diff --git a/include/osmocom/trau/osmo_ortp.h b/include/osmocom/trau/osmo_ortp.h
index cec0d2e..eb53a41 100644
--- a/include/osmocom/trau/osmo_ortp.h
+++ b/include/osmocom/trau/osmo_ortp.h
@@ -22,6 +22,15 @@
 /*! \brief Osmocom pseudo-static paylaod type for Adaptive Multi Rate (AMR) */
 #define RTP_PT_AMR 98
 
+#define GSM_VOICE_SAMPLE_RATE_HZ 8000
+#define GSM_VOICE_SAMPLES_PER_MS (GSM_VOICE_SAMPLE_RATE_HZ / 1000)
+#define GSM_VOICE_MULTIFRAME 26
+#define GSM_RTP_FRAME_DURATION_MS 20
+#define GSM_SAMPLES_PER_RTP_FRAME (GSM_RTP_FRAME_DURATION_MS * GSM_VOICE_SAMPLES_PER_MS)
+#define GSM_TDMA_FRAME_MS (120 / GSM_VOICE_MULTIFRAME)
+#define GSM_MS_TO_SAMPLES(ms) (ms * GSM_VOICE_SAMPLES_PER_MS)
+#define GSM_FN_TO_MS(fn) (fn * GSM_TDMA_FRAME_MS)
+
 /*! \brief Parameter to osmo_rtp_socket_param_set() */
 enum osmo_rtp_param {
 	OSMO_RTP_P_JITBUF = 1,