Beginner's Question - Schema Validation Problem

I am trying to validate a schema and getting the error "Element 'qos' is used but not declared in DTD/Schema". 'qos' is my root element. I have looked at several books and at similar messages on this list with no luck.

This is how I am declaring it in my XML file:

<qos
     xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
     xsi:noNamespaceSchemaLocation="qos.xsd"
     service_key="d5921160-3e16-11d5-98bf-002035229c64">

    ...............

</qos> 

And this is how it is declared in the schema file ('qos.xsd' located at the same directory as the XML file):

<xsd:schema xmlns:xsd="http://www.w3.org/2001/XMLSchema">

    .............................

     <xsd:element name="qos" maxOccurs="1">
          <xsd:complexType>
               <xsd:element ref="performance" minOccurs="0"    maxOccurs="1"/>
           <xsd:element ref="information" minOccurs="0" maxOccurs="1"/>
           <xsd:element ref="configuration" minOccurs="0" maxOccurs="1"/>
           <xsd:element ref="security" minOccurs="0" maxOccurs="1"/>   
          </xsd:complexType>
      <xsd:attribute ref="service_key" use="required"/>
 </xsd:element>

</xsd:schema>

I have tried qualifying the path of the xsd file as it was explained in a previous message and got the same result. I am using the XRay editor from Architag.

I would appreciate any help and guidance.

Thank you.

Ayman