My android banner ad not showing?

this my main activity xml code i dont know whats wrong with this please help me anyone?

<!-- Framelayout to display Fragments -->
<FrameLayout
    android:id="@+id/frame_container"
    android:layout_width="match_parent"
    android:layout_height="match_parent" />

 <com.google.android.gms.ads.AdView
    android:id="@+id/adView"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:layout_centerHorizontal="true"
    android:layout_alignParentBottom="true"
    ads:adSize="BANNER"
    ads:adUnitId="@string/banner_ad_unit_id">
</com.google.android.gms.ads.AdView>

<!-- Listview to display slider menu -->
<ListView
    android:id="@+id/list_slidermenu"
    android:layout_width="240dp"
    android:layout_height="match_parent"
    android:layout_gravity="start"
    android:choiceMode="singleChoice"
    android:divider="@color/list_divider"
    android:dividerHeight="1dp"        
    android:listSelector="@drawable/list_selector"
    android:background="@color/list_background"/>

i added all permissions also in manifest too