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

Rob nomail at example.com
Thu Sep 11 23:02:39 UTC 2014


David Woolley <david at ex.djwhome.demon.invalid> wrote:
> 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.

The poster had no DSL, he mentioned a cable modem.
In a cable modem there is another issue: the subscribers share the
same uplink channel in an alternating fashion, while the downlink channel
is used only by the cable headend.  To avoid collisions, there will
usually be some user timeslot allocation by the headend.



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