var start = function() {
  request = getHTTPObject();
  if (!request || !url) {
   return false;
  } else {
   initiateRequest();
   return true;
  }
 };

 var getHTTPObject = function() {
  var xmlhttp = false;
  if (window.XMLHttpRequest) {
   xmlhttp = new XMLHttpRequest();
  } else if(window.ActiveXObject) {
   try {
    xmlhttp = new ActiveXObject("Msxml2.XMLHTTP");
   } catch (e) {
    try {
     xmlhttp = new ActiveXObject("Microsoft.XMLHTTP");
    } catch (e) {
     xmlhttp = false;
    }
   }
  }
  return xmlhttp;
 };

 var initiateRequest = function() {
  if (loading) {
   loading();
  }
  request.onreadystatechange = completeRequest;
  if (data) {
   request.open("POST", url, true);
   request.setRequestHeader("Content-Type","application/x-www-form-urlencoded");
   request.send(data);
  } else {
   request.open("GET", url, true);
   request.send(null);
  }
 };

 var completeRequest = function() {
  if (request.readyState == 4) {
   if (request.status == 200 || request.status == 304) {
    if (canvas) {
     canvas.innerHTML = request.responseText;
    }
    if (callback) {
     callback();
    }
   }
  }
 };