var timeout = 3 * 60 * 1000;

function getHTTPObject()
{
	if ( window.XMLHttpRequest )		// Gecko
		return new XMLHttpRequest() ;
	else if ( window.ActiveXObject )	// IE
		return new ActiveXObject("MsXml2.XmlHttp") ;
}
var http = getHTTPObject();
function pingServer()
{
	try{
		http.open("GET", location.protocol+'//'+location.hostname+'/index.php', false);
		http.send(null);
	}catch (e){}

	setTimeout(pingServer, timeout);
}

setTimeout(pingServer, timeout);