jquery - How to pass the xml to web service and get back the json reply

Question

url: '<spring:url value="/service/org/fixt/assignlist"/>',      
    mtype: "POST",
    postData: fxXML,
    datatype: "json",
    autoencode: true,

I am able to post xml using ajax call and get the reply back in json format. But now I don't know how to post this xml to the web service and get the json back using jqgrid.