On Wed, Sep 15, 2010 at 9:07 AM, Jonathan Guerin <jonathan@xxxxxxxxxxxx> wrote:
> On Wed, Sep 15, 2010 at 3:39 AM, Bob Copeland <me@xxxxxxxxxxxxxxx> wrote:
>> On Tue, Sep 14, 2010 at 1:51 AM, Jonathan Guerin <jonathan@xxxxxxxxxxxx> wrote:
>>> as well as 1360 frames which came in with a negative CONTENTION_TIME.
>>>
>>> Ignoring the fact that some frames are coming up with a negative
>>> CONTENTION_TIME (which potentially points to another problem)
>>
>> How negative are these? Are you subtracting more for, say, txtime
>> than you should be?
>
> Actually, I've found that they were exactly one slot time negative
> (give or take 1us). That said, I was doing these calculations using
> the 802.11a OFDM spec timings. I looked at the driver and realised
> that SIFS is actually being initialised to 14us, whereas it should be
> set to 16us. This will also affect DIFS. Once I take these into
> consideration, they come up as approximately 4-5us out. Now that I
> think about it, it could make sense.
>
> With regards to the tx_time, I'm using the OFDM tx_time formula from
> the 802.11-2007 spec. I'm trying to lock down enough aspects of the
> timing in order to verify it. From what I've seen now, my txtime is
> almost always 4us short, but I can't work out why. I will get the code
> for the txtime when I get into my office, I don't have it on this
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 }
};
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);
}
>
>>
>> --
>> Bob Copeland %% www.bobcopeland.com
>>
>
> Thanks,
>
> Jonathan
>
Thanks,
--
Jonathan Guerin
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