diff --git a/library/GSM_Types.ttcn b/library/GSM_Types.ttcn
index ee6635b..bcf2794 100644
--- a/library/GSM_Types.ttcn
+++ b/library/GSM_Types.ttcn
@@ -46,6 +46,15 @@
 	}
 }
 
+const GprsTlli TLLI_UNUSED := 'FFFFFFFF'O;
+function f_gen_tlli() return GprsTlli {
+	var GprsTlli tlli := f_rnd_octstring(4);
+	if (tlli == TLLI_UNUSED) {
+		tlli := 'EEEEEEEE'O;
+	}
+	return tlli;
+}
+
 /* 10.5.2.8 */
 type enumerated ChannelNeeded {
 	CHAN_NEED_ANY	(0),