[ns] Can a node receive a packet while carrier sensing?

[Shen Ren]

it discard the packet if the channel is busy. as u could see right starting at the swtich sentence

int
Mac802_11::check_pktCTRL()
{
	struct hdr_mac802_11 *mh;
	double timeout;

	if(pktCTRL_ == 0)
		return -1;
	if(tx_state_ == MAC_CTS || tx_state_ == MAC_ACK)
		return -1;

	mh = HDR_MAC802_11(pktCTRL_);
							  
	switch(mh->dh_fc.fc_subtype) {
	/*
	 *  If the medium is not IDLE, don't send the CTS.
	 */
	case MAC_Subtype_CTS:
		if(!is_idle()) {
			discard(pktCTRL_, DROP_MAC_BUSY); pktCTRL_ = 0;
			return 0;
		}
		SET_TX_STATE(MAC_CTS);
		
		/*
		 * timeout:  cts + data tx time calculated by
		 *           adding cts tx time to the cts duration
		 *           minus ack tx time -- this timeout is
		 *           a guess since it is unspecified
		 *           (note: mh->dh_duration == cf->cf_duration)
		 */
		timeout = txtime(ETHER_CTS_LEN, basicRate_)
			+ DSSS_MaxPropagationDelay			// XXX
			+ sec(mh->dh_duration)
			+ DSSS_MaxPropagationDelay			// XXX
			- sifs_
			- txtime(ETHER_ACK_LEN, basicRate_);
		
		break;
		/*
		 * IEEE 802.11 specs, section 9.2.8
		 * Acknowledments are sent after an SIFS, without regard to
		 * the busy/idle state of the medium.
		 */
	case MAC_Subtype_ACK:
		SET_TX_STATE(MAC_ACK);

		timeout = txtime(ETHER_ACK_LEN, basicRate_);
		
		break;
	default:
		fprintf(stderr, "check_pktCTRL:Invalid MAC Control subtype\n");
		exit(1);
	}
        TRANSMIT(pktCTRL_, timeout);
	return 0;
}

-----Original Message-----
From:	ns-users-bounces at ISI.EDU on behalf of Athanasia Tsertou
Sent:	Tue 11/16/2004 7:26 PM
To:	ns-users at ISI.EDU
Cc:	
Subject:	[ns] Can a node receive a packet while carrier sensing?

Hello everyone,

I have a technical question regarding the IEEE 802.11 standard. Suppose that we
have the following situation.

A >---------------------< B >---------------------< C

A sends an RTS to B which is received correctly. Now, during the SIFS period
that B waits before answering with a CTS, C decides to transmit an RTS packet.
The standard says that due to Carrier Sensing B will find the channel busy and
will not send a CTS back to A. My question is: What does sensing really mean?
Does it mean that B actually receives the RTS from C correctly, so after
RTS+SIFS it responds to C with a CTS?

Thank you in advance

Athanasia


--
Athanasia Tsertou
Signals and Systems Group
School of Engineering and Electronics
The University of Edinburgh
King's Buildings, Mayfield Rd,Edinburgh EH9 3JL