+# to start the HTML page, insert the header, 
+# navigation and news if appropriate.
+
+require("../include/dvdisaster.php");
+begin_page();
+?>
+
+<!-- Insert actual page content below -->
+
+<h3 class="top">The RS01, RS02 and RS03 methods</h3>
+
+dvdisaster contains three error correction methods.
+RS01 and RS02 are the existing and proven methods while RS03 is still
+under development.<p>
+
+<b>Comparison of the methods.</b>
+
+All three methods build on the same
+<a href="qa31.php">Reed-Solomon</a> error correction.
+They calculate error correction information for ISO images which
+is used to recover unreadable sectors if the disc becomes damaged afterwards.<p>
+
+The methods differ in the way the error correction information is stored:<p>
+
+<ul>
+<li>
+<a name="file"> </a>
+RS01 creates <b>error correction files</b> which are stored separately from 
+the image they belong to. Since data protection at the
+<a href="qa32.php#file">file level</a> is difficult,
+error correction files must be stored on media which are protected
+against data loss by dvdisaster, too.<p></li>
+
+<li>
+<a name="image"> </a>
+To apply the RS02 method an image is first created on hard disc using a
+CD/DVD writing software. Before the image is written on the medium,
+dvdisaster is used to <b>augment the image</b> with error correction data.
+Therefore the data to be protected and the error correction information
+are located at the same medium. Damaged sectors in the error correction
+information reduce the data recovery capacity, but do not make recovery
+impossible - a second medium for keeping or protecting the error correction
+information is not required.<p></li>
+</ul>
+
+RS03 is a further development of RS01 and RS02. It can create both
+error correction files and augmented images:
+<ul>
+<li>RS03 can distribute work over multiple processor cores and is therefore
+much faster than RS01/RS02 on modern hardware.</li>
+<li>RS03 <b>error correction files</b> are - contrary to RS01 - robust against
+damage. This should not delude you into careless handling of your error
+correction files though - the disadvantages of <a href="qa32.php#file">reading
+at the filesystem level</a> are still valid.</li>
+<li>RS03 <b>augmented images</b> do not require so-called master blocks
+holding important information. This makes RS03 a bit more robust, but also
+more restrictive: The augmented image must completely fill the medium now
+while the size of augmented image can be freely chosen in RS02.
+</li>
+</ul>
+The changes for parallel computation and higher robustness make RS03 a bit less
+space efficient, e.g. RS03 error correction data has slighly less error
+correction capacity than its RS01/RS02 counterpart with equal size.<p>
+
+<a name="table"> </a>
+<b>Comparison of error correction storage.</b><p>
+The following table summarizes the differences between 
+error correction files (RS01, RS03) and augmented images (RS02, RS03):<p>
+
+<table width="100%" border="1" cellspacing="0" cellpadding="5">
+<tr>
+<td class="w50p"><i>Error correction files</i></td>
+<td class="w50p"><i>Image augmented with error correction data</i></td>
+</tr>
+<tr valign="top">
+<td> 
+any possible redundancy can be chosen</td>
+<td> redundancy is limited by free space on medium<br>
+(= medium capacity - size of data image)</td>
+</tr>
+
+<tr valign="top">
+<td>already effective at 15% redundancy
+since error correction files are required to be free of damage</td>
+<td>requires more redundancy (recommended: 20-30%)
+to compensate defects in the error correction data</td> 
+</tr>
+
+<tr valign="top">
+<td>medium can be completely filled with data</td>
+<td>usable medium capacity is reduced by amount of error correction data</td> 
+</tr>
+
+<tr valign="top">
+<td> can be created for already existing media</td>
+<td> only applicable before writing the new medium since
+the image must be augmented with error correction information in advance</td>
+</tr>
+
+<tr valign="top">
+<td> separately storing the error correction file from user data
+strengthens data protection</td>
+<td> common storage of user data and error correction data may reduce error
+correction capacity</td>
+</tr>
+
+<tr valign="top">
+<td>Mapping between error correction files and media must be kept.
+Error correction files must be protected against damage.</td>
+<td>Easy one-medium solution; error correction information
+needs not to be cataloged or explicitly protected.</td></tr>
+
+<tr valign="top">
+<td> no compatibilty issues with play-back units</td>
+<td> media with augmented images may not play correctly on all units</td>
+</tr>
+</table><p>
+
+<!-- do not change below -->
+
+<?php
+# end_page() adds the footer line and closes the HTML properly.
+
+end_page();
+?>