webservice ask for Username and Password For server insted of redirecting to login page

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i have a asp.net page which retrive all food item when click on a div .i done this using web service and $.ajax(). i m using form authentication and form authentication ticket method for my web.problem is when authentication expires web service ask for Username and Password.It does not redirect to login window as button click or link click redirect.If i open url for this page in two tab and sign out page in one tab,then try to access web service from next tab then also page ask for credentials for server


use authentication

<system.web>
        <authentication mode="Forms">
            <forms loginUrl="Index.aspx" protection="All" name=".OTS"></forms>
        </authentication>
    </system.web>

i want to redirect to login page when authentication is sign out. i am a beginner in Web service plz Help ,also sorry for my bad english