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From: Bruno Randolf <br1@einfach.org>
To: Jonathan Guerin <jonathan@guerin.id.au>
Subject: Re: [ath5k-devel] [support] ath5k contention windows
Date: Wed, 15 Sep 2010 10:47:38 +0900
Cc: Bob Copeland <me@bobcopeland.com>,
	"linux-wireless" <linux-wireless@vger.kernel.org>,
	"ath5k-devel" <ath5k-devel@lists.ath5k.org>
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On Wed September 15 2010 09:44:09 Jonathan Guerin wrote:
> The timings I used were from the 802.11-2007 spec.
> SIFS, DIFS & Slot Time: Table 17-15—OFDM PHY characteristics
> NDPS values: Table 17-3—Modulation-dependent parameters
> txtime calculations: Section 17.4.3 OFDM TXTIME calculation
> 
> Here are the functions I use to do the basics:
> int16_t ndbps(uint8_t rate_Mbps)
> {
>   //Radiotap returns rate in 500kbps units
>   rate_Mbps/=2;
> 
>    static const uint8_t rate_nsyms[][2] = {
>       {  6,  24 },
>       {  9,  36 },
>       { 12,  48 },
>       { 18,  72 },
>       { 24,  96 },
>       { 36, 144 },
>       { 48, 192 },
>       { 54, 216 }

this is rate * 4

>    };
>    static const size_t nof_rate_nsyms = sizeof(rate_nsyms) /
> sizeof(rate_nsyms[0]);
>    for(size_t i = 0; i < nof_rate_nsyms; ++i) {
>       if(rate_Mbps == rate_nsyms[i][0]) {
>          return rate_nsyms[i][1];
>       }
>    }
>    return -1;
> }
> 
> int getTXTime(int rate, int noctets) {
>   uint16_t usecs = 0;
>   const int16_t nof_bps = ndbps(rate);
> 
>   int chan_rate = 1;
> 
>   if(-1 != nof_bps) {
>     const uint16_t preamble = 16 * chan_rate;
>     const uint16_t signal = 4 * chan_rate;
>     const uint16_t tsym = 4 * chan_rate;
>     const uint16_t nsyms = ceill((16.0 + 8.0 * (noctets) + 6.0) / nof_bps);
>     usecs = preamble + signal + tsym * nsyms;
>   }
> 
>   return(usecs);

if you compare that to the code in mac80211/util.c ieee80211_frame_duration():

		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 */

it is equivalent, except that you don't add 16 /* SIFS + signal ext */. is 
this intended?

bruno