[time-nuts] wifi with time sync
jimlux at earthlink.net
Sat Jan 14 11:21:37 EST 2017
On 1/14/17 7:53 AM, John Hawkinson wrote:
> I tried to engage with you off-list and give you some pointers on this, but
> that does not seem to be working. Consumer wifi driver problems are manifestly
> inappropriate for this list, and trying to do both at once leads to gross
> confusion :( I know this is my personal opinion and I don't speak with any
I think that precision time distribution, over whatever medium, *is* the
subject of the list. And to the extent we are cursed with hardware that
is designed for mass markets and didn't give any thought to time
distribution, figuring out how to make a rayon purse out of a pig's ear
is probably worth discussing.
NTP does a fine job at the millisecond-ey level with commodity network
hardware, and a lot of the techniques that are used by NTP - clock
modeling, various statistical filters to estimate delays from
measurements, etc. might have applicability to packet radio links (which
WiFi is but one instance of).
It might be intriguing to contemplate *wireless* precision time
distribution with other 802.xx systems - Zigbee, WiMax, etc.
This is an area of significant interest in the space community: We're
trying to get away from artisanally hand-crafted individual works of
technology to better utilize mass produced products. When you're
contemplating building a radio telescope with 50 receivers, each on a
small satellite, or deployed as a node on the far side of the moon,
trying to get time and frequency sync among the receivers is a
non-trivial problem. Unlike the Allen Telescope Array, it's hard to
string fiber in environmentally controlled ducts, etc. If I could use an
off the shelf WiFi or Zigbee chip to do so, that would be a wonderful
thing. Or even if we have to design our own chip, leveraging existing
protocols and designs means we don't have to do that part of the job again.
Just to put some numerical desires to it..
Say we're contemplating HF signals.. Call it 50 MHz at the top, or 20
ns/cycle. Say you want 1/1000 of a cycle phase knowledge, so you need
20 ps timing knowledge - for integration times of, say a few seconds.
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