Re: [ntp:questions] Windows and Wi-Fi - starts well, frequency steps?
In article jdmht9$jc2$1...@synge.maths.tcd.ie, David Malone dwmal...@walton.maths.tcd.ie wrote: r...@panix.com (Rod Dorman) writes: I dont see anything to support the claim that UDP is treated as guaranteed by WiFi It says unicast frames are retransmitted - that's as close as you'll get. But thats my point, it says nothing about transport layer protocols. I'm just trying to understand Dave Hart's statement In article CAMbSiYDKdnWenOK=sqwo_zrs9u0d02a8m5qeaj+rvcxcjfr...@mail.gmail.com, Dave Hart davehart_gmail_exchange_...@davehart.net wrote: ... I do indeed, but UDP is treated as guaranteed by WiFi, and I expect the reason is DNS over UDP otherwise becomes a user experience killer due to extra seconds of wait for each loss. which appears to claim the UDP over WiFi is guaranteed which I've never seen stated before. -- -- Rod -- rodd(at)polylogics(dot)com ___ questions mailing list questions@lists.ntp.org http://lists.ntp.org/listinfo/questions
Re: [ntp:questions] Windows and Wi-Fi - starts well, frequency steps?
Rod Dorman wrote: But thats my point, it says nothing about transport layer protocols. I'm just trying to understand Dave Hart's statement As it says nothing about them, it means that all transport protocols get the same resilience, other things being equal (UDP opens the possibility of multicast). which appears to claim the UDP over WiFi is guaranteed which I've never seen stated before. In a network with a WiFi element, the WiFi element is the most likely one to lose packets and force retransmissions, and therefore cause NTP packets to arrive with large delays. To a large extent it does guarantee delivery compared with what would happen if it didn't retransmit. ___ questions mailing list questions@lists.ntp.org http://lists.ntp.org/listinfo/questions
Re: [ntp:questions] Accuracy of NTP - Advice Needed
On 12/29/2011 8:38 PM, Dennis Ferguson wrote: On 29 Dec, 2011, at 23:26 , Terje Mathisen wrote: Danny Mayer wrote: No, they use synchronized Cesium atomic clocks for time accuracy. GPS is only used to get a fix on the location and I'm not sure that 10's of centimeters is good enough for what they are trying to prove. I'd have to look closely at the methods used and the data to even have a clue as to what is needed and I have touched that stuff in years. Danny, how do you think they keep those atomic clocks synchronized? How do they _verify_ that they actually stay in sync (to a single-digit ns level) over the entire length of the experiment(many months)? Even Hydrogen Masers won't give you that performance over a year or so, you have to have some way to sync them either to each other or to UTC. Yes, they use GPS to compare the clocks to each other. One of the articles I read even identified the GPS receiver they use. I think it was a Septentrio PolaRx3eTR PRO (or maybe the older model which that one replaced). Those receivers take a 10 MHz and 1 PPS reference in from the atomic clock so that they can produce GPS carrier phase measurements with respect to the local clock's time. Making these measurements simultaneously at both locations gives you data you can post-process to determine the time difference between the two clocks, independent of the GPS system time. The GPS signals are used only as markers that can be measured at both locations. They used Septentrio PolaRx2e GPS receivers in both places along with a Symmetricom Cs4000 Cs atomic clock. All of this raises additional questions for which I'd have to dig into the references for answers. For example, both ends are underground and they are likely to use heavy shielding around the sites of the source and target so how are they even getting a GPS signal through in the first place? Are they getting signal or did they set up an external antenna in which case they would have to also figure out the distance of the antenna from the receiver (which part of the antenna?). This is not an easy physics experiment and the errors involved can easily overwhelm the result. It used to be that detecting neutrinos was very hard never mind generating them in a reliable way. Now if only we could send NTP packets via neutrino... Danny ___ questions mailing list questions@lists.ntp.org http://lists.ntp.org/listinfo/questions
Re: [ntp:questions] Accuracy of NTP - Advice Needed
Danny Mayer ma...@ntp.org wrote: On 12/29/2011 8:38 PM, Dennis Ferguson wrote: On 29 Dec, 2011, at 23:26 , Terje Mathisen wrote: Danny Mayer wrote: No, they use synchronized Cesium atomic clocks for time accuracy. GPS is only used to get a fix on the location and I'm not sure that 10's of centimeters is good enough for what they are trying to prove. I'd have to look closely at the methods used and the data to even have a clue as to what is needed and I have touched that stuff in years. Danny, how do you think they keep those atomic clocks synchronized? How do they _verify_ that they actually stay in sync (to a single-digit ns level) over the entire length of the experiment(many months)? Even Hydrogen Masers won't give you that performance over a year or so, you have to have some way to sync them either to each other or to UTC. Yes, they use GPS to compare the clocks to each other. One of the articles I read even identified the GPS receiver they use. I think it was a Septentrio PolaRx3eTR PRO (or maybe the older model which that one replaced). Those receivers take a 10 MHz and 1 PPS reference in from the atomic clock so that they can produce GPS carrier phase measurements with respect to the local clock's time. Making these measurements simultaneously at both locations gives you data you can post-process to determine the time difference between the two clocks, independent of the GPS system time. The GPS signals are used only as markers that can be measured at both locations. They used Septentrio PolaRx2e GPS receivers in both places along with a Symmetricom Cs4000 Cs atomic clock. All of this raises additional questions for which I'd have to dig into the references for answers. For example, both ends are underground and they are likely to use heavy shielding around the sites of the source and target so how are they even getting a GPS signal through in the first place? Are they getting signal or did they set up an external antenna in which case they would have to also figure out the distance of the antenna from the receiver (which part of the antenna?). This is not an easy physics experiment and the errors involved can easily overwhelm the result. Given the size of a GPS antenna, which part they measure from is down in the noise level. And yes, if you actually were to read all the documents available, you would find the antennas are outside and they did measure the distance to the antenna. How else would they be able to know were the expirement was in relation to a known point, i.e. the antenna, and the time delay down the coax if they didn't make such measurements. -- Jim Pennino Remove .spam