diff --git a/openbsc/src/gprs/gprs_gmm.c b/openbsc/src/gprs/gprs_gmm.c
index 9d94c24..5f0a5fd 100644
--- a/openbsc/src/gprs/gprs_gmm.c
+++ b/openbsc/src/gprs/gprs_gmm.c
@@ -1753,7 +1753,7 @@
 	uint8_t req_qos_len, req_pdpa_len;
 	uint8_t *req_qos, *req_pdpa;
 	struct tlv_parsed tp;
-	uint8_t transaction_id = (gh->proto_discr >> 4);
+	uint8_t transaction_id = gsm48_hdr_trans_id(gh);
 	struct sgsn_ggsn_ctx *ggsn;
 	struct sgsn_pdp_ctx *pdp;
 	enum gsm48_gsm_cause gsm_cause;
@@ -1923,7 +1923,7 @@
 	msg = gprs_msgb_copy(_msg, __func__);
 	if (!msg) {
 		struct gsm48_hdr *gh = (struct gsm48_hdr *) msgb_gmmh(_msg);
-		uint8_t transaction_id = (gh->proto_discr >> 4);
+		uint8_t transaction_id = gsm48_hdr_trans_id(gh);
 
 		LOGMMCTXP(LOGL_ERROR, mmctx, "-> ACTIVATE PDP CONTEXT REQ failed copy.\n");
 		/* Send reject with GSM_CAUSE_INV_MAND_INFO */
@@ -1941,7 +1941,7 @@
 static int gsm48_rx_gsm_deact_pdp_req(struct sgsn_mm_ctx *mm, struct msgb *msg)
 {
 	struct gsm48_hdr *gh = (struct gsm48_hdr *) msgb_gmmh(msg);
-	uint8_t transaction_id = (gh->proto_discr >> 4);
+	uint8_t transaction_id = gsm48_hdr_trans_id(gh);
 	struct sgsn_pdp_ctx *pdp;
 
 	LOGMMCTXP(LOGL_INFO, mm, "-> DEACTIVATE PDP CONTEXT REQ (cause: %s)\n",
@@ -1962,7 +1962,7 @@
 static int gsm48_rx_gsm_deact_pdp_ack(struct sgsn_mm_ctx *mm, struct msgb *msg)
 {
 	struct gsm48_hdr *gh = (struct gsm48_hdr *) msgb_gmmh(msg);
-	uint8_t transaction_id = (gh->proto_discr >> 4);
+	uint8_t transaction_id = gsm48_hdr_trans_id(gh);
 	struct sgsn_pdp_ctx *pdp;
 
 	LOGMMCTXP(LOGL_INFO, mm, "-> DEACTIVATE PDP CONTEXT ACK\n");
diff --git a/openbsc/src/libmsc/gsm_04_08.c b/openbsc/src/libmsc/gsm_04_08.c
index 9e70ba9..1a1d6f2 100644
--- a/openbsc/src/libmsc/gsm_04_08.c
+++ b/openbsc/src/libmsc/gsm_04_08.c
@@ -3487,7 +3487,7 @@
 {
 	struct gsm48_hdr *gh = msgb_l3(msg);
 	uint8_t msg_type = gsm48_hdr_msg_type(gh);
-	uint8_t transaction_id = ((gh->proto_discr & 0xf0) ^ 0x80) >> 4; /* flip */
+	uint8_t transaction_id = gsm48_hdr_trans_id_flip_ti(gh);
 	struct gsm_trans *trans = NULL;
 	int i, rc = 0;
 
diff --git a/openbsc/src/libmsc/gsm_04_11.c b/openbsc/src/libmsc/gsm_04_11.c
index bf256c3..20d18a9 100644
--- a/openbsc/src/libmsc/gsm_04_11.c
+++ b/openbsc/src/libmsc/gsm_04_11.c
@@ -780,7 +780,7 @@
 {
 	struct gsm48_hdr *gh = msgb_l3(msg);
 	uint8_t msg_type = gh->msg_type;
-	uint8_t transaction_id = ((gh->proto_discr >> 4) ^ 0x8); /* flip */
+	uint8_t transaction_id = gsm48_hdr_trans_id_flip_ti(gh);
 	struct gsm_trans *trans;
 	int new_trans = 0;
 	int rc = 0;
diff --git a/openbsc/src/osmo-bsc_nat/bsc_ussd.c b/openbsc/src/osmo-bsc_nat/bsc_ussd.c
index 2905c85..e080905 100644
--- a/openbsc/src/osmo-bsc_nat/bsc_ussd.c
+++ b/openbsc/src/osmo-bsc_nat/bsc_ussd.c
@@ -409,7 +409,7 @@
 
 	proto = gsm48_hdr_pdisc(hdr48);
 	msg_type = gsm48_hdr_msg_type(hdr48);
-	ti = (hdr48->proto_discr & 0x70) >> 4;
+	ti = gsm48_hdr_trans_id_no_ti(hdr48);
 	if (proto != GSM48_PDISC_NC_SS)
 		return 0;