Updated Contiki X-MAC to use hardware acks, which allows for tighter timing and results in a lower duty cycle. Hardware acks also make X-MAC more robust against packet losses, as there is a direct way for the CSMA layer to detect loss of data packets. Before, Contiki X-MAC only detected loss of strobes.

core/net/mac/xmac.c

@@ -28,7 +28,7 @@
  *
  * This file is part of the Contiki operating system.
  *
- * $Id: xmac.c,v 1.59 2010/10/03 22:46:53 joxe Exp $
+ * $Id: xmac.c,v 1.60 2011/01/25 14:31:09 adamdunkels Exp $
  */
 
 /**
@@ -137,7 +137,7 @@ struct xmac_hdr {
    cycle. */
 #define ANNOUNCEMENT_TIME (random_rand() % (ANNOUNCEMENT_PERIOD))
 
-#define DEFAULT_STROBE_WAIT_TIME (7 * DEFAULT_ON_TIME / 8)
+#define DEFAULT_STROBE_WAIT_TIME (5 * DEFAULT_ON_TIME / 8)
 
 struct xmac_config xmac_config = {
   DEFAULT_ON_TIME,
@@ -415,6 +415,40 @@ register_encounter(const rimeaddr_t *neighbor, rtimer_clock_t time)
 #endif /* WITH_ENCOUNTER_OPTIMIZATION */
 /*---------------------------------------------------------------------------*/
 static int
+detect_ack(void)
+{
+#define INTER_PACKET_INTERVAL              RTIMER_ARCH_SECOND / 5000
+#define ACK_LEN 3
+#define AFTER_ACK_DETECTECT_WAIT_TIME      RTIMER_ARCH_SECOND / 1000
+  rtimer_clock_t wt;
+  uint8_t ack_received = 0;
+  
+  wt = RTIMER_NOW();
+  leds_on(LEDS_GREEN);
+  while(RTIMER_CLOCK_LT(RTIMER_NOW(), wt + INTER_PACKET_INTERVAL)) { }
+  leds_off(LEDS_GREEN);
+  /* Check for incoming ACK. */
+  if((NETSTACK_RADIO.receiving_packet() ||
+      NETSTACK_RADIO.pending_packet() ||
+      NETSTACK_RADIO.channel_clear() == 0)) {
+    int len;
+    uint8_t ackbuf[ACK_LEN + 2];
+    
+    wt = RTIMER_NOW();
+    while(RTIMER_CLOCK_LT(RTIMER_NOW(), wt + AFTER_ACK_DETECTECT_WAIT_TIME)) { }
+    
+    len = NETSTACK_RADIO.read(ackbuf, ACK_LEN);
+    if(len == ACK_LEN) {
+      ack_received = 1;
+    }
+  }
+  if(ack_received) {
+    leds_toggle(LEDS_RED);
+  }
+  return ack_received;
+}
+/*---------------------------------------------------------------------------*/
+static int
 send_packet(void)
 {
   rtimer_clock_t t0;
@@ -422,7 +456,8 @@ send_packet(void)
   rtimer_clock_t encounter_time = 0;
   int strobes;
   struct xmac_hdr *hdr;
-  int got_strobe_ack = 0;
+  uint8_t got_strobe_ack = 0;
+  uint8_t got_ack = 0;
   uint8_t strobe[MAX_STROBE_SIZE];
   int strobe_len, len;
   int is_broadcast = 0;
@@ -456,6 +491,8 @@ send_packet(void)
   }
   is_reliable = packetbuf_attr(PACKETBUF_ATTR_RELIABLE) ||
     packetbuf_attr(PACKETBUF_ATTR_ERELIABLE);
+
+  packetbuf_set_attr(PACKETBUF_ATTR_MAC_ACK, 1);
   len = NETSTACK_FRAMER.create();
   strobe_len = len + sizeof(struct xmac_hdr);
   if(len == 0 || strobe_len > sizeof(strobe)) {
@@ -518,6 +555,9 @@ send_packet(void)
 
       now = RTIMER_NOW();
       wait = ((rtimer_clock_t)(e->time - now)) % (DEFAULT_PERIOD);
+      if(wait < 2 * DEFAULT_ON_TIME) {
+        wait = DEFAULT_PERIOD;
+      }
       expected = now + wait - 2 * DEFAULT_ON_TIME;
 
