diff --git a/core/net/mac/xmac.c b/core/net/mac/xmac.c
index 22e6d6a77..2f1c40508 100644
--- a/core/net/mac/xmac.c
+++ b/core/net/mac/xmac.c
@@ -28,7 +28,7 @@
  *
  * This file is part of the Contiki operating system.
  *
- * $Id: xmac.c,v 1.55 2010/03/01 13:30:23 nifi Exp $
+ * $Id: xmac.c,v 1.56 2010/04/30 07:32:39 adamdunkels Exp $
  */
 
 /**
@@ -61,7 +61,7 @@
 #include <string.h>
 
 #ifndef WITH_ACK_OPTIMIZATION
-#define WITH_ACK_OPTIMIZATION        1
+#define WITH_ACK_OPTIMIZATION        0
 #endif
 #ifndef WITH_ENCOUNTER_OPTIMIZATION
 #define WITH_ENCOUNTER_OPTIMIZATION  1
@@ -298,7 +298,7 @@ powercycle(struct rtimer *t, void *ptr)
     schedule_powercycle(t, xmac_config.on_time);
     PT_YIELD(&pt);
 
-    if(xmac_config.off_time > 0) {
+    if(xmac_config.off_time > 0 && !NETSTACK_RADIO.receiving_packet()) {
       powercycle_turn_radio_off();
       if(waiting_for_packet != 0) {
 	waiting_for_packet++;