diff --git a/pySim-prog.py b/pySim-prog.py
index 0abd190..4c85be7 100755
--- a/pySim-prog.py
+++ b/pySim-prog.py
@@ -146,7 +146,7 @@
 	parser.add_option("--opmode", dest="opmode", type="choice",
 			help="Set UE Operation Mode in EF.AD (Administrative Data)",
 			default=None,
-			choices=['{:02X}'.format(m) for m in list(EF_AD.OP_MODE.keys())],
+			choices=['{:02X}'.format(int(m)) for m in EF_AD.OP_MODE],
 		)
 	parser.add_option("--epdgid", dest="epdgid",
 			help="Set Home Evolved Packet Data Gateway (ePDG) Identifier. (Only FQDN format supported)",
diff --git a/pySim-read.py b/pySim-read.py
index 59c5762..8e4a512 100755
--- a/pySim-read.py
+++ b/pySim-read.py
@@ -28,7 +28,7 @@
 import random
 import re
 import sys
-from pySim.ts_51_011 import EF, DF, EF_SST_map, EF_AD_mode_map
+from pySim.ts_51_011 import EF, DF, EF_SST_map, EF_AD
 from pySim.ts_31_102 import EF_UST_map, EF_USIM_ADF_map
 from pySim.ts_31_103 import EF_IST_map, EF_ISIM_ADF_map
 
@@ -230,11 +230,10 @@
 	(res, sw) = card.read_binary('AD')
 	if sw == '9000':
 		print("Administrative data: %s" % (res,))
-		if res[:2] in EF_AD_mode_map:
-			print("\tMS operation mode: %s" % (EF_AD_mode_map[res[:2]],))
-		else:
-			print("\tMS operation mode: (unknown 0x%s)" % (res[:2],))
-		if int(res[4:6], 16) & 0x01:
+		ad = EF_AD()
+		decoded_data = ad.decode_hex(res)
+		print("\tMS operation mode: %s" % decoded_data['ms_operation_mode'])
+		if decoded_data['ofm']:
 			print("\tCiphering Indicator: enabled")
 		else:
 			print("\tCiphering Indicator: disabled")
diff --git a/pySim-shell.py b/pySim-shell.py
index 2a7c377..a8db263 100755
--- a/pySim-shell.py
+++ b/pySim-shell.py
@@ -30,7 +30,7 @@
 import sys
 from pathlib import Path
 
-from pySim.ts_51_011 import EF, DF, EF_SST_map, EF_AD_mode_map
+from pySim.ts_51_011 import EF, DF, EF_SST_map
 from pySim.ts_31_102 import EF_UST_map, EF_USIM_ADF_map
 from pySim.ts_31_103 import EF_IST_map, EF_ISIM_ADF_map
 
diff --git a/pySim/cards.py b/pySim/cards.py
index 5a39bda..9a19eed 100644
--- a/pySim/cards.py
+++ b/pySim/cards.py
@@ -175,29 +175,27 @@
 
 		# read from card
 		raw_hex_data, sw = self._scc.read_binary(EF['AD'], length=None, offset=0)
-		raw_bin_data = h2b(raw_hex_data)
-		abstract_data = ad.decode_bin(raw_bin_data)
+		abstract_data = ad.decode_hex(raw_hex_data)
 
 		# perform updates
-		if mnc:
+		if mnc and abstract_data['extensions']:
 			mnclen = len(str(mnc))
 			if mnclen == 1:
 				mnclen = 2
 			if mnclen > 3:
 				raise RuntimeError('invalid length of mnc "{}"'.format(mnc))
-			abstract_data['len_of_mnc_in_imsi'] = mnclen
+			abstract_data['extensions']['mnc_len'] = mnclen
 		if opmode:
-			opmode_symb = ad.OP_MODE.get(int(opmode, 16))
-			if opmode_symb:
-				abstract_data['ms_operation_mode'] = opmode_symb
+			opmode_num = int(opmode, 16)
+			if opmode_num in [int(v) for v in EF_AD.OP_MODE]:
+				abstract_data['ms_operation_mode'] = opmode_num
 			else:
 				raise RuntimeError('invalid opmode "{}"'.format(opmode))
 		if ofm:
-			abstract_data['specific_facilities']['ofm'] = bool(int(ofm, 16))
+			abstract_data['ofm'] = bool(int(ofm, 16))
 
 		# write to card
-		raw_bin_data = ad.encode_bin(abstract_data)
-		raw_hex_data = b2h(raw_bin_data)
+		raw_hex_data = ad.encode_hex(abstract_data)
 		data, sw = self._scc.update_binary(EF['AD'], raw_hex_data)
 		return sw
 
diff --git a/pySim/construct.py b/pySim/construct.py
index 839497c..b0f03b7 100644
--- a/pySim/construct.py
+++ b/pySim/construct.py
@@ -47,3 +47,44 @@
 
