Asynchronous postback is still a full form postback? The amount of data sent?

Question

I am an expereinced Java programmer and I am learning ASP.NET and ASP.NET Ajax now.

I believe the asynchronous postback in ASP.NET Ajax is still a full form postback. Is this correct? I did experiments about this. I have a web form with two independent UpdatePanels, each of which has ASP.NET controls. Besides, the web form also has controls in outside the two UpdatePlanels. At the time of asynchronous postback originated from a UpdatePanel, the lastest value of controls outside the UpdatePanel is also available on the server-side. I feel this is proof.

If an asynchronous postback is still a full form postback, then the amount of data sent to a server is the sames as for a normal postback, no reduction. However, I am not sure about the amount of data sent to a browser from the server as response to an asynchronous postback. Is the amount of data same as from a normal response or just the data needed to update the UpdatePanel that initiated the asynchronous postback? I know I can update the content of the UpdatePanel in server-side code. Does ASP.NET Ajax send data (no changes) for another UpdatePanel down the wire too?

Thanks!