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
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