diff --git a/include/grgsm/flow_control/burst_sdcch_subslot_filter.h b/include/grgsm/flow_control/burst_sdcch_subslot_filter.h
index 150d76a..5bcf829 100644
--- a/include/grgsm/flow_control/burst_sdcch_subslot_filter.h
+++ b/include/grgsm/flow_control/burst_sdcch_subslot_filter.h
@@ -54,6 +54,13 @@
        * creating new instances.
        */
       static sptr make(subslot_filter_mode mode, unsigned int subslot);
+
+      /* External API */
+      virtual unsigned int get_ss(void) = 0;
+      virtual unsigned int set_ss(unsigned int ss) = 0;
+
+      virtual subslot_filter_mode get_mode(void) = 0;
+      virtual subslot_filter_mode set_mode(subslot_filter_mode mode) = 0;
     };
   } // namespace gsm
 } // namespace gr