diff --git a/GSM411.st b/GSM411.st
index 7bccadc..2ecc6f1 100644
--- a/GSM411.st
+++ b/GSM411.st
@@ -195,7 +195,6 @@ Object subclass: GSMRpInformationElement [
         len printNl.
         ^self new
             readFrom: (aStream next: len);
-            inspect;
             yourself.
     ]
 ]
@@ -212,6 +211,12 @@ GSMRpInformationElement subclass: GSMRpOrigantorAddress [
     readFrom: anArray [
         data := anArray
     ]
+
+    writeOn: aMessage with: anAttribute [
+        aMessage
+            putByte: data size;
+            putByteArray: data
+    ]
 ]
 
 GSMRpInformationElement subclass: GSMRpDestinationAddress [
@@ -226,6 +231,12 @@ GSMRpInformationElement subclass: GSMRpDestinationAddress [
     readFrom: anArray [
         data := anArray
     ]
+
+    writeOn: aMessage with: anAttribute [
+        aMessage
+            putByte: data size;
+            putByteArray: data
+    ]
 ]
 
 GSMRpInformationElement subclass: GSMRpUserData [
@@ -240,6 +251,12 @@ GSMRpInformationElement subclass: GSMRpUserData [
     readFrom: anArray [
         data := anArray
     ]
+
+    writeOn: aMessage with: anAttribute [
+        aMessage
+            putByte: data size;
+            putByteArray: data
+    ]
 ]
 
 Osmo.TLVParserBase subclass: GSMRpMessage [
@@ -274,6 +291,13 @@ Osmo.TLVParserBase subclass: GSMRpMessage [
                 [self parseOptional: attr tag: aStream peek stream: aStream].
         ].
     ]
+
+    writeHeaderOn: aMessage [
+        direction = #msn
+            ifTrue: [aMessage putByte: self class messageTypeToNetwork]
+            ifFalse: [aMessage putByte: self class messageTypeToMS].
+        aMessage putByte: reference.
+    ]
 ]
 
 GSMRpMessage subclass: GSMRpData [
@@ -281,6 +305,14 @@ GSMRpMessage subclass: GSMRpData [
     <category: 'OsmoGSM-SMS-Message'>
     <comment: '7.3.1'>
 
+    GSMRpData class >> messageTypeToNetwork [
+        ^2r000
+    ]
+
+    GSMRpData class >> messageTypeToMS [
+        ^2r001
+    ]
+
     GSMRpData class >> tlvDescription [
         ^OrderedCollection new
             add: GSMRpOrigantorAddress asTLVDescription;
@@ -309,6 +341,10 @@ GSMRpMessage subclass: GSMRpAck [
             add: GSMRpUserData asTLVDescription beOptional yourself;
             yourself.
     ]
+
+    GSMRpAck class >> messageTypeToNetwork [
+        ^2r010
+    ]
 ]
 
 
diff --git a/SMSTests.st b/SMSTests.st
index 47e33c8..0ba290d 100644
--- a/SMSTests.st
+++ b/SMSTests.st
@@ -35,7 +35,7 @@ TestCase subclass: GSM411Test [
             assert: dec toMessage asByteArray = inp.
 
         self
-            assert: dec rpMessage toMessage asByteArray = dec userData data.
+            assert: dec rpMessage toMessage asByteArray equals: dec userData data.
     ]
 
     testCPData2 [
@@ -46,5 +46,8 @@ TestCase subclass: GSM411Test [
         self
             assert: dec type = GSMCpData messageType;
             assert: dec toMessage asByteArray = inp.
+
+        self
+            assert: dec rpMessage toMessage asByteArray equals: dec userData data.
     ]
 ]