diff -r ed0b2d9301a1 src/devices/wifi/dcf-manager.cc
--- a/src/devices/wifi/dcf-manager.cc	Tue Dec 01 18:34:11 2009 +0300
+++ b/src/devices/wifi/dcf-manager.cc	Wed Dec 02 17:55:15 2009 +0300
@@ -375,7 +375,7 @@
    * by notifying the collision to the user.
    */
   if (state->GetBackoffSlots () == 0 && 
-      IsBusy ())
+      GetBackoffStartFor (state) > Simulator::Now ())
     {
       MY_DEBUG ("medium is busy: collision");
       /* someone else has accessed the medium.