[ns] mac-802_11.cc ACKTimeout looks incorrect & sifs/difs used incorectly?

["Susan Tsao" <SusanTsao@WoodsideNet.Com>]

Hello,

I have 2 quesiton relating to NS implementation of the Mac 802.11
protocol.

1) In mac-802_11.h, it defines ACKTimeout to be:

	#define ACKTimeout(len) (DATA_Time(len) + ACK_Time + sifs_ +
difs_)

    But according to standard, it should be:
	#define ACKTimeout(len) (DATA_Time(len) + ACK_Time + sifs_ +
sifs_)

2) In mac-802_11.cc:
		
	void Mac802_11::tx_resume()  
	{
	      .........
	    else if(pktTx_) {
               	 if(mhBackoff_.busy() == 0)
                        	mhDefer_.start(difs_);	
	   .........
	}  

According to standards:  Upon receiving a CTS frame, station will wait
for SIFS period before sending DATA
However, according to the above code, station waits for DIFS period
before sending DATA.
Can someone please explain to me whether this is a bug in the code or
did I misinterpret the standard.
Thank you.
Susan