diff --git a/gprs_gb/BSSGP_Helper_Functions.ttcn b/gprs_gb/BSSGP_Helper_Functions.ttcn
index 731d2ab..3085648 100644
--- a/gprs_gb/BSSGP_Helper_Functions.ttcn
+++ b/gprs_gb/BSSGP_Helper_Functions.ttcn
@@ -3,4 +3,9 @@
 	external function f_BSSGP_compact_len(in octetstring inp) return octetstring;
 	external function f_NS_expand_len(in octetstring inp) return octetstring;
 	external function f_NS_compact_len(in octetstring inp) return octetstring;
+	external function f_LLC_compute_fcs(in octetstring inp) return octetstring;
+
+	function f_LLC_append_fcs(in octetstring inp) return octetstring {
+		return inp & f_LLC_compute_fcs(inp);
+	}
 };