// Record the start time of the new schedule.
	start_time_ = NOW;
	/* Seperate slot_num_ and the node id: 
	   we may have flexibility as node number changes.
	*/
	slot_num_ = slot_pointer++;
	tdma_schedule_[slot_num_] = (char) index_;
}

/* To handle incoming packet. */
void MacTdma::recv(Packet* p, Handler* h) {
	struct hdr_cmn *ch = HDR_CMN(p);
	
	/* Incoming packets from phy layer, send UP to ll layer. 
	   Now, it is in receiving mode. 
	*/
	if (ch->direction() == hdr_cmn::UP) {
		// Since we can't really turn the radio off at lower level, 
		// we just discard the packet.
		if (!radio_active_) {
			free(p);
			//printf("<%d>, %f, I am sleeping...\n", index_, NOW);
			return;
		}

		sendUp(p);
		//printf("<%d> packet recved: %d\n", index_, tdma_pr_++);
		return;
	}
	
	/* Packets coming down from ll layer (from ifq actually),
	   send them to phy layer. 
	   Now, it is in transmitting mode. */
	callback_ = h;
	state(MAC_SEND);
	sendDown(p);
	//printf("<%d> packet sent down: %d\n", index_, tdma_ps_++);
}

void MacTdma::sendUp(Packet* p) 
{
	struct hdr_cmn *ch = HDR_CMN(p);

	/* Can't receive while transmitting. Should not happen...?*/
	if (tx_state_ && ch->error() == 0) {
		printf("<%d>, can't receive while transmitting!\n", index_);
		ch->error() = 1;
	};

	/* Detect if there is any collision happened. should not happen...?*/
	if (rx_state_ == MAC_IDLE) {
		SET_RX_STATE(MAC_RECV);     // Change the state to recv.
		pktRx_ = p;                 // Save the packet for timer reference.

		/* Schedule the reception of this packet, 
		   since we just see the packet header. */
		double rtime = TX_Time(p);
		assert(rtime >= 0);

		/* Start the timer for receiving, will end when receiving finishes. */
		mhRxPkt_.start(p, rtime);
	} else {
		/* Note: we don't take the channel status into account, 
		   as collision should not happen...
		*/
		printf("<%d>, receiving, but the channel is not idle....???\n", index_);
	}
}

/* Actually receive data packet when RxPktTimer times out. */
void MacTdma::recvDATA(Packet *p){
	/*Adjust the MAC packet size: strip off the mac header.*/
	struct hdr_cmn *ch = HDR_CMN(p);
	ch->size() -= ETHER_HDR_LEN;
	ch->num_forwards() += 1;

	/* Pass the packet up to the link-layer.*/
	uptarget_->recv(p, (Handler*) 0);
}

/* Send packet down to the physical layer. 
   Need to calculate a certain time slot for transmission. */
void MacTdma::sendDown(Packet* p) {
	u_int32_t dst, src, size;
  
	struct hdr_cmn* ch = HDR_CMN(p);
	struct hdr_mac_tdma* dh = HDR_MAC_TDMA(p);

	/* Update the MAC header, same as 802.11 */
	ch->size() += ETHER_HDR_LEN;

	dh->dh_fc.fc_protocol_version = MAC_ProtocolVersion;
	dh->dh_fc.fc_type       = MAC_Type_Data;
	dh->dh_fc.fc_subtype    = MAC_Subtype_Data;
	
	dh->dh_fc.fc_to_ds      = 0;
	dh->dh_fc.fc_from_ds    = 0;
	dh->dh_fc.fc_more_frag  = 0;
	dh->dh_fc.fc_retry      = 0;
	dh->dh_fc.fc_pwr_mgt    = 0;
	dh->dh_fc.fc_more_data  = 0;
	dh->dh_fc.fc_wep        = 0;
	dh->dh_fc.fc_order      = 0;

	if((u_int32_t)ETHER_ADDR(dh->dh_da) != MAC_BROADCAST)
		dh->dh_duration = DATA_DURATION;
	else
		dh->dh_duration = 0;

	dst = ETHER_ADDR(dh->dh_da);
	src = ETHER_ADDR(dh->dh_sa);
	size = ch->size();

	/* buffer the packet to be sent. */
	pktTx_ = p;
}

/* Actually send the packet. */
void MacTdma::send() 
{
	u_int32_t dst, src, size;
	struct hdr_cmn* ch;
	struct hdr_mac_tdma* dh;
	double stime;

	/* Check if there is any packet buffered. */
	if (!pktTx_) {
		printf("<%d>, %f, no packet buffered.\n", index_, NOW);
		return;
	}

	/* Perform carrier sence...should not be collision...? */
	if(!is_idle()) {
		/* Note: we don't take the channel status into account, ie. no collision,
		   as collision should not happen...
		*/
		printf("<%d>, %f, transmitting, but the channel is not idle...???\n", index_, NOW);
		return;
	}

	ch = HDR_CMN(pktTx_);
	dh = HDR_MAC_TDMA(pktTx_);  

	dst = ETHER_ADDR(dh->dh_da);
	src = ETHER_ADDR(dh->dh_sa);
	size = ch->size();
	stime = TX_Time(pktTx_);
	ch->txtime() = stime;
	
	/* Turn on the radio and transmit! */
	SET_TX_STATE(MAC_SEND);						     
	radioSwitch(ON);

