考虑链路丢失率或冲突的Mac802_11仿真 (for NS2)

In Mac802_11 simulations, packets are assumed to be received correctly within transmission range, hence a RTS will definitely get a CTS response. However, when you use shadowing propagation model, or consider link loss ratio in opportunistic routing, or consider
using competition CTS, a sender may lose CTS due to channel contention or dynamic channel condition and a RTS will be retransmitted. In this case, the specified receiver should not refuse to respond to it; otherwise any algorithm which needs RTS/CTS negotiation
may compromise its performance a lot.

Therefore, following lines need to be modified in mac/mac-802_11.cc

/* [PH] following lines will be replaced =======================

if(tx_state_ != MAC_IDLE) {

discard(p, DROP_MAC_BUSY);

return;

}

*/

/*

* If I'm responding to someone else, discard this RTS.

*/

/* if(pktCTRL_) {

discard(p, DROP_MAC_BUSY);

return;

}

*/

//=================replace by==========================

// [PH] this node may receive a re-transmitting RTS

// from its sender who didn't receive the previous CTS

// due to collision or dynamic channel condition

// thus it should cancel the waiting timer for data

// and retransmit CTS

// However, if pktCTRL_ != CTS then I should discard the received RTS

if(tx_state_ != MAC_IDLE) {

if(tx_state_ == MAC_CTS) {

hdr_mac802_11 *mh = HDR_MAC802_11(pktCTRL_);

if( ETHER_ADDR(mh->dh_ra) == ETHER_ADDR(rf->rf_ta) ) {

if(mhSend_.busy()) { mhSend_.stop();}

setTxState(MAC_IDLE);

Packet::free(pktCTRL_);

pktCTRL_ = 0;

} else {

discard(p, DROP_MAC_BUSY); //received a new RTS

return;

}

} else {

discard(p, DROP_MAC_BUSY); //I'm not waiting for data. I'm busy in sending RTS/DATA/ACK, cannot handle another connection request.

return;

}

}