Using server side processing on datatable

Hi,
I am fairly new to datatable and trying to explore. I was able to get client side datatable woking. However we have large dataset to be used hence I am trying to explore server side processing.
Our server side is a struts based java servlet application application.following are my questions.
1) When we are using server side processing how do the parameters like iDisplayStart,sSearch get passed to server ? Do thye get passed automatically or we have to pass explicitly though some code ?
2) I am trying to understand the diffrence between fnServerData and sAjaxSource . if our server side is a servlet what do I need to set these values to ?

Any sample code will be appreciated.
vj