	/* Start a timer that expires when the packet transmission is complete. */
	mhTxPkt_.start(pktTx_->copy(), stime);
	downtarget_->recv(pktTx_, this);        

	pktTx_ = 0;
}

// Turn on / off the radio
void MacTdma::radioSwitch(int i) 
{
	radio_active_ = i;
	//EnergyModel *em = netif_->node()->energy_model();
	if (i == ON) {
		//if (em && em->sleep())
		//em->set_node_sleep(0);
		//    printf("<%d>, %f, turn radio ON\n", index_, NOW); 
		Phy *p;
		p = netif_;
		((WirelessPhy *)p)->node_wakeup();
		return;
	}

	if (i == OFF) {
		//if (em && !em->sleep()) {
		//em->set_node_sleep(1);
		//    netif_->node()->set_node_state(INROUTE);
		Phy *p;
		p = netif_;
		((WirelessPhy *)p)->node_sleep();
		//    printf("<%d>, %f, turn radio OFF\n", index_, NOW);
		return;
	}
}

// make the new preamble.
void MacTdma::makePreamble() 
{
	u_int32_t dst;
	struct hdr_mac_tdma* dh;
	
	// If there is a packet buffered, file its destination to preamble.
	if (pktTx_) {
		dh = HDR_MAC_TDMA(pktTx_);  
		dst = ETHER_ADDR(dh->dh_da);
		//printf("<%d>, %f, write %d to slot %d in preamble\n", index_, NOW, dst, slot_num_);
		tdma_preamble_[slot_num_] = dst;
	} else {
		//printf("<%d>, %f, write NO_PKT to slot %d in preamble\n", index_, NOW, slot_num_);
		tdma_preamble_[slot_num_] = NOTHING_TO_SEND;
	}
}

/* Timers' handlers */
/* Slot Timer:
   For the preamble calculation, we should have it:
   occupy one slot time,
   radio turned on for the whole slot.
*/
void MacTdma::slotHandler(Event *e) 
{
	// Restart timer for next slot.
	mhSlot_.start((Packet *)e, slot_time_);

	// Make a new presamble for next frame.
	if ((slot_count_ == active_node_) || (slot_count_ == FIRST_ROUND)) {
		//printf("<%d>, %f, make the new preamble now.\n", index_, NOW);
		// We should turn the radio on for the whole slot time.
		radioSwitch(ON);

		makePreamble();
		slot_count_ = 0;
		return;
	}

	// If it is the sending slot for me.
	if (slot_count_ == slot_num_) {
		//printf("<%d>, %f, time to send.\n", index_, NOW);
		// We have to check the preamble first to avoid the packets coming in the middle.
		if (tdma_preamble_[slot_num_] != NOTHING_TO_SEND)
			send();
		else
			radioSwitch(OFF);

		slot_count_++;
		return;
	}
 
	// If I am supposed to listen in this slot
	if ((tdma_preamble_[slot_count_] == index_) || ((u_int32_t)tdma_preamble_[slot_count_] == MAC_BROADCAST)) {
		//printf("<%d>, %f, preamble[%d]=%d, I am supposed to receive now.\n", index_, NOW, slot_count_, tdma_preamble_[slot_count_]);
		slot_count_++;

		// Wake up the receive packets.
		radioSwitch(ON);
		return;
	}

	// If I dont send / recv, do nothing.
	//printf("<%d>, %f, preamble[%d]=%d, nothing to do now.\n", index_, NOW, slot_count_, tdma_preamble_[slot_count_]);
	radioSwitch(OFF);
	slot_count_++;
	return;
}

void MacTdma::recvHandler(Event *e) 
{
	u_int32_t dst, src; 
	int size;
	struct hdr_cmn *ch = HDR_CMN(pktRx_);
	struct hdr_mac_tdma *dh = HDR_MAC_TDMA(pktRx_);

	/* Check if any collision happened while receiving. */
	if (rx_state_ == MAC_COLL) 
		ch->error() = 1;

	SET_RX_STATE(MAC_IDLE);
  
	/* check if this packet was unicast and not intended for me, drop it.*/   
	dst = ETHER_ADDR(dh->dh_da);
	src = ETHER_ADDR(dh->dh_sa);
	size = ch->size();

	//printf("<%d>, %f, recv a packet [from %d to %d], size = %d\n", index_, NOW, src, dst, size);

	// Turn the radio off after receiving the whole packet
	radioSwitch(OFF);

	/* Ordinary operations on the incoming packet */
	// Not a pcket destinated to me.
	if ((dst != MAC_BROADCAST) && (dst != (u_int32_t)index_)) {
		drop(pktRx_);
		return;
	}
  
	/* Now forward packet upwards. */
	recvDATA(pktRx_);
}

/* After transmission a certain packet. Turn off the radio. */
void MacTdma::sendHandler(Event *e) 
{
	//  printf("<%d>, %f, send a packet finished.\n", index_, NOW);

	/* Once transmission is complete, drop the packet. 
	   p  is just for schedule a event. */
	SET_TX_STATE(MAC_IDLE);
	Packet::free((Packet *)e);
  
	// Turn off the radio after sending the whole packet
	radioSwitch(OFF);

	/* unlock IFQ. */
	if(callback_) {
		Handler *h = callback_;
		callback_ = 0;
		h->handle((Event*) 0);
	} 
}