diff --git a/include/grgsm/flow_control/burst_timeslot_filter.h b/include/grgsm/flow_control/burst_timeslot_filter.h
index f23fcf7..2f16b77 100644
--- a/include/grgsm/flow_control/burst_timeslot_filter.h
+++ b/include/grgsm/flow_control/burst_timeslot_filter.h
@@ -48,6 +48,10 @@
        * creating new instances.
        */
       static sptr make(unsigned int timeslot);
+
+      /* External API */
+      virtual unsigned int get_tn(void) = 0;
+      virtual unsigned int set_tn(unsigned int tn) = 0;
     };
   } // namespace gsm
 } // namespace gr