[ntp:questions] Compensating for asymmetric delay on a per-peer/server basis?

David Woolley david at ex.djwhome.demon.invalid
Thu Sep 11 22:43:32 UTC 2014


On 11/09/14 22:11, Rob wrote:
> mike cook <michael.cook at sfr.fr> wrote:
>>
>> Le 11 sept. 2014 à 21:08, Paul a écrit :
>>
>>> On Thu, Sep 11, 2014 at 2:08 PM, mike cook <michael.cook at sfr.fr> wrote:
>>>>   Did I miss something?
>>>
>>> On Tue, Sep 9, 2014 at 3:17 PM, Rich Wales <richw at richw.org> wrote:
>>>> My home LAN is connected to my school's network via a cable modem.
>>>
>>> If we make the (safe) assumption of a common cable ISP/FiOS in the
>>> Palo Alto area the path is asymmetric.
>>
>>    Yup, AsymmetricDSL does have different up/down bit rates. What I really meant was that the difference would not explain his issue.
>>    ex: with a 12Mbps down rate and 1.3Mbps up rate, the ratio is around 40usec to 300usec transfer of a 48byte NTP packet.
>
> Bitrate is not the only modem parameter.
>

The baud rate is 4kHz (approx).  That means that the absolute minimum 
packet time is 125 microseconds.  As packets probably don't align with 
signalling units, there is a good chance that you will need to add 
another 125 microseconds.  There is also going to be a delay of, on 
average, 63 microseconds waiting for the start of a signalling unit.

To this you need to allow for any expansion due to FEC coding, and the 
likely use of interleaving, which, to be effective, would need to spread 
adjacent bits over quite a lot of signalling units.  I can't find a 
figure at the moment, but my guess is that it pushes the minimum delay 
into the milliseconds range.

(Gamers tend to turn off interleaving, if they can, as they want low 
latency at all costs.  Businesses are most likely to want it on.)



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