为什么总是出现Ext未定义的错误！高手赐教呀！

<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml">
<head>
<meta http-equiv="Content-Type" content="text/html; charset=iso-8859-1"/>
    <title></title>
</head>
<body>  
 </table>
</body>
 <script language="javascript" type="text/javascript">
      Ext.onReady(function () {
         
                          Ext.Ajax.request({
                          url: 'core/WebService.asmx/Time',
                          headers: { 'Content-Type': 'application/json; charset=utf-8' },
                          method: 'POST',
                          jsonData: { ID:id},
                          success: function (response) {
                          }
                        failure: function () { Ext.Msg.alert("错误"); }              });
          
      });
       
    </script>
</html>