XMLHTTP Request

Question

Hi

    I am working in AJAX and use a DropDownList at this time i face probleam in  XMLHTTP Request. Define XMLHTTP  request and use ?

xmlHttp = getXMLHTTP();  if (xmlHttp)  {              xmlHttp.onreadystatechange = doReadyStateChange;              xmlHttp.open("GET", requestUrl, true);              xmlHttp.send();
}