GPS NTP Stratum (6 vs. 2)

Other topics MikroTik hardware questions
1

I am curious if anyone knows why when the time is set via GPS (using a serial connection), the stratum reported by the NTP server is 6. (I would presume if we’re getting our time from a stratum 1 source such as GPS, it would be serving out stratum 2 time).

2

I’ve ran into this same issue many times and emailed support about it many times. Never got a good resolution on it. They told me that it was because it hadn’t been GPS synced for enough time, however I never saw anything other than 6 … and windows won’t sync with something larger than 2, so it’s useless for windows most of the time.

Sam

3

Its suppose to be 0, maybe 0 isn’t an option.

4

MT - please fix this in the next RC. If GPS is used the stratum should be 1. Please please please with sugar on top. I’d like to quit using 3rd party NTP since I do have a GPS sitting here syncing time on the mikrotik. It’s useless with a stratum 6 - no one will use it but other Mikrotik devices.

5

we have changed it to 4, which windows supports (tested with the new setting). this number indicates how close you are to the time source. as GPS is connected over serial, the precision is not so high, so 0 is out of question.

6

First of all, stratum 0 IS the time source itself so unless you have a cesium clock or satellite counting pulsar bursts or something you should never be stratum 0.

Stratum 1 is a device connected directly to a stratum 0 device. For stratum 1 it should be perfectly acceptable to use rs232 for GPS communication of time data. Connecting a GPS to a computer and pulling time off of it makes the box the GPS is connected to a stratum 1 device.

7

Thanks for at least making it 4 … however, it should be 1 if its coming from a GPS device. I know 4800 baud isn’t that fast, but its fast enough for time sync. I don’t have it handy but there is an RFC on this.

Anyhow - 4 is okay for most windows systems, however I believe there are times when windows adds a stratum to each hop and if youre using AD I believe downstream systems still won’t believe it. Somewhere I read that windows will mostly need a 2 stratum or higher for things to work smoothly.

8

This is related to a previous debate, “GPS wireless sync”. To crudely summarise (I am not a developer) as I understand:

  • Low cost GPS devices have RS232 interface
  • RS232 interface has latency, and add drivers+OS, is hard to determing precisely the timings across the interface
  • Some “clever code” is needed to calibrate timings across RS232 interface, and a “timing loop” to average-out “per reading” variations in latencies across the interface, to derive “true time” compared to the router’s internal clock.
  • This HAS been done by other software companies, I guess mostly the ones who sell GPS systems.
  • There might be some open-source code to do this, I don’t know.

If MT people can work out how to do this, which I guess is “measuring timing across the RS232 interface, time-averaging responses, and measuring against local router-time-clock” there could be major benefits.
A major one being, “GPS wireless sync” -
(note: WiMax base stations use similar GPS devices, Linux OS, and manage this. It can be done. Also, the Moto Canopy system has GPS sync, but not sure how it’s implemented in hardware/software.)

Am sure more people more knowledgable on this topic can comment more -

Regards

9

apart from wireless sync, I’m just interested in NTP stratums here ; )

http://www.mikrotik.com/testdocs/ros/2.9/

The manual even says the following:

GPS Synchronization
Stratum 1 NTP time source and location retrieval from GPS

10

Mikrotik, can you please make a GPS synced clock be a stratum 1? Active Directory won’t believe anything higher than 2. Is there a reason that it can’t be 1 when it has a GPS fix?

11
  1. RouterOS clock doesn’t support that kind of precision, so it would be incorrect to make it stratum 1
  2. RouterBOARD serial port doesn’t have the pins that are required for increased precision time from GPS.
12

okay, i understand. I guess in my case I dont care about millisecond precision, only that all of my clocks are sychronized. I am having to run a windows service on another box to provide this now because my RouterOS GPS isn’t trusted by anything. Maybe you could just include a warning about ‘its not all that accurate but provided anyways’ or something.

thanks for your reply however : )

Sam

13

or maybe add an option to select desired stratum?

14

lately on this forum, all you want to do is break standards :slight_smile:

15

i opt that person who uses RouterOS knows what it does :slight_smile: it may be a “hidden” option only available through console… :slight_smile:

16

Thanks for answering this, normis!

17

No. A GPS device should only be Stratum 1 if its sending PPS (Pulse Per Second) and from what I’ve read so far the RouterOS does not support PPS.

Mikrotik only uses NMEA so even though its from a GPS, Stratum 4 is probably as high as it should go. If you want a Stratum 1 time server you really should build your own dedicated Linux/ntpd time server with PPS.

18

Thanks for clarification. It’s what I meant.

19

"Typical GPS Performance

Since the deactivation of the Selective Availability program on May 2, 2000, most GPS receivers now produce a 1 pps output with a standard deviation of 10 ns or less. Many receivers produce frequency with an uncertainty of < 1 x 10-12 when averaged for one day. Two key factors that contribute to receiver performance are the quality of the receiver’s internal oscillator, and the quality of the software algorithms that process data acquired from the satellites."

I can handle a 10ns delay… so just let me override the stratum manually if I trust it.

20

it’s not about the GPS, it’s about RouterOS clock