 #if WITH_ACK_OPTIMIZATION
@@ -549,7 +589,7 @@ send_packet(void)
   /* Send a train of strobes until the receiver answers with an ACK. */
 
   /* Turn on the radio to listen for the strobe ACK. */
-  on();
+  //  on();
   collisions = 0;
   if(!is_already_streaming) {
     watchdog_stop();
@@ -563,6 +603,8 @@ send_packet(void)
       while(got_strobe_ack == 0 &&
 	    RTIMER_CLOCK_LT(RTIMER_NOW(), t + xmac_config.strobe_wait_time)) {
 	rtimer_clock_t now = RTIMER_NOW();
+
+#if 0
 	/* See if we got an ACK */
 	packetbuf_clear();
 	len = NETSTACK_RADIO.read(packetbuf_dataptr(), PACKETBUF_SIZE);
@@ -588,6 +630,7 @@ send_packet(void)
 	    PRINTF("xmac: send failed to parse %u\n", len);
 	  }
 	}
+#endif /* 0 */
       }
       
       t = RTIMER_NOW();
@@ -605,8 +648,9 @@ send_packet(void)
           off();
 	} else {
 	  rtimer_clock_t wt;
+          on();
 	  NETSTACK_RADIO.send(strobe, strobe_len);
-#if 1
+#if 0
 	  /* Turn off the radio for a while to let the other side
 	     respond. We don't need to keep our radio on when we know
 	     that the other side needs some time to produce a reply. */
@@ -614,7 +658,12 @@ send_packet(void)
 	  wt = RTIMER_NOW();
 	  while(RTIMER_CLOCK_LT(RTIMER_NOW(), wt + WAIT_TIME_BEFORE_STROBE_ACK));
 #endif /* 0 */
-	  on();
+
+          if(detect_ack()) {
+            got_strobe_ack = 1;
+          } else {
+            off();
+          }
         }
       }
     }
@@ -633,8 +682,6 @@ send_packet(void)
   } else {
     off();
   }
-#else /* WITH_ACK_OPTIMIZATION */
-  off();
 #endif /* WITH_ACK_OPTIMIZATION */
 
   /* restore the packet to send */
@@ -644,7 +691,14 @@ send_packet(void)
   /* Send the data packet. */
   if((is_broadcast || got_strobe_ack || is_streaming) && collisions == 0) {
     NETSTACK_RADIO.send(packetbuf_hdrptr(), packetbuf_totlen());
+
+    if(!is_broadcast) {
+      if(detect_ack()) {
+        got_ack = 1;
       }
+    }
+  }
+  off();
 
 #if WITH_ENCOUNTER_OPTIMIZATION
   if(got_strobe_ack && !is_streaming) {
@@ -675,7 +729,7 @@ send_packet(void)
 
   LEDS_OFF(LEDS_BLUE);
   if(collisions == 0) {
-    if(!is_broadcast && !got_strobe_ack) {
+    if(is_broadcast == 0 && got_ack == 0) {
       return MAC_TX_NOACK;
     } else {
       return MAC_TX_OK;
@@ -781,6 +835,8 @@ input_packet(void)
 	   acknowledge the strobe and wait for the packet. By using
 	   the same address as both sender and receiver, we flag the
 	   message is a strobe ack. */
+        waiting_for_packet = 1;
+#if 0
 	hdr->type = TYPE_STROBE_ACK;
 	packetbuf_set_addr(PACKETBUF_ADDR_RECEIVER,
 			   packetbuf_addr(PACKETBUF_ADDR_SENDER));
@@ -797,6 +853,7 @@ input_packet(void)
 	} else {
 	  PRINTF("xmac: failed to send strobe ack\n");
 	}
+#endif /* 0 */
       } else if(rimeaddr_cmp(packetbuf_addr(PACKETBUF_ADDR_RECEIVER),
                              &rimeaddr_null)) {
 	/* If the receiver address is null, the strobe is sent to