 # here we collect some shared / common definitions of data types
 LV = Prefixed(Int8ub, HexAdapter(GreedyBytes))
+
+# Default value for Reserved for Future Use (RFU) bits/bytes
+# See TS 31.101 Sec. "3.4 Coding Conventions"
+__RFU_VALUE = 0
+
+# Field that packs Reserved for Future Use (RFU) bit
+FlagRFU = Default(Flag, __RFU_VALUE)
+
+# Field that packs Reserved for Future Use (RFU) byte
+ByteRFU = Default(Byte, __RFU_VALUE)
+
+# Field that packs all remaining Reserved for Future Use (RFU) bytes
+GreedyBytesRFU = Default(GreedyBytes, b'')
+
+def BitsRFU(n=1):
+    '''
+    Field that packs Reserved for Future Use (RFU) bit(s)
+    as defined in TS 31.101 Sec. "3.4 Coding Conventions"
+
+    Use this for (currently) unused/reserved bits whose contents
+    should be initialized automatically but should not be cleared
+    in the future or when restoring read data (unlike padding).
+
+    Parameters:
+        n (Integer): Number of bits (default: 1)
+    '''
+    return Default(BitsInteger(n), __RFU_VALUE)
+
+def BytesRFU(n=1):
+    '''
+    Field that packs Reserved for Future Use (RFU) byte(s)
+    as defined in TS 31.101 Sec. "3.4 Coding Conventions"
+
+    Use this for (currently) unused/reserved bytes whose contents
+    should be initialized automatically but should not be cleared
+    in the future or when restoring read data (unlike padding).
+
+    Parameters:
+        n (Integer): Number of bytes (default: 1)
+    '''
+    return Default(Bytes(n), __RFU_VALUE)
diff --git a/pySim/ts_51_011.py b/pySim/ts_51_011.py
index c21e86c..48649cd 100644
--- a/pySim/ts_51_011.py
+++ b/pySim/ts_51_011.py
@@ -319,22 +319,12 @@
 	59: 'MMS User Connectivity Parameters',
 }
 
-# 10.3.18 "EF.AD (Administrative data) "
-EF_AD_mode_map = {
-	'00' : 'normal operation',
-	'80' : 'type approval operations',
-	'01' : 'normal operation + specific facilities',
-	'81' : 'type approval operations + specific facilities',
-	'02' : 'maintenance (off line)',
-	'04' : 'cell test operation',
-}
-
-
 from pySim.utils import *
 from struct import pack, unpack
 from construct import *
 from construct import Optional as COptional
-from pySim.construct import HexAdapter, BcdAdapter
+from pySim.construct import HexAdapter, BcdAdapter, FlagRFU, ByteRFU, GreedyBytesRFU, BitsRFU, BytesRFU
+import enum
 
 from pySim.filesystem import *
 import pySim.ts_102_221
@@ -553,30 +543,34 @@
 
 # TS 51.011 Section 10.3.18
 class EF_AD(TransparentEF):
-    OP_MODE = {
-            0x00: 'normal',
-            0x80: 'type_approval',
-            0x01: 'normal_and_specific_facilities',
-            0x81: 'type_approval_and_specific_facilities',
-            0x02: 'maintenance_off_line',
-            0x04: 'cell_test',
-        }
-    OP_MODE_reverse = dict(map(reversed, OP_MODE.items()))
+    class OP_MODE(enum.IntEnum):
+        normal                                  = 0x00
+        type_approval                           = 0x80
+        normal_and_specific_facilities          = 0x01
+        type_approval_and_specific_facilities   = 0x81
+        maintenance_off_line                    = 0x02
+        cell_test                               = 0x04
+    #OP_MODE_DICT = {int(v) : str(v) for v in EF_AD.OP_MODE}
+    #OP_MODE_DICT_REVERSED = {str(v) : int(v) for v in EF_AD.OP_MODE}
+
     def __init__(self, fid='6fad', sfid=None, name='EF.AD', desc='Administrative Data', size={3,4}):
         super().__init__(fid, sfid=sfid, name=name, desc=desc, size=size)
-    def _decode_bin(self, raw_bin):
-        u = unpack('!BH', raw_bin[:3])
-        ofm = True if u[1] & 1 else False
-        res = {'ms_operation_mode': self.OP_MODE.get(u[0], u[0]), 'specific_facilities': { 'ofm': ofm } }
-        if len(raw_bin) > 3:
-            res['len_of_mnc_in_imsi'] = int(raw_bin[3]) & 0xf
-        return res
-    def _encode_bin(self, abstract):
-        op_mode = self.OP_MODE_reverse[abstract['ms_operation_mode']]
-        res = pack('!BH', op_mode, abstract['specific_facilities']['ofm'])
-        if 'len_of_mnc_in_imsi' in abstract:
-            res += pack('!B', abstract['len_of_mnc_in_imsi'])
-        return res
+        self._construct = BitStruct(
+            # Byte 1
+            'ms_operation_mode'/Bytewise(Enum(Byte, EF_AD.OP_MODE)),
+            # Byte 2
+            'rfu1'/Bytewise(ByteRFU),
+            # Byte 3
+            'rfu2'/BitsRFU(7),
+            'ofm'/Flag,
+            # Byte 4 (optional),
+            'extensions'/COptional(Struct(
+                'rfu3'/BitsRFU(4),
+                'mnc_len'/BitsInteger(4),
+                # Byte 5..N-4 (optional, RFU)
+                'extensions'/Bytewise(GreedyBytesRFU)
+            ))
+        )
 
