var keepSessionTimeout = 3 * 60 * 1000 //every 5 minutes to be safe

setTimeout("keepSession();", keepSessionTimeout);

var req = null;

function keepSession() {
    var url = "/graphics/add.gif?" + Math.random();
    
    if (window.XMLHttpRequest) {
        req = new XMLHttpRequest();
    }
    else if (window.ActiveXObject) {
        req = new ActiveXObject("Microsoft.XMLHTTP");
    }

    req.open("GET", url, true);
    req.onreadystatechange = keepSessionCallback;
    req.send(null);
    setTimeout("keepSession();", keepSessionTimeout);
}

function keepSessionCallback() {
    if (req.readyState == 4) {
        if (req.status == 200) {
            //the page request was a success                                   
        }
    }
}