diff -r d5cfe99001f0 src/devices/wifi/wifi-net-device.cc
--- a/src/devices/wifi/wifi-net-device.cc	Tue Jul 20 07:34:14 2010 -0700
+++ b/src/devices/wifi/wifi-net-device.cc	Wed Jul 21 13:28:23 2010 +0200
@@ -43,7 +43,7 @@
     .SetParent<NetDevice> ()
     .AddConstructor<WifiNetDevice> ()
     .AddAttribute ("Mtu", "The MAC-level Maximum Transmission Unit",
-                   UintegerValue (MAX_MSDU_SIZE),
+                   UintegerValue (MAX_MSDU_SIZE - LLC_SNAP_HEADER_SIZE),
                    MakeUintegerAccessor (&WifiNetDevice::SetMtu,
                                          &WifiNetDevice::GetMtu),
                    MakeUintegerChecker<uint16_t> (1,MAX_MSDU_SIZE))
diff -r d5cfe99001f0 src/node/llc-snap-header.cc
--- a/src/node/llc-snap-header.cc	Tue Jul 20 07:34:14 2010 -0700
+++ b/src/node/llc-snap-header.cc	Wed Jul 21 13:28:23 2010 +0200
@@ -43,7 +43,7 @@
 uint32_t 
 LlcSnapHeader::GetSerializedSize (void) const
 {
-  return 1 + 1 + 1 + 3 + 2;
+  return LLC_SNAP_HEADER_SIZE;
 }
 
 TypeId 
diff -r d5cfe99001f0 src/node/llc-snap-header.h
--- a/src/node/llc-snap-header.h	Tue Jul 20 07:34:14 2010 -0700
+++ b/src/node/llc-snap-header.h	Wed Jul 21 13:28:23 2010 +0200
@@ -27,6 +27,8 @@
 
 namespace ns3 {
 
+static const uint16_t LLC_SNAP_HEADER_SIZE = 8;
+
 /**
  * \ingroup node
  *