Question on ieee80211_frame_duration in util.c

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