function pingServer(){
    var url = "tool_sessionPing.jsp";
    var tStr = new Date().getTime();
    var dataStr = "t=" + tStr;
    Request.send(url, "POST", dataStr, pingServerCallBack);
}
function pingServerCallBack(resp){
    setTimeout ( "pingServer()", 900000 );
}
setTimeout ( "pingServer()", 900000 );