function sessionKeepAlive() {
 var wRequest = new Sys.Net.WebRequest();
 wRequest.set_url("/KeepAlive.aspx");
 wRequest.set_httpVerb("POST");
 wRequest.add_completed(sessionKeepAlive_Callback);
 wRequest.set_body();
 wRequest.get_headers()["Content-Length"] = 0;
 wRequest.invoke();
}

function sessionKeepAlive_Callback(executor, eventArgs)
{

}

window.setInterval('sessionKeepAlive()', 5 * 60 * 1000);