Jquery response when retrieving Data

Jquery is getting server side data using any of the methods ($.getJSON , $.ajax or $.get) when data is retrieving many sites show some GIf or animation even some show the exact % prgress bar (yahoo email uploading , downloading) how can we encorporate this using jquery how can we know the content lenth dat is coming so that i can put some progress bar or gif animation until the data is complete retrieved.

According to need,

function GetCurrentUser() { 
 
    $.ajax({ 
        type: "POST", 
        url: "TestService.asmx/GetCurrentUser", 
        contentType: "application/json; charset=utf-8", 
        dataType: "json", 
        success: onSuccessGetCurrentUser, 
        error: onError, 
        beforeSend: startingAjax, 
        complete: ajaxCompleted, 
        cache: false 
    }); 
} 
function onSuccessGetCurrentUser(data, status) { 
 
    if (data.d != "") { 
 
        alert('Employee id: ' + data.d); 
    } 
} 
function onError(request, status, error) { 
 
    var msg = 'Error occoured'; 
    alert('Error: ' + msg); 
} 
function startingAjax() { 
 
    $('#imgLoading').show(); 
} 
function ajaxCompleted() { 
 
    $('#imgLoading').hide(); 
}

Where beforeSend is the function to show gif image and complete is the function when your data return in json format.