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

unruh unruh at invalid.ca
Mon Dec 26 19:56:15 UTC 2011


On 2011-12-26, Charles Elliott <elliott.ch at verizon.net> wrote:
>  
>
>  
>
>> -----Original Message-----
>
>> From: questions-bounces+elliott.ch=verizon.net at lists.ntp.org
>
>> [mailto:questions-bounces+elliott.ch=verizon.net at lists.ntp.org] On
>
>> Behalf Of unruh
>
>> Sent: Sunday, December 25, 2011 4:46 PM
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>> To: questions at lists.ntp.org
>
>> Subject: Re: [ntp:questions] Windows and Wi-Fi - starts well, frequency
>
>> steps?
>
>> 
>
>> On 2011-12-25, Charles Elliott < <mailto:elliott.ch at verizon.net>
> elliott.ch at verizon.net> wrote:
>
>> ntp assumes that the outgoing and incoming trips are the same time. If
>> not, you get an offset. Thus if the return trip takes 10 min  and the
>> outgoing 1ms, ntp is going to say that the remote clock is five minutes
>> further behind than it is. On a wireless I could easily imagine that
>> the
>> trip times are not the same, and that one way of the other is
>> preferentially got a higher jitter than the other.
>
> That's true.  But that the offset becomes more negative as the delay
> increases implies that the response trip always takes longer than the
> request trip.  Isn't that somewhat unlikely?  Why would not the request and
> response times be equal on average?  That is, on average why don't the path
> asymmetries cancel each other out?

Why is it unlikely? And even if it is, why could it not be happening to
you? the two paths ARE assymetric--in one case it is the modem that is
receiving the wireless signal, in the other your machine. If your
modem's wireelss is weak or noisier than your machine's it could case an
assymetry. But this is irrelevant. Your measurements indictate that this
is what is happening.



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