diff --git a/csma.c b/csma.c
index dd1c00b..aef0120 100644
--- a/csma.c
+++ b/csma.c
@@ -2,9 +2,11 @@
 #include <linux/slab.h>
 #include <linux/hrtimer.h>
 #include <linux/spinlock.h>
+#include <linux/random.h>
 
 #include "csma.h"
 #include "cc2520.h"
+#include "radio.h"
 
 struct cc2520_interface *csma_top;
 struct cc2520_interface *csma_bottom;
@@ -16,12 +18,24 @@ static int backoff_max_cong;
 static struct hrtimer backoff_timer;
 
 static u8* cur_tx_buf;
+static u8 cur_tx_len;
+
+static spinlock_t state_sl;
+
+enum cc2520_csma_state_enum {
+	CC2520_CSMA_IDLE,
+	CC2520_CSMA_TX,
+	CC2520_CSMA_CONG
+};
+
+static int csma_state;
 
 static int cc2520_csma_tx(u8 * buf, u8 len);
 static void cc2520_csma_tx_done(u8 status);
 static void cc2520_csma_rx_done(u8 *buf, u8 len);
 static enum hrtimer_restart cc2520_csma_timer_cb(struct hrtimer *timer);
 static void cc2520_csma_start_timer(int us_period);
+static int cc2520_csma_get_backoff(int min, int max);
 
 int cc2520_csma_init()
 {
@@ -33,6 +47,8 @@ int cc2520_csma_init()
 	backoff_max_init = CC2520_DEF_INIT_BACKOFF;
 	backoff_max_cong = CC2520_DEF_CONG_BACKOFF;
 
+	csma_state = CC2520_CSMA_IDLE;
+
 	cur_tx_buf = kmalloc(PKT_BUFF_SIZE, GFP_KERNEL);
 	if (!cur_tx_buf) {
 		goto error;
@@ -62,6 +78,16 @@ void cc2520_csma_free()
 	hrtimer_cancel(&backoff_timer);
 }
 
+static int cc2520_csma_get_backoff(int min, int max)
+{
+	uint rand_num;
+	int span;
+
+	span = max - min;
+	get_random_bytes(&rand_num, 4);
+	return min + (rand_num % span);
+}
+
 static void cc2520_csma_start_timer(int us_period)
 {
     ktime_t kt;
@@ -71,19 +97,71 @@ static void cc2520_csma_start_timer(int us_period)
 
 static enum hrtimer_restart cc2520_csma_timer_cb(struct hrtimer *timer)
 {
+    ktime_t kt;
+    int new_backoff;
 
-	return HRTIMER_NORESTART;
-}
-
+    //printk(KERN_INFO "[cc2520] - csma timer fired. \n");
+	if (cc2520_radio_is_clear()) {
+		//printk(KERN_INFO "[cc2520] - channel clear, sending.\n");
+		csma_bottom->tx(cur_tx_buf, cur_tx_len);
+		return HRTIMER_NORESTART;		
+	}
+	else {
+		spin_lock(&state_sl);
+		if (csma_state == CC2520_CSMA_TX) {
+			csma_state = CC2520_CSMA_CONG;
+			spin_unlock(&state_sl);
+
+			new_backoff = 
+				cc2520_csma_get_backoff(backoff_min, backoff_max_cong);
+
+			INFO((KERN_INFO "[cc2520] - channel still busy, waiting %d uS\n", new_backoff));
+			kt=ktime_set(0,1000 * new_backoff);
+			hrtimer_forward_now(&backoff_timer, kt);
+			return HRTIMER_RESTART;
+		}
+		else {
+			csma_state = CC2520_CSMA_IDLE;
+			spin_unlock(&state_sl);
 
+			INFO((KERN_INFO "[cc2520] - csma/ca: channel busy. aborting tx\n"));
+			csma_top->tx_done(-CC2520_TX_BUSY);
+			return HRTIMER_NORESTART;
+		}
+	}
+}
 
 static int cc2520_csma_tx(u8 * buf, u8 len)
 {
-	return csma_bottom->tx(buf, len);
+	int backoff;
+
+	spin_lock(&state_sl);
+	if (csma_state == CC2520_CSMA_IDLE) {
+		csma_state = CC2520_CSMA_TX;
+		spin_unlock(&state_sl);
+
+		memcpy(cur_tx_buf, buf, len);
+		cur_tx_len = len;
+
+		backoff = cc2520_csma_get_backoff(backoff_min, backoff_max_init);
+
+		//printk(KERN_INFO "[cc2520] - waiting %d uS to send.\n", backoff);
+		cc2520_csma_start_timer(backoff);
+	}
+	else {
+		spin_unlock(&state_sl);
+		csma_top->tx_done(-CC2520_TX_BUSY);
+	}
+
+	return 0;
 }
 
 static void cc2520_csma_tx_done(u8 status)
 {
+	spin_lock(&state_sl);
+	csma_state = CC2520_CSMA_IDLE;
+	spin_unlock(&state_sl);
+	//printk(KERN_INFO "[cc2520] - tx done and successful.\n");
 	csma_top->tx_done(status);
 }