diff -r 9b43310e9840 src/devices/wifi/dca-txop.cc
--- a/src/devices/wifi/dca-txop.cc	Thu Oct 01 17:30:39 2009 +0400
+++ b/src/devices/wifi/dca-txop.cc	Tue Oct 06 19:30:31 2009 +0400
@@ -118,6 +118,8 @@
   m_queue = CreateObject<WifiMacQueue> ();
   m_rng = new RealRandomStream ();
   m_txMiddle = new MacTxMiddle ();
+  m_dcf->ResetCw ();
+  m_dcf->StartBackoffNow (m_rng->GetNext (0, m_dcf->GetCw ()));
 }
 
 DcaTxop::~DcaTxop ()
@@ -190,6 +192,8 @@
 {
   NS_LOG_FUNCTION (this << minCw);
   m_dcf->SetCwMin (minCw);
+  m_dcf->ResetCw ();
+  m_dcf->StartBackoffNow (m_rng->GetNext (0, m_dcf->GetCw ()));
 }
 void 
 DcaTxop::SetMaxCw (uint32_t maxCw)