jQuery Grid Plugin - jqGrid

Hi,

In My Asp.net project

I am not able to sort on  the columns

function successFunction(jsondata) {
 
            var jsonObj = JSON.parse(jsondata.d);
 
            var stringObj = "{total:'',page:'',records:'',rows:[";
            for (var itr = 0; itr < jsonObj.length; itr++) {
                stringObj += "{ id:'" + itr + "',";
                stringObj += " cell:[";
                stringObj += "'" + jsonObj[itr].Empid + "',";
                stringObj += "'" + jsonObj[itr].Name + "',";
                stringObj += "'" + jsonObj[itr].Age + "'";
                stringObj += "]";
                if (itr == jsonObj.length - 1) {
                    stringObj += "}";
                }
                else {
                    stringObj += "},";
                }
            }
            stringObj += " ]}";
 
 
            var thegrid = jQuery("#JqGridBody")[0];
            thegrid.addJSONData(JSON.parse(stringObj));
 
        }
 
        function getProducts() {
            $.ajax({
                url: "Default.aspx/GetJsonDataFromService",
                data: "{}",
                dataType: "json",
                type: "POST",
                async: false,
                contentType: "application/json",
                success: successFunction,
                error: function (data) { alert(data); }
            });
        }
 
        function LoadJqGrid() {
 
            jQuery("#JqGridBody").jqGrid({
                datatype: getProducts,
                colNames: colHeader,
                colModel: colBody,
                rowNum: 10,
                rowList: [5, 10, 20, 50, 100],
                jsonReader:
                {
                    root: "rows",
                    page: "page",
                    total: "total",
                    records: "records",
                    repeatitems: true,
                    cell: "cell",
                    id: "id",
                    userdata: "userdata",
                    subgrid:
                    {
                        root: "rows",
                        repeatitems: true,
                        cell: "cell"
                    }
                },
                pager: '#JqGridPager',
                sortorder: "desc",
                multiselect: false,
                viewrecords: true,
                treeGrid: true,
                ExpandColumn: 'name',
                caption: "Json Data"
        })
        .filterToolbar()           
        .navGrid('#JqGridPager', { edit: true, add: true, del: true });
 
    }