diff -urw ns-2.26/mac/mac-802_11.cc ns-2.26-std/mac/mac-802_11.cc
--- ns-2.26/mac/mac-802_11.cc	2003-10-10 09:56:53.000000000 -0700
+++ ns-2.26-std/mac/mac-802_11.cc	2003-10-10 10:24:53.000000000 -0700
@@ -302,6 +302,8 @@
 		return 0;
 	if(nav_ > Scheduler::instance().clock())
 		return 0;
+	if(pcs_ > Scheduler::instance().clock())
+		return 0;
 	
 	return 1;
 }
@@ -366,7 +368,7 @@
 			   (u_int32_t)ETHER_ADDR(mh->dh_sa) == \
 			   (u_int32_t)index_ ||
 			   (u_int32_t)ETHER_ADDR(mh->dh_da) == MAC_BROADCAST) {
-				drop(p);
+				drop(p, why);
 				return;
 			}
 			break;
@@ -1111,7 +1113,7 @@
 	 *  interface.
 	 *
 	 */
-
+	set_pcs(txtime(p));
 	/*
 	 *  If the interface is currently in transmit mode, then
 	 *  it probably won't even see this packet.  However, the
Only in ns-2.26-std/mac: mac-802_11.cc~
diff -urw ns-2.26/mac/mac-802_11.h ns-2.26-std/mac/mac-802_11.h
--- ns-2.26/mac/mac-802_11.h	2003-02-26 14:08:56.000000000 -0800
+++ ns-2.26-std/mac/mac-802_11.h	2003-10-10 10:10:16.000000000 -0700
@@ -327,6 +327,13 @@
 		}
 	}
 
+	inline void set_pcs(u_int16_t us) {
+		double now = Scheduler::instance().clock();
+		double t = us * 1e-6;
+		if((now + t) > pcs_) 
+			pcs_ = now + t;
+	}
+
 protected:
 	PHY_MIB		*phymib_;
 	MAC_MIB		*macmib_;
@@ -350,6 +357,7 @@
 	   Internal MAC State
 	   ============================================================ */
 	double		nav_;		// Network Allocation Vector
+	double          pcs_;           // Physical Carrier Sense
 
 	MacState	rx_state_;	// incoming state (MAC_RECV or MAC_IDLE)
 	MacState	tx_state_;	// outgoint state