Is it possible to get viewstate only for related update panel controls in asp.net ajax framework. I know that the browser sends the entire viewstate during post back, even if we don't need the whole viewstate for ajax calls.
No, It is Immpossible. this is how asp.net ajax works. you should probably use jquery ajax instead
Look at this excellent Codeplex case study that also used the UpdatePanel and suffered from a bloated ViewState.
http://encosia.com/2008/12/03/have-you-ever-wondered-why-codeplex-is-so-slow/