[HN Gopher] Graham: Synchronizing Clocks by Leveraging Local Clo...
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Graham: Synchronizing Clocks by Leveraging Local Clock Properties
(2022) [pdf]
Author : mlerner
Score : 54 points
Date : 2025-08-11 04:54 UTC (18 hours ago)
(HTM) web link (www.usenix.org)
(TXT) w3m dump (www.usenix.org)
| RossBencina wrote:
| From TFA: "Some commercial PTP implementations use packet delay
| variation (PDV) filters [27], and compensate for known latencies
| in the receive and transmit paths."
|
| This is phrased vaguely. All gPTP implementations are required to
| compensate for link delay (not necessarily rx/tx asymmetry).
| RossBencina wrote:
| I wonder whether there are clock generators with on-chip
| temperature sensing...
|
| Yes: "an advanced, low-power, high-performance mobile clock
| generator with four clock outputs. The device integrates a MEMS
| resonator, temperature sensor and a temperature-to-digital
| converter (TDC), which eliminates the need for external crystal
| and temperature-sensing crystal resonators." --
| https://www.sitime.com/products/clock-generators/clock-gener...
| bux93 wrote:
| The paper mentions TCXOs and OCXOs, respectively temperature
| and oven-controlled crystal oscillator. You can get PCIe cards
| that have these on them, and devices for audio studios with a
| rubidium oscillator like the Tascam CG-1800.
|
| Jane Street has a podcast that lifts the veil a little bit on
| how they keep their gear synced up, which is pretty
| interesting: https://signalsandthreads.com/clock-
| synchronization/
| RossBencina wrote:
| The CG-1800 uses "a high-precision OCXO (oven-controlled
| crystal oscillator)" https://tascam.com/us/product/cg-1800/
| only mention of rubidium is: "An external input connector
| that supports a 10MHz signal enables the CG-1800 to be
| connected to a rubidium clock or GPS clock for even higher
| precision."
|
| On the other hand, Antelope Audio 10MX is a rubidium word
| clock source: https://en.antelopeaudio.com/products/10mx/
|
| Surplus rubidium frequency standards are cheap on Ebay:
| https://www.diyphysics.com/2012/02/14/d-i-y-10-mhz-atomic-
| cl...
| adrian_b wrote:
| Cheap second hand rubidium frequency standards may be
| worthwhile, unlike new rubidium oscillators, which are not
| that good in comparison with good OCXOs to make their much
| higher price acceptable.
|
| Traditional rubidium frequency standards still have an
| aging rate that is high enough to be non-negligible. While
| from the datasheet of a rubidium oscillator it appears that
| it is better than an OCXO, that can be misleading, because
| many OCXOs have very predictable aging drifts. If you
| compensate the aging in software, you may reach a similar
| performance to a rubidium oscillator.
|
| In recent years there have been invented several new kinds
| of rubidium frequency standards, with much better
| performances than the traditional models. However these are
| either not available yet as commercial products or they may
| have a single provider which demands a steep price.
| RossBencina wrote:
| The Jane Street podcast was a very approachable explanation,
| and was interesting to me for a few reasons: (1) that EU
| regulation requires their timestamps to be within 100
| microseconds of UTC, and (2) they claim to achieve 20
| microsecond accuracy using hardware-timestamped NTP synched
| to local NTP time servers, the NTP servers then being synced
| to off-the-shelf GPS masters using PTP, (3) they really
| didn't like the idea of running PTP on all of their switching
| infrastructure (even though running PTP boundary clocks on
| all of your switches is the obvious way to go), (4) they
| weren't doing anything fancy or hardcore. Very pragmatic. I
| was expecting lasers, custom FPGA systems, and a dev team
| dedicated to time synchronisation.
|
| They did mention that to go below 20 microseconds they'd need
| a different approach. And mentioned white rabbit
| https://ohwr.org/projects/white-rabbit/ From NTP to white-
| rabbit sounds like a big jump to me.
|
| In the context of digital audio, 20 microseconds is an entire
| sample period at 48kHz. AVB using gPTP is capable of locking
| up all devices on the network to some small fraction of a
| sample period. That requires all network switches to
| propagate time information. Start here:
| https://en.wikipedia.org/wiki/Time-Sensitive_Networking
| pclmulqdq wrote:
| I was personally pretty surprised at the idea that Jane
| Street found PTP to be too difficult to administer or run.
| Hardware support for PTP is nearly ubiquitous in NICs and
| switches (unless you build this gear for yourself, I
| guess), so it is not that hard to administer. PTP done
| poorly gets you ~100 ns time sync across your cluster, and
| if you do everything correctly you can get time within
| about 10 ns given how small a trading network is.
| AdamN wrote:
| This time accuracy would need to propagate to all their
| hosts, not just the ones in a single DC. I presume they
| have hosts in the EU, London, NYC/NJ, Tokyo, Chicago,
| etc... I imagine 100ns accuracy with that kind of global
| installation diversity isn't straightforward.
| namibj wrote:
| Yes it is, GPS is 1foot ~ 1ns; thus a few ns are trivial
| with a vaguely decent receiver.
| pclmulqdq wrote:
| The gold standard for PTP is to use separate GPS-
| disciplined atomic clocks in your separate points of
| presence, and PTP within each one. These are about $10k
| each, so it is not that expensive even if you have 100
| datacenters.
| hermitdev wrote:
| > that EU regulation requires their timestamps to be within
| 100 microseconds of UTC,
|
| It's the same in the US. It's covered under CAT NMS
| (Consolidated Audit Trail, National Market System).
| Probably too much information at:
| https://www.catnmsplan.com/
| jeffbee wrote:
| Yes, and the fact that this has been a global regulation
| for years suggests that it isn't all that special.
| Outfits like Goldman Sachs meet this regulation with
| fewer blogs.
| adrian_b wrote:
| Nitpick: the C in TCXO stands for "Compensated", not for
| "Controlled", like in OCXO.
|
| That means that the resonator of an OCXO is held at a
| constant temperature, while that of a TCXO is at the ambient
| temperature, but the temperature is monitored and a
| compensation circuit adjusts the resonance frequency,
| maintaining it as constant as possible.
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(page generated 2025-08-11 23:01 UTC)