diff --git a/library/GSM_Types.ttcn b/library/GSM_Types.ttcn
index 5884c27..b86bfd7 100644
--- a/library/GSM_Types.ttcn
+++ b/library/GSM_Types.ttcn
@@ -247,5 +247,10 @@
 	return GSM51_MFRAMES_PER_SEC * int2float(bs_ag_blks_res);
 }
 
+/* compute TC as per 45.002 6.3.1.3 */
+function f_gsm_compute_tc(integer fn) return integer {
+	return (fn / 51) mod 8;
+}
+
 
 } with { encode "RAW"; variant "FIELDORDER(msb)" }