diff --git a/sysinfo/GSM_SystemInformation.ttcn b/sysinfo/GSM_SystemInformation.ttcn
index 552463f..8413ee3 100644
--- a/sysinfo/GSM_SystemInformation.ttcn
+++ b/sysinfo/GSM_SystemInformation.ttcn
@@ -6,6 +6,7 @@
 module GSM_SystemInformation {
 
 	import from General_Types all;
+	import from GSM_Types all;
 	import from Osmocom_Types all;
 
 	type record L2PseudoLength {
@@ -32,10 +33,10 @@
 		L2PseudoLength	l2_plen,
 		uint4_t		skip_indicator,
 		uint4_t		rr_protocol_discriminator,
-		OCT1		message_type
+		RrMessageType	message_type
 	} with { variant "" };
 
-	template SystemInformationHeader t_SiHeader(OCT1 msg_type, uint6_t len) := {
+	template SystemInformationHeader t_SiHeader(RrMessageType msg_type, uint6_t len) := {
 		l2_plen := t_L2Pseudolength(len),
 		rr_protocol_discriminator := 6,
 		message_type := msg_type
@@ -234,16 +235,16 @@
 		SystemInformationType5bis	si5bis,
 		SystemInformationType5ter	si5ter,
 		SystemInformationType6		si6
-	} with { variant "TAG(si1, header.message_type = '19'O;
-			      si2, header.message_type = '1A'O;
-			      si2bis, header.message_type = '02'O;
-			      si2ter, header.message_type = '03'O;
-			      si3, header.message_type = '1B'O;
-			      si4, header.message_type = '1C'O;
-			      si5, header.message_type = '1D'O;
-			      si5bis, header.message_type = '05'O;
-			      si5ter, header.message_type = '06'O;
-			      si6, header.message_type = '1E'O;
+	} with { variant "TAG(si1, header.message_type = SYSTEM_INFORMATION_TYPE_1;
+			      si2, header.message_type = SYSTEM_INFORMATION_TYPE_2;
+			      si2bis, header.message_type = SYSTEM_INFORMATION_TYPE_2bis;
+			      si2ter, header.message_type = SYSTEM_INFORMATION_TYPE_2ter;
+			      si3, header.message_type = SYSTEM_INFORMATION_TYPE_3;
+			      si4, header.message_type = SYSTEM_INFORMATION_TYPE_4;
+			      si5, header.message_type = SYSTEM_INFORMATION_TYPE_5;
+			      si5bis, header.message_type = SYSTEM_INFORMATION_TYPE_5bis;
+			      si5ter, header.message_type = SYSTEM_INFORMATION_TYPE_5ter;
+			      si6, header.message_type = SYSTEM_INFORMATION_TYPE_6;
 			)" };
 
