Communication between js client and asp.net server

I write web site in VS 2008. I use extjs for make side with gridView and button. Now I want to send request to server. I add event to click on button:

    function getData()
    {
        var xmlhttp;
        if (window.XMLHttpRequest)
        {
            xmlhttp = new XMLHttpRequest();
        } else
        {
            xmlhttp = new ActiveXObject("Mic开发者_高级运维rosoft.XMLHTTP");
        }

        xmlhttp.open("GET", "Default.aspx", true);
        xmlhttp.setRequestHeader("data", "true");
        xmlhttp.send();
    }

And my question is how can I receive this request on server? I found in the Internet that I should to do it like this:

protected void Page_Load(object sender, EventArgs e)
{
    if (Request.Headers["data"] != null)
    {
        // do something
    }
}

but it doesn't work. I don't understand it, if I use JS page doesn't reload, so why I must check it in method page_load?

Client side:

For sending GET-request use :

xmlhttp.open("GET", "Default.aspx?data=true");

Server side:

Use HttpRequest.QueryString for getting params.