How to handle ajax call when there is no response from the server

You can handle something like this

if ( request.readyState == 4 ){  // 4 is "complete" 
  if ( request.status == 200 ){
    // HTTP OK, carry out your normal Ajax processing
    // ... 
  }else{
    // something went wrong, report the error
    error( "HTTP "+request.status+".  An error was »
             encountered: "+ request.statusText );
  }
}

(or)

$.ajax({
    type: "post", 
    url: "somepage.html",
    success: function (data, text) {
        //...
    },
    error: function (request, status, error) {
        alert(request.responseText);
    }
});

when you send a request to a server. a connection is opened and its kept open unless the server responds.
1.if due to some error on the server side it cannot respond then a response code of 5xx is sent back generally (503)
2.if due to some connection issues the connection is terminated prematurely then also jquery would take that as an error.

1.so if you wanna wait for the server to send a request or connection termination (which ever occurs earlier) then u can use the completed option in the jquery ajax.
2.and if you are in a condition in which server isnt responding even after 20 secs and you think that it should have responded by now use timeout.
3.finally if your problem is that you are using some kind of customized(hand made http server) which doesn't end a request even if it encounters some error then atleast customize it enough so that it sends back some response code(because this is HTTP model of request and response)