diff --git a/source/MAC/IEEE802_15_4/mac_mcps_sap.c b/source/MAC/IEEE802_15_4/mac_mcps_sap.c
index 07a9e65da18..b62fead6746 100644
--- a/source/MAC/IEEE802_15_4/mac_mcps_sap.c
+++ b/source/MAC/IEEE802_15_4/mac_mcps_sap.c
@@ -1395,6 +1395,15 @@ static void mcps_generic_sequence_number_allocate(protocol_interface_rf_mac_setu
 }
 
 
+static uint32_t mcps_generic_backoff_calc(protocol_interface_rf_mac_setup_s *rf_ptr)
+{
+    uint32_t random_period = mac_csma_backoff_get(rf_ptr);
+    if (rf_ptr->fhss_api) {
+        return mcps_calculate_tx_time(rf_ptr, random_period);
+    }
+    return random_period;
+}
+
 static int8_t mcps_generic_packet_build(protocol_interface_rf_mac_setup_s *rf_ptr, mac_pre_build_frame_t *buffer)
 {
     phy_device_driver_s *dev_driver = rf_ptr->dev_driver->phy_driver;
@@ -1411,6 +1420,7 @@ static int8_t mcps_generic_packet_build(protocol_interface_rf_mac_setup_s *rf_pt
         tx_buf->len = buffer->mac_payload_length;
 
         memcpy(ptr, buffer->mac_payload, buffer->mac_payload_length );
+        buffer->tx_time = mcps_generic_backoff_calc(rf_ptr);
         return 0;
     }
 
@@ -1466,13 +1476,7 @@ static int8_t mcps_generic_packet_build(protocol_interface_rf_mac_setup_s *rf_pt
 
     tx_buf->len = frame_length;
     uint8_t *mhr_start = ptr;
-
-    uint32_t random_period = mac_csma_backoff_get(rf_ptr);
-    if (rf_ptr->fhss_api) {
-        buffer->tx_time = mcps_calculate_tx_time(rf_ptr, random_period);
-    } else {
-        buffer->tx_time = random_period;
-    }
+    buffer->tx_time = mcps_generic_backoff_calc(rf_ptr);
 
     ptr = mac_generic_packet_write(rf_ptr, ptr, buffer);
 
@@ -1506,6 +1510,7 @@ static int8_t mcps_generic_packet_rebuild(protocol_interface_rf_mac_setup_s *rf_
         tx_buf->len = buffer->mac_payload_length;
 
         memcpy(ptr, buffer->mac_payload, buffer->mac_payload_length );
+        buffer->tx_time = mcps_generic_backoff_calc(rf_ptr);
         return 0;
     }
 
@@ -1533,12 +1538,8 @@ static int8_t mcps_generic_packet_rebuild(protocol_interface_rf_mac_setup_s *rf_
     tx_buf->len = frame_length;
     uint8_t *mhr_start = ptr;
 
-    uint32_t random_period = mac_csma_backoff_get(rf_ptr);
-    if (rf_ptr->fhss_api) {
-        buffer->tx_time = mcps_calculate_tx_time(rf_ptr, random_period);
-    } else {
-        buffer->tx_time = random_period;
-    }
+    buffer->tx_time = mcps_generic_backoff_calc(rf_ptr);
+
     ptr = mac_generic_packet_write(rf_ptr, ptr, buffer);
 
     if (buffer->fcf_dsn.securityEnabled) {