diff --git a/src/bts.cpp b/src/bts.cpp
index 3eb41d2..e72f4c7 100644
--- a/src/bts.cpp
+++ b/src/bts.cpp
@@ -40,6 +40,7 @@
 	#include <osmocom/core/stats.h>
 	#include <osmocom/gsm/protocol/gsm_04_08.h>
 	#include <osmocom/gsm/gsm_utils.h>
+	#include <osmocom/gsm/gsm0502.h>
 	#include <osmocom/gsm/gsm48.h>
 	#include <osmocom/core/gsmtap_util.h>
 	#include <osmocom/core/application.h>
@@ -1092,7 +1093,7 @@
 		trx_no, tbf->trx->arfcn, ts_no, tbf->ta());
 	plen = Encoding::write_immediate_assignment(&bts->trx[trx_no].pdch[ts_no],
 						    tbf, immediate_assignment, true, 125,
-						    (tbf->pdch[ts_no]->last_rts_fn + 21216) % GSM_MAX_FN,
+						    GSM_TDMA_FN_SUM(tbf->pdch[ts_no]->last_rts_fn, 21216),
 						    tbf->ta(), 7, false, 0,
 						    bts_get_ms_pwr_alpha(bts), bts->pcu->vty.gamma, -1,
 						    GSM_L1_BURST_TYPE_ACCESS_0);