 # TS 51.011 Section 10.3.20 / 10.3.22
 class EF_VGCS(TransRecEF):
diff --git a/pysim-testdata/Fairwaves-SIM.ok b/pysim-testdata/Fairwaves-SIM.ok
index e6fcfe3..f83f415 100644
--- a/pysim-testdata/Fairwaves-SIM.ok
+++ b/pysim-testdata/Fairwaves-SIM.ok
@@ -43,7 +43,7 @@
 ACC: 0008
 MSISDN: Not available
 Administrative data: 00000002
-	MS operation mode: normal operation
+	MS operation mode: normal
 	Ciphering Indicator: disabled
 SIM Service Table: ff3cc3ff030fff0f000fff03f0c0
 	Service 1 - CHV1 disable function
diff --git a/pysim-testdata/Wavemobile-SIM.ok b/pysim-testdata/Wavemobile-SIM.ok
index 1c78cc9..0682a70 100644
--- a/pysim-testdata/Wavemobile-SIM.ok
+++ b/pysim-testdata/Wavemobile-SIM.ok
@@ -50,7 +50,7 @@
 ACC: abce
 MSISDN: Not available
 Administrative data: 00000102
-	MS operation mode: normal operation
+	MS operation mode: normal
 	Ciphering Indicator: enabled
 SIM Service Table: ff33ff0f3c00ff0f000cf0c0f0030000
 	Service 1 - CHV1 disable function
diff --git a/pysim-testdata/fakemagicsim.ok b/pysim-testdata/fakemagicsim.ok
index 1d1714f..11296f5 100644
--- a/pysim-testdata/fakemagicsim.ok
+++ b/pysim-testdata/fakemagicsim.ok
@@ -17,7 +17,7 @@
 ACC: ffff
 MSISDN: Not available
 Administrative data: 000000
-	MS operation mode: normal operation
+	MS operation mode: normal
 	Ciphering Indicator: disabled
 SIM Service Table: ff3fff0f0300f003000c
 	Service 1 - CHV1 disable function
diff --git a/pysim-testdata/sysmoISIM-SJA2.ok b/pysim-testdata/sysmoISIM-SJA2.ok
index ae332a8..dc7b865 100644
--- a/pysim-testdata/sysmoISIM-SJA2.ok
+++ b/pysim-testdata/sysmoISIM-SJA2.ok
@@ -55,7 +55,7 @@
 ACC: 0200
 MSISDN (NPI=1 ToN=3): 6766266
 Administrative data: 00000002
-	MS operation mode: normal operation
+	MS operation mode: normal
 	Ciphering Indicator: disabled
 SIM Service Table: ff33ffff3f003f0f300cf0c3f00000
 	Service 1 - CHV1 disable function
diff --git a/pysim-testdata/sysmoUSIM-SJS1.ok b/pysim-testdata/sysmoUSIM-SJS1.ok
index 95f6967..bce3c9d 100644
--- a/pysim-testdata/sysmoUSIM-SJS1.ok
+++ b/pysim-testdata/sysmoUSIM-SJS1.ok
@@ -55,7 +55,7 @@
 ACC: 0008
 MSISDN (NPI=1 ToN=1): +77776336143
 Administrative data: 00000002
-	MS operation mode: normal operation
+	MS operation mode: normal
 	Ciphering Indicator: disabled
 SIM Service Table: ff3fffff3f003f1ff00c00c0f00000
 	Service 1 - CHV1 disable function
diff --git a/pysim-testdata/sysmosim-gr1.ok b/pysim-testdata/sysmosim-gr1.ok
index f4b09c8..3aff2a3 100644
--- a/pysim-testdata/sysmosim-gr1.ok
+++ b/pysim-testdata/sysmosim-gr1.ok
@@ -17,7 +17,7 @@
 ACC: 0008
 MSISDN: Not available
 Administrative data: 000000
-	MS operation mode: normal operation
+	MS operation mode: normal
 	Ciphering Indicator: disabled
 SIM Service Table: ff3fff0f0f0000030000
 	Service 1 - CHV1 disable function