Hi, I got some problems about the madwifi driver and mac80211 framework.
I. DIFS time problem.
In minstrel imiplementation on the Madwifi 0.9.4 driver, the DIFS time is
28ms as shown in the following code obtained from the
calc_usecs_unicast_packet() function.
case IEEE80211_T_OFDM:
t_slot = 9;
t_sifs = 16;
t_difs = 28;
However, according to the IEEE 802.11 2007 standard, the DIFS time for OFDM
should be 34, which is 2 slot time plus a SIFS time shown in table 17-15 in
page 626 or described in page 271.
Which one is right ? Does the DIFS depend on the hardware?
2. time needed for a frame transmission
In ieee80211_frame_duration() function in util.c on the mac80211 framework,
the frame time is calculated as the following, which adds a SIFS time.
dur = 16; /* SIFS + signal ext */
dur += 16; /* T_PREAMBLE = 16 usec */
dur += 4; /* T_SIGNAL = 4 usec */
dur += 4 * DIV_ROUND_UP((16 + 8 * (len + 4) + 6) * 10, 4 * rate); /* T_SYM x
N_SYM */
why is there a SIFS time?
If there is no SIFS time, the function returns the time which is needed for
transmitting the frame.
Another problem arrives because of this, Minstrel implementation on the
mac80211 uses this function to calculate the perfect_tx_time for a frame
delivered. However, this time should be DIFS + backoff + Frametime+ SIFS +
ACK time, as shown in Minstrel's original Madwifi implementation. THis time
should be the total time needed for a successful transmitted frame.
Correct me if I am wrong. And I think you may have other considerations.
Thanks for telling.
-
Wei YIN
School of Information Technology and Electrical Engineering,
the University of Queensland,
Brisbane, QLD 4072, Australia
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