NetAccessorException

[Neil]

Hello Everyone,
While parsing my VXML files using Xerces i am getting
"NetAccessorException"
The error Message is as follows:

Message: An exception occurred! Type:NetAccessorException,
Message:Could not
open file: http://www.w3.org/TR/voicexml21/grammar-core.xsd
Fatal Error at file "http://www.w3.org/TR/voicexml21/vxml-grammar-
restriction.xsd",
line 28, column 52 Message: Fatal error encountered during schema
scan

My VXML File is:

<?xml version="1.0" encoding="UTF-8"?>
<vxml version="2.1" xmlns="http://www.w3.org/2001/vxml"
xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
xsi:schemaLocation="http://www.w3.org/2001/vxml http://www.w3.org/TR/voicexml21/vxml.xsd">
<catch event="error.semantic">
<log>Pass Event<value expr="_event"/>caught.</log>
<prompt>Pass Event<value expr="_event"/>caught.</prompt>
</catch>
<catch>
<log>FAIL Event<value expr="_event"/>caught.</log>
<prompt>FAIL Event<value expr="_event"/>caught.</prompt>
</catch>
<var name="v1" expr="'mark'"/>
<form id="assigntest">
<block>
<prompt>Testing the assign tag with name case sensitive and a
semantic error should be thrown <value expr="V1"/>
</prompt>
<assign name="V1" expr="'mark'"/>
<if cond="V1=='mark'">
<log>FAIL</log>
<prompt>FAIL</prompt>
</if>
</block>
</form>
</vxml>
Please Help me out as soon as possible

Thanks in advance
~Neil