diff --git a/src/ranap_msg_factory.c b/src/ranap_msg_factory.c
index 94eb1c5..16a9d03 100644
--- a/src/ranap_msg_factory.c
+++ b/src/ranap_msg_factory.c
@@ -705,7 +705,10 @@
 	id->bits_unused = 0;
 }
 
-/*! \brief generate RANAP RAB ASSIGNMENT REQUEST message for CS (voice) */
+/*! \brief generate RANAP RAB ASSIGNMENT REQUEST message for CS (voice).
+ * See 3GPP TS 25.413 8.2.
+ * RAB ID: 3GPP TS 25.413 9.2.1.2
+ */
 struct msgb *ranap_new_msg_rab_assign_voice(uint8_t rab_id, uint32_t rtp_ip,
 					    uint16_t rtp_port,
 					    bool use_x213_nsap)