[ntp:questions] ARRGH!!! I woke up to a 50 SECOND clock error.

Chuck Swiger cswiger at mac.com
Thu Mar 15 22:57:13 UTC 2012

On Mar 15, 2012, at 2:06 PM, unruh wrote:
> Before hitting on 13 different servers, it would be better tofigure out
> what the problem is that is giving you such lousy results.

It's not hard to make an educated guess as to the cause; since the topology is:

"PC Wifi --> Wifi router --> wired router --> cable modem --> ISP -- internet"

WiFi radios are half-duplex since only one radio can be sending at a time, else you get a backoff similar to ethernet collisions on a hub.  If the WiFi link is idle (and nobody else is using 2.4 GHz bands nearby!), one ought to be able to get reasonably good timing, but as soon as there is any significant congestion from other machines (or phones, tablets, etc), a wireless NTP client will experience heavy jitter on the level of tens of milliseconds.

[ That doesn't get Ron up to 120ms, but if double-NAT is going on, or the routers aren't playing nice with each other for some other reason, it wouldn't be surprising to end up with marginal timekeeping results.  Plug something directly into ethernet jack on the wired router and retry.... ]


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