diff --git a/library/L1CTL_Types.ttcn b/library/L1CTL_Types.ttcn
index 44f0490..5c90966 100644
--- a/library/L1CTL_Types.ttcn
+++ b/library/L1CTL_Types.ttcn
@@ -140,7 +140,13 @@
 	} with { variant "" };
 
 	/* gsm48_chan_mode */
-	type uint8_t L1ctlTchMode;
+	type enumerated L1ctlTchMode {
+		L1CTL_CHAN_MODE_SIGN		('00000000'B),	/* Signalling */
+		L1CTL_CHAN_MODE_SPEECH_V1	('00000001'B),	/* FR or HR codec */
+		L1CTL_CHAN_MODE_SPEECH_V2	('00100001'B),	/* EFR codec */
+		L1CTL_CHAN_MODE_SPEECH_V3	('01000001'B)	/* AMR codec */
+		/* Other modes are not supported for now */
+	} with { variant "FIELDLENGTH(8)" };
 
 	type record L1ctlAudioMode {
 		BIT4		padding,
@@ -572,7 +578,7 @@
 		payload := {
 			dm_est_req := {
 				tsc := tsc,
-				tch_mode := 0,
+				tch_mode := L1CTL_CHAN_MODE_SIGN,
 				audio_mode := t_L1CTL_AudioModeNone
 			}
 		}