On Thursday 07 February 2008 08:52:35 Johannes Berg wrote:
> Looks fine, one last question:
> > + if (mactime <= timestamp) {
> > + if (CONFIG_MAC80211_IBSS_DEBUG || net_ratelimit())
> > + printk(KERN_DEBUG "%s: beacon TSF higher than "
> > + "local TSF - IBSS merge with BSSID %s\n",
> > + dev->name, print_mac(mac, mgmt->bssid));
>
> I'd rewrite that as timestamp >= mactime and rename the two variables
> (maybe beacon_timestamp and phy_timestamp or so?), but that's not too
> important.
i'll do that, sure.
> However, in any case "<=" seems wrong, shouldn't we only
> merge when it's actually higher? If your hardware/firmware has synced
> properly and the PHY delay is accounted for properly etc. then every
> beacon from the own BSS will fulfil the == part of the condition, no?
true. i was not aware that the beacon timestamp is taking PHY delays into
account and wrongly asumed that the RX timestamp should always be a bit
later.
(which is also what i saw on my atheros hardware: we usually get the RX
timestamp about 60 to 300 usec later than the beacon timestamp, but it's
quite likely that we have not calibrated everything 100% correctly.)
> Also, the beacon timestamp is defined as follows:
>
> A STA sending a beacon shall set the value of the beacon’s
> timestamp so that it equals the value of the STA’s TSF timer at
> the time that the data symbol containing the first bit of the
> timestamp is transmitted to the PHY plus the transmitting STA’s
> delays through its local PHY from the MAC-PHY interface to its
> interface with the WM [e.g., antenna, light-emitting diode (LED)
> emission surface].
>
> (IEEE 802.11 11.1.2)
>
> Since we define the RX timestamp to be when the first data symbol of the
> frame hits the PHY, we here have to take into account the offset between
> the two, at 1 MBit that's 192 (=24 bytes * 8 usecs/byte) usecs earlier
> than the beacon timestamp.
>
> On the other hand, you're unlikely to hit that window, but I suppose it
> warrants at least a comment.
thats a very good point! if everything was calibrated properly and all delays
taken into account *exactly* that would cause us to go thru a merge all the
time, on every received beacon. i think we have to take that into account,
for different bitrates. what about something like:
timestamp >= ( mactime + (24 * 8 / beacon_phy_rate_in_mbps ))
i'll resend my patch adding that and the other proposed changes soon.
bruno
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