problems about the madwifi driver and mac80211 framework

[Peizhao Hu <Peizhao.Hu@nicta.com.au>]

Hi all,

I am studying the minstrel rate control mechinsim in mac80211. I have
observed a few strange problem with the current implementation of the
minstrel on the mac80211. They are explained below:

1. Different DIFS values were used by madwifi 0.9.4 and mac80211.
In madwifi 0.9.4, the DIFS is set to 28 from the source code in the
calc_usecs_unicast_packet() function.

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.

Why we have different DIFS values?

2. Another question is on how we calculate the frame transmission time

In the function "ieee80211_frame_duration()" within util.c on the
mac80211 framework, the frame time is calculated as the following.
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 */

In mac80211 based minstrel implementation, minstrel uses the able
fuction to calculate the time for sending a data frame as well as
sending an ack.

to our understanding the normal transmission time is calculated as below
(assume no retransmission):
DIFS + contentionWindow + dataTime + SIFS + ackTime

where

dataTime=T_PREAMBLE + T_SIGNAL + T_SYM * N_SYM
ackTime is similar

but in Minstrel's implementation (using the above
"ieee80211_frame_duration()" function), we effectively calculate the
transmission time as

SIFS + contentionWindow + dataTime + SIFS + ackTime

So why there are two SIFS???

Any idea?

--
regards;

Peizhao

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