Question on ieee80211_frame_duration in util.c

Question on ieee80211_frame_duration in util.c

[Peizhao Hu]

Hi all,

I am trying to understand why this function is implemented as such 
currently.

within the util.c in mac80211, if the coding is OFDM:

         /*
	 * OFDM:
	 *
	 * N_DBPS = DATARATE x 4	
	 * N_SYM = Ceiling((16+8xLENGTH+6) / N_DBPS)
	 *    (16 = SIGNAL time, 6 = tail bits)
	 * TXTIME = T_PREAMBLE + T_SIGNAL + T_SYM x N_SYM + Signal Ext
	 *
	 * T_SYM = 4 usec
	 * 802.11a - 17.5.2: aSIFSTime = 16 usec
	 * 802.11g - 19.8.4: aSIFSTime = 10 usec +
	 *    signal ext = 6 usec
	 */
	dur = 16; /* SIFS + signal ext */
	dur += 16; /* 17.3.2.3: T_PREAMBLE = 16 usec */
	dur += 4; /* 17.3.2.3: T_SIGNAL = 4 usec */
	dur += 4 * DIV_ROUND_UP((16 + 8 * (len + 4) + 6) * 10,
	            4 * rate); /* T_SYM x N_SYM */

so the Tx time for a frame is calculated as

SIFS + T_PREAMBLE + T_SIGNAL + T_SYM * N_SYM

What is this Tx time calculation for? Data or the ack? I looks like it 
is for the ack, since it has an SIFS in front. Because the my 
understanding of the standard IEEE std 802.11 2007 Section 9. a full 
transmission consists of

DIFS + Data + SIFS + Ack

 From the rc80211_minstrel.c implementation,

calc_rate_durations(struct minstrel_sta_info *mi, struct ieee80211_local *local,
struct minstrel_rate *d, struct ieee80211_rate *rate)
{
     int erp = !!(rate->flags&  IEEE80211_RATE_ERP_G);
     d->perfect_tx_time = ieee80211_frame_duration(local, 1200, rate->bitrate, erp, 1);
     d->ack_time = ieee80211_frame_duration(local, 10, rate->bitrate, erp, 1);
}

It looks like minstrel treating the Tx time calculation for data frame 
and ack to be the same (as shown above).

If this is right, then we have

SIFS + Data + SIFS + Ack

Anyone has an idea on this?

-- 
regards;

Peizhao Hu