About the TSFT field
Hi,
I'm planning to use the TSFT field present in the radiotap header in order to reconstruct a timeline of packet reception at different AP.
As far as I've understood the TSFT is generated from the MAC, however I do not know to which instant the TSFT is relative, the first bit of the MAC payload, the beginning of the PLCP, etc.
Also I do not know from which reference the TSFT is generated, I mean two AP synchronized using NTP will have consistent TSFT, which kind of error should I expect?
R.
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-------------------------------------------------------- Roberto Riggio, Ph.D. CREATE-NET Future Networks (FuN) Senior Researcher Via alla Cascata 56/D - 38123 Povo Trento (Italy) e-mail: [email protected] Tel: (+39) 0461 408400 - interno/extension 1708 Fax: (+39) 0461 421157 http://www.create-net.org/~rriggio/ -------------------------------------------------------- The information transmitted is intended only for the person or entity to which it is addressed and may contain confidential and/or privileged material. Any review, retransmission, dissemination or other use of, or taking of any action in reliance upon, this information by persons or entities other than the intended recipient is prohibited according to the Italian Law 196/2003 of the Legislature. If you received this in error, please contact the sender and delete the material from any computer. Le informazioni contenute in questo messaggio di posta elettronica e nei file allegati sono da considerarsi strettamente riservate. Il loro utilizzo e' consentito esclusivamente al destinatario del messaggio, per le finalita' indicate nel messaggio stesso. Qualora riceveste questo messaggio senza esserne il destinatario, Vi preghiamo cortesemente di darcene notizia via e-mail e di procedere alla cancellazione del messaggio stesso dal Vostro sistema. Trattenere il messaggio stesso, divulgarlo anche in parte, distribuirlo ad altri soggetti, copiarlo, od utilizzarlo per finalita' diverse, costituisce comportamento contrario ai principi dettati dal D. Lgs. 196/2003.
Quoted 4 lines · Roberto Riggio
As far as I've understood the TSFT is generated from the MAC, however I do not know to which instant the TSFT is relative, the first bit of the MAC payload, the beginning of the PLCP, etc.
It's documented in the radiotap spec, but not all drivers are really doing the right thing here.
Quoted 3 lines
Also I do not know from which reference the TSFT is generated, I mean two AP synchronized using NTP will have consistent TSFT, which kind of error should I expect?
It's purely local time - no synchronisation happens.
johannes
Inline...
Quoted 2 lines · Johannes Berg
It's documented in the radiotap spec, but not all drivers are really doing the right thing here.
Found it. Thanks.
Quoted 5 lines
Quoted 3 lines
Also I do not know from which reference the TSFT is generated, I mean two AP synchronized using NTP will have consistent TSFT, which kind of error should I expect?
It's purely local time - no synchronisation happens.
Meaning that is the system time. right? What if I create an adhoc interface together with the AP? According to the standard devices in IBSS shall synch with each other using the tsf info in the beacon frames. Is this behavior implemented by mac80211?
Thanks
R.
Quoted 3 lines · Roberto Riggio
Quoted 1 line
It's purely local time - no synchronisation happens.
Meaning that is the system time. right?
No, it's wifi time. TSF in TUs. Whatever that means. It may or may not even have a fixed reference, and it's not even guaranteed to sync with the network.
Quoted 4 lines
What if I create an adhoc
interface together with the AP? According to the standard devices in IBSS shall synch with each other using the tsf info in the beacon frames. Is this behavior implemented by mac80211?
That behaviour is implemented, but it's not guaranteed that the TSF will be used for the radiotap timestamp field. In fact radiotap isn't really specified to be used while operating, so it seems to me that any free-running clock with the right frequency (TUs) would be acceptable.
In that sense, the only real use for the timestamp field (barring external information, like knowing what your driver does) would be for relative time inside a single capture file.
johannes
Quoted 5 lines · Johannes Berg
On Wed, 2014-08-13 at 19:33 +0200, Roberto Riggio wrote:
That behaviour is implemented, but it's not guaranteed that the TSF will be used for the radiotap timestamp field. In fact radiotap isn't really specified to be used while operating, so it seems to me that any free-running clock with the right frequency (TUs) would be acceptable.
Do you (or somebody else on this ML) happen to know if TSF is used for the radiotap TS for the ath9k driver?
Quoted 3 lines
In that sense, the only real use for the timestamp field (barring external information, like knowing what your driver does) would be for relative time inside a single capture file.
Like above, does anybody know if the ath9k driver synchronizes with other devices in ad hoc mode in the network.
Quoted 1 line
johannes
r.