[ntp:questions] frequency adjusting only

Greg Dowd GDowd at symmetricom.com
Wed Apr 30 18:23:28 UTC 2008


As noted, these are really stability measurements of the difference
between two clocks.  The absolute accuracies, particularly once you
reach the submillisecond domain, are impacted by the sum of all biases
in the measurement system, os, stack, driver, dma controller, bus, mac,
phy, physical layer, switching/routing matrix and protocols
(ARP/STP/QoS) and phy,mac,bus,driver,stack,os,app on the other end.  Not
just jitter and delay variation, but biases. Sometimes the biases are
complentary and cancel and sometimes they don't.

There is a real difference available which is the followup message.  It
is possible to have the system record the timestamp of actual
transmission and send it in a followup in ptp.  I did some testing with
this a few years ago and achieved the same results in timestamp
transmission with both protocols.  Having said that, I presume that one
REAL benefit for time transfer is that PTP can, and does, run at a
higher sync rate than ntp.  It is also synchronizing to a single clock.
Also, the default ptp app is using multicast "broadcasts" with ttl 1 and
the client uses a slightly funky "point to point" multicast transmission
as a ranging request to calculate propagation delay.  The delay is then
added to sync to arrive at value for local clock comparison.  However, I
don't think that there is a multi tap filter.  In fact, in the open
source ptp, I think the servo is just pretty much a jam hack.  The point
was to show the protocol.

All of this is good dialogue but it is VERY important to note that what
you test in a small LAN has very little bearing on the performance
possible in various types of real networks of greater scale..  

  
Greg Dowd
gdowd at symmetricom dot com (antispam format)
Symmetricom, Inc.
www.symmetricom.com
"Everything should be made as simple as possible, but no simpler" Albert
Einstein

-----Original Message-----
From: questions-bounces+gdowd=symmetricom.com at lists.ntp.org
[mailto:questions-bounces+gdowd=symmetricom.com at lists.ntp.org] On Behalf
Of Bill Unruh
Sent: Wednesday, April 30, 2008 1:20 PM
To: questions at lists.ntp.org
Subject: Re: [ntp:questions] frequency adjusting only

m.louvel at gmail.com (maxime louvel) writes:

>On Tue, Apr 29, 2008 at 6:27 PM, Unruh <unruh-spam at physics.ubc.ca>
wrote:

>> m.louvel at gmail.com (maxime louvel) writes:
>>
>> >Hi,
>>
>> >I have know run a lot of tests.
>> >Just to let you know what I've got so far.
>> >I have tried NTP, and NTP + PTP (Precision Time Protocol).
>> >I haven't tried Chrony nor TSClock.
>> >I have used the software only implementation of PTP (ptpd).
>>
>> >With NTP only I have got an accuracy around 1ms,

Actually, I have no idea what the difference is between the "software
implimentation" of PTP and standard NTP is. The advantage of PTP is the
HARDWARE timestamping of the packets as they come into the ethernet card
(special purpose ethernet cards with clocks on board) and possibly PTP
aware switches which race through the PTP packets without delay.
 Software only means
that PTP uses exactly the same kernel routines, etc. to read the
computer clock as does ntp I assume. I cannot see how it can be better
unless there are some severe bugs in NTP. 
What version of NTP are you running?


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