 	external function enc_SystemInformation(in SystemInformation si) return octetstring
diff --git a/sysinfo/GSM_Types.ttcn b/sysinfo/GSM_Types.ttcn
new file mode 100644
index 0000000..70051bd
--- /dev/null
+++ b/sysinfo/GSM_Types.ttcn
@@ -0,0 +1,106 @@
+/* Encoding/Decoding routines for GSM System Information messages
+ * according to 3GPP TS 44.018 Version 12.3.0 Release 12 */
+
+/* (C) 2017 by Harald Welte <laforge@gnumonks.org> */
+
+module GSM_Types {
+
+	import from General_Types all;
+	import from Osmocom_Types all;
+
+	/* Table 10.4.1 of Section 10.4 / 3GPP TS 44.018 */
+	type enumerated RrMessageType {
+		ADDITIONAL_ASSIGNMENT		('00111011'B),
+		IMMEDIATE_ASSIGNMENT		('00111111'B),
+		IMMEDIATE_ASSIGNMENT_EXTENDED	('00111001'B),
+		IMMEDIATE_ASSIGNMENT_REJECT	('00111010'B),
+		IMMEDIATE_PACKET_ASSIGNMENT	('01101001'B),
+
+		CIPHERING_MODE_COMMAND		('00110101'B),
+		CIPHERING_MODE_COMPLETE		('00110010'B),
+
+		CONFIGURATION_CHANGE_COMMAND	('00110000'B),
+		CONFIGURATION_CHANGE_ACK	('00110001'B),
+		CONFIGURATION_CHANGE_REJECT	('00110011'B),
+
+		ASSIGNMENT_COMMAND		('00101110'B),
+		ASSIGNMENT_COMPLETE		('00101001'B),
+		ASSIGNMENT_FAILURE		('00101111'B),
+		HANDOVER_COMMAND		('00101011'B),
+		HANDOVER_COMPLETE		('00101100'B),
+		HANDOVER_FAILURE		('00101000'B),
+		PHYSICAL_INFORMATION		('00101101'B),
+
+		CHANNEL_RELEASE			('00001101'B),
+		PARTIAL_RELEASE			('00001010'B),
+		PARTIAL_RELEASE_COMPLETE	('00001111'B),
+
+		PAGING_REQUEST_TYPE_1		('00100001'B),
+		PAGING_REQUEST_TYPE_2		('00100010'B),
+		PAGING_REQUEST_TYPE_3		('00100100'B),
+		PAGING_RESPONSE			('00100111'B),
+		NOTIFICATION_NCH		('00100000'B),
+		NOTIFICATION_RESPOSNE		('00100110'B),
+
+		SYSTEM_INFORMATION_TYPE_8	('00011000'B),
+		SYSTEM_INFORMATION_TYPE_1	('00011001'B),
+		SYSTEM_INFORMATION_TYPE_2	('00011010'B),
+		SYSTEM_INFORMATION_TYPE_3	('00011011'B),
+		SYSTEM_INFORMATION_TYPE_4	('00011100'B),
+		SYSTEM_INFORMATION_TYPE_5	('00011101'B),
+		SYSTEM_INFORMATION_TYPE_6	('00011110'B),
+		SYSTEM_INFORMATION_TYPE_7	('00011111'B),
+		SYSTEM_INFORMATION_TYPE_2bis	('00000010'B),
+		SYSTEM_INFORMATION_TYPE_2ter	('00000011'B),
+		SYSTEM_INFORMATION_TYPE_2quater	('00000111'B),
+		SYSTEM_INFORMATION_TYPE_5bis	('00000101'B),
+		SYSTEM_INFORMATION_TYPE_5ter	('00000110'B),
+		SYSTEM_INFORMATION_TYPE_9	('00000100'B),
+		SYSTEM_INFORMATION_TYPE_13	('00000000'B),
+
+		SYSTEM_INFORMATION_TYPE_16	('00111101'B),
+		SYSTEM_INFORMATION_TYPE_17	('00111110'B),
+
+		CHANNEL_MODE_MODIFY		('00010000'B),
+		RR_STATUS			('00010010'B),
+		CHANNEL_MODE_MODIFY_ACKNOWLEDGE	('00010111'B),
+		FREQUENCY_REDEFINITION		('00010100'B),
+		MEASUREMENT_REPORT		('00010101'B),
+		CLASSMARK_CHANGE		('00010110'B),
+		CLASSMARK_ENQUIRY		('00010011'B),
+		EXTENDED_MEASUREMENT_REPORT	('00110110'B),
+		EXTENDED_MEASUREMENT_ORDER	('00110111'B),
+		GPRS_SUSPENSION_REQUEST		('00110100'B),
+		//MBMS_ANNOUNCEMENT		('00010110'B), duplicate?
+		//SERVICE_INFORMATION		('00110110'B), duplicate?
+
+		APPLICATION_INFORMATION		('00111000'B),
+
+		SYSTEM_INFORMATION_TYPE_14	('00000001'B),
+		SYSTEM_INFORMATION_TYPE_15	('01000011'B),
+		SYSTEM_INFORMATION_TYPE_18	('01000000'B),
+		SYSTEM_INFORMATION_TYPE_19	('01000001'B),
+		SYSTEM_INFORMATION_TYPE_20	('01000010'B),
+		SYSTEM_INFORMATION_TYPE_13alt	('01000100'B),
+		SYSTEM_INFORMATION_TYPE_2n	('01000101'B),
+		SYSTEM_INFORMATION_TYPE_21	('01000110'B),
+		SYSTEM_INFORMATION_TYPE_22	('01000111'B),
+		SYSTEM_INFORMATION_TYPE_23	('01001111'B),
+
+		DTM_ASSIGNMENT_FAILURE		('01001000'B),
+		DTM_REJECT			('01001001'B),
+		DTM_REQUEST			('01001010'B),
+		PACKET_ASSIGNMENT		('01001011'B),
+		DTM_ASSIGNMENT_COMMAND		('01001100'B),
+		DTM_INFORMATION			('01001101'B),
+		PACKET_INFORMATION		('01001110'B),
+
+		UTRAN_CLASSMARK_CHANGE		('01100000'B),
+		CDMA2000_CLASSMARK_CHANGE	('01100010'B),
+		INTERSYS_TO_UTRAN_HO_CMD	('01100011'B),
+		INTERSYS_TO_CDMA2000_HO_CMD	('01100100'B),
+		GERAN_IU_MODE_CLASSMARK_CHG	('01100101'B),
+		INTERSYS_TO_EUTRAN_HO_CMD	('01100110'B)
+	} with { variant "FIELDLENGTH(8)" };
+
+} with { encode "RAW"; variant "FIELDORDER(msb)" }
diff --git a/sysinfo/Test.ttcn b/sysinfo/Test.ttcn
index 5fa89fa..c2ffa4d 100644
--- a/sysinfo/Test.ttcn
+++ b/sysinfo/Test.ttcn
@@ -16,12 +16,6 @@
 	};
 
 	testcase TC_si1() runs on dummy_CT {
-		var SystemInformationHeader hdr := {
-			l2_plen := valueof(t_L2Pseudolength(21)),
-			skip_indicator := 0,
-			rr_protocol_discriminator := 6,
-			message_type := '19'O
-		};
 		log("SI: ", dec_SystemInformation(si1));
 		log("SI: ", dec_SystemInformation(si2));
 		log("SI: ", dec_SystemInformation(si3));