jQuery DataTables Initialization Server-Side

I've this code bellow to load contents from ashx handler wich generate the json to the table, but it doesn't work, it even don't call it, so this function is not doing a ajax call, only initializing the data table withou anything.

$('#tabela').dataTable({
 serverSide: true,
 ajax: {
  url: "handlers/ultimosTrabalhos.ashx",
  type: "GET"
 }
});

How can i call this in order to retrive the json data to the table?