var LoadedAt = new Date(); //when the page was loaded
var LoginIFrame = null;

function refreshSession() {
  if ( (new Date()).valueOf() - LoadedAt.valueOf() < 60*60*1000) { //stop refreshing after 30 minutes
    if (LoginIFrame) {
      document.body.removeChild(LoginIFrame);
    }
    LoginIFrame = document.createElement("iframe");
    LoginIFrame.style.display = "none";
    LoginIFrame.src="refresh.aspx"; //ping the webserver to restart another 2 minute session
    document.body.appendChild(LoginIFrame);
    window.setTimeout("refreshSession()",1.6*60*1000); //try again in 1.6 minutes
  }
}
window.setTimeout("refreshSession()",1.6*60*1000); //1.6 minutes