jqGrid parameters. Can't be passed

I call web-service to fill jqGrid and wants to pass parameters to it

I use the following code (client side):

jQuery('#EmployeeTable').jqGrid({
    datatype: function () {
        var params = new Object();
        params.page = 10;
        params.Filter = true;
        params.DateStart = null;
        params.DateEnd = null;
        params.TagID = null;
        params.CategoryID = 3;
        params.StatusID = 1;
        params.IsDescription = true;
        $.ajax({
            url: '/Admin/IdeasJSON',
            type: "POST",
            contentType: "application/json; charset=utf-8",
            data: JSON.stringify(params),
            dataType: "json",
            success: function (data, st) {
                if (st == "success") {
                    var grid = $("#EmployeeTable")[0];
                    grid.addJSONData(data);
                }
            },
            error: function () {
                alert("Error with AJAX callback");
            }
        });
    },

also, there is propotype of web-method (MVC):

public JsonResult IdeasJSON(int? page, bool? Filter, DateTime? DateStart, DateTime? DateEnd, int? TagID, int? CategoryID, int? StatusID, bool? IsDescription)

Why all these parameters are null?

[ADDED 04/11]

    jQuery(document).ready(function () {
        var StatusID = null, Filter = null, page = null, DateStart = null, DateEnd = null, TagID = null, CategoryID = null, IsDescription = null;
        if (jQuery.url.param('StatusID') != null) {
            StatusID = jQuery.url.param('StatusID');
        }

        if (jQuery.url.param('Filter') != null) {
            Filter = jQuery.url.param('Filter');
        }

        if (jQuery.url.param('page') != null) {
            page = jQuery.url.param('page');
        }

        if (jQuery.url.param('DateStart') != null) {
            DateStart = jQuery.url.param('DateStart');
        }

        if (jQuery.url.param('DateEnd') != null) {
            DateEnd = jQuery.url.param('DateEnd');
        }

        if (jQuery.url.param('TagID') != null) {
            TagID = jQuery.url.param('TagID');
        }

        if (jQuery.url.param('CategoryID') != null) {
            CategoryID = jQuery.url.param('CategoryID');
        }

        if (jQuery.url.param('IsDescription') != null) {
            IsDescription = jQuery.url.param('IsDescription');
        }



        jQuery('#EmployeeTable').jqGrid({
            url: '/Admin/IdeasJSON',
            datatype: 'json',
            postData: { page: page, Filter: Filter, DateStart: DateStart, DateEnd: DateEnd, TagID: TagID, StatusID: StatusID, CategoryID: CategoryID, IsDescription: IsDescription },
            jsonReader: {
                page: "page",
                total: "total",
                records: "records",
                root: "rows",
                repeatitems: false,
                id: ""
            },
            colNames: ['Logged By', 'Logging Agency (ID)', 'Title', 'Status', 'Points', 'Categories', 'Created Date', 'Description', 'Jira ID#', 'Portal Name', '', '', '', '', '', '', ''],
            colModel: [
                        { name: 'LoggedBy', width: 100 },
                        { name: 'LoggingAgencyID', width: 85 },
                        { name: 'Title', width: 100 },
                        { name: 'Status', width: 100 },
                        { name: 'Points', width: 40, align: 'center' },
                        { name: 'Categories', width: 100 },
                        { name: 'CreatedDate', width: 80, formatter: ndateFormatter },
                        { name: 'Description', width: 300 },
                        { name: 'JiraID', width: 55 },
                        { name: 'PortalName', width: 100 },
                        { name: 'IdeaID', width: 25, formatter: ActionDescriptionFormatter },
                        { name: 'IdeaID', width: 25, formatter: ActionEditFormatter },
                        { name: 'IdeaID', width: 25, formatter: ActionDeleteFormatter },
                        { name: 'IdeaGridCommentsAndSubideas', width: 25, formatter: ActionIdeaGridCommentsAndSubideas },
                        { name: 'IdeaGridCountVotes', width: 25, formatter: ActionIdeaGridCountVotes },
                        { name: 'IdeaGridCountVotes', width: 25, formatter: ActionIdeaGridLinkIdeas },
                        { name: 'IdeaGr开发者_高级运维idCountVotes', width: 25, formatter: ActionIdeaGridIdeaType },
                    ],
            pager: '#EmployeeTablePager',
            width: 1000,
            viewrecords: true,
            caption: 'Idea List',
            excel: true
        }).jqGrid('navGrid', '#EmployeeTablePager', {
            add: false,
            edit: false,
            del: false,
            search: false,
            refresh: false
        }).jqGrid('navButtonAdd', '#EmployeeTablePager', {
            caption: " Export to Excel ",
            buttonicon: "ui-icon-bookmark",
            onClickButton: ReturnExcel,
            position: "last"
        }).jqGrid('navButtonAdd', '#EmployeeTablePager', {
            caption: " Export to CSV ",
            buttonicon: "ui-icon-bookmark",
            onClickButton: ReturnCSV,
            position: "last"
        });
    });

You should not use datatype as the function to use the JSON data. Probably you used template of very old examples.

For example in the "UPDATED" part of the answer you could find full demo project which demonstrate how to use jqGrid in ASP.MVC 2.0 inclusive paging, sorting and filtering/advanced searching.

If you want post some additional parameters to the server as a part of jqGrid request you should use postData parameters in the form postData: {CategoryID: 3, StatusID: 1, IsDescription:true, Filter: true}