[ntp:questions] Windows and Wi-Fi - starts well, frequency steps?

Dave Hart davehart at gmail.com
Sun Dec 25 01:11:00 UTC 2011


On Sat, Dec 24, 2011 at 18:18, unruh <unruh at invalid.ca> wrote:
> On 2011-12-24, David J Taylor <david-taylor at blueyonder.co.uk.invalid> wrote:
>> - one Netbook PC worked very well on a LAN connection (about 1 ms steady
>> jitter).  However, when moving to a Wi-Fi connection after a power-down
>> reboot, the reported jitter gradually built up over about a 30 minute
>> period, ending up with a 5 ms peak, later decaying to a value between 1.3
>> and 2.5 ms.  The offset also appeared to have spikes which because much
>> worse after about 30 minutes.
>
> I would expect wifi to be much worse than a lan for jitter. The signal
> has to be converted, broadcast, reconverted and then sent on down the
> lan. That all takes time, and can have aproblem with dropped bits,
> retransmission, etc.

Retransmission is the killer issue for NTP performance over 802.11.
For practical interop with software developed on wired networks, WiFi
equipment detects packet loss and triggers retransmission invisibly to
higher layers.  I suspect NTP would do better if the 802.11 layer
differentiated its handling of UDP 53 and 123 :)  Where dropping DNS
queries has an awful impact on user experience, it would be preferable
for NTP compared to introducing the extra delay and thereby jitter.
I'd love to see more DNS over TCP, so that perhaps one day layer 2
wireless networks will do better letting UDP drop rather than
retransmit at layer 2.  NTP is like VoIP in this regard, dropping the
traffic is likely better than unbounded delay for retransmission.

I wonder if the 90 minute periodicity to the -0.4 PPM shifts aligns
with some WiFi security renegotiation.

Cheers,
Dave Hart


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