5 ms·

Interesting. I might have to get this PTP board, as I've never played with that. How much more precision do you get on the client side by eliminating the chro

Interesting. I might have to get this PTP board, as I've never played with that. How much more precision do you get on the client side by eliminating the chronyd <-> network packet latency? There is still switching latency (I'm sure those things have buffers, because everything has a buffer these days) and the scheduling latency on the client side. Will I be able to notice this improvement with the right tools? (In the past, I did this: https://github.com/jrockway/beaglebone-gps-clock https://github.com/jrockway/beaglebone-gps-clock, so I have a testbench for this sort of thing.) Also, does anyone know if RTK helps much with timing? Every so often I connect up my clock to the NYS DOT RTK network (free!) and ... it works, but I don't know if it helps with anything.

Calwestjobs 2y ago
Little bit controversial post - does not matter that much, few miliseconds either way, consistency is more important. just think what those clocks are used for inside of your hardware and you get your answer. even "old" time signals like DCF77, WWV.... are sufficient enough for home lab. they can be heard even inside of a buildings, where GPS can have problems. And you can think about RTK signal as a signal in this more available signal category. Most strange thing to me is that i have to set correct time in long wave radios (receiver), when they can just receive any of all those time signals.
michaelt 2y ago
> and the scheduling latency on the client side. You might be surprised by how many PCs already have PTP capable NICs. I think it's standard on Intel's network chipsets. On Linux, just look for /dev/ptp0 However, PTP support is far from standard on network switches; and it doesn't work over wifi, or with USB-to-ethernet converters. So for sure plenty of clients can't use it. > Will I be able to notice this improvement with the right tools? If you're a precision timing aficionado? Possibly! Just set up two devices with ethernet and GPIO pins, one that takes in a GPS 1PPS signal and acts as a PTP server, then a second that acts as a PTP client and outputs 1PPS. Then connect your two 1PPS signals to the two inputs of an oscilloscope, trigger on one and watch how much the other is off by. Make sure the PTP client is downloading big files, under heavy CPU load and suchlike, so you can see the effects of those.
_moof 2y ago
PTP is mostly used for industrial control systems that need to have very precisely synchronized timing. The precision you can achieve depends a great deal on the clock hardware you're using. Network hardware, i.e. whether timing in network hardware is deterministic, also plays an important role. So the question is less "what can PTP do," it's more "what can my hardware do with PTP." Fundamentally PTP is just measuring network latency and applying that correction. But the quality of that measurement is critical, and there's nothing PTP can do to mask a crummy oscillator or a switch with non-deterministic timing.
- sciencesama 2y ago
  You will be amazed how many things ptp ! Having ptp signals to test networks is a god send many errors are due to wrogn timing syncs ! Spirent loves ptp ! They partnered a lot with calnex !! We saw 80% data base write miss reduction in postgres once we started using ptp master clock across servers !
- jpc0 2y ago
  From my industry PTP is used for video and audio sync down to single sample accuracy for audio. See AES67/SMPTE ST 2110/IPMX.
- immibis 2y ago
  PTP is used in cellphone networks to coordinate radio timeslots between multiple base stations. They need at least microsecond accuracy.
- ssl-3 2y ago
  In my little universe, PTP gets used for audio-over-IP systems like AES67, Dante, and AVB, which in turn get used for stage and studio (both very big and also quite small). These systems are commonly used to trunk audio between points. Using them and a strand or two of glass or a cat6 cable is most of all that is needed to distribute dozens (or hundreds, and potentially thousands) of low-latency uncompressed audio signals. PTP is part of how latency is kept low, and audio in-phase (which is a task that, itself, requires accuracy on the order of microseconds).

Calwestjobs 2y ago

Little bit controversial post - does not matter that much, few miliseconds either way, consistency is more important. just think what those clocks are used for inside of your hardware and you get your answer. even "old" time signals like DCF77, WWV.... are sufficient enough for home lab. they can be heard even inside of a buildings, where GPS can have problems. And you can think about RTK signal as a signal in this more available signal category. Most strange thing to me is that i have to set correct time in long wave radios (receiver), when they can just receive any of all those time signals.

michaelt 2y ago

> and the scheduling latency on the client side. You might be surprised by how many PCs already have PTP capable NICs. I think it's standard on Intel's network chipsets. On Linux, just look for /dev/ptp0 However, PTP support is far from standard on network switches; and it doesn't work over wifi, or with USB-to-ethernet converters. So for sure plenty of clients can't use it. > Will I be able to notice this improvement with the right tools? If you're a precision timing aficionado? Possibly! Just set up two devices with ethernet and GPIO pins, one that takes in a GPS 1PPS signal and acts as a PTP server, then a second that acts as a PTP client and outputs 1PPS. Then connect your two 1PPS signals to the two inputs of an oscilloscope, trigger on one and watch how much the other is off by. Make sure the PTP client is downloading big files, under heavy CPU load and suchlike, so you can see the effects of those.

_moof 2y ago

PTP is mostly used for industrial control systems that need to have very precisely synchronized timing. The precision you can achieve depends a great deal on the clock hardware you're using. Network hardware, i.e. whether timing in network hardware is deterministic, also plays an important role. So the question is less "what can PTP do," it's more "what can my hardware do with PTP." Fundamentally PTP is just measuring network latency and applying that correction. But the quality of that measurement is critical, and there's nothing PTP can do to mask a crummy oscillator or a switch with non-deterministic timing.

sciencesama 2y ago
You will be amazed how many things ptp ! Having ptp signals to test networks is a god send many errors are due to wrogn timing syncs ! Spirent loves ptp ! They partnered a lot with calnex !! We saw 80% data base write miss reduction in postgres once we started using ptp master clock across servers !
jpc0 2y ago
From my industry PTP is used for video and audio sync down to single sample accuracy for audio. See AES67/SMPTE ST 2110/IPMX.
immibis 2y ago
PTP is used in cellphone networks to coordinate radio timeslots between multiple base stations. They need at least microsecond accuracy.
ssl-3 2y ago
In my little universe, PTP gets used for audio-over-IP systems like AES67, Dante, and AVB, which in turn get used for stage and studio (both very big and also quite small). These systems are commonly used to trunk audio between points. Using them and a strand or two of glass or a cat6 cable is most of all that is needed to distribute dozens (or hundreds, and potentially thousands) of low-latency uncompressed audio signals. PTP is part of how latency is kept low, and audio in-phase (which is a task that, itself, requires accuracy on the order of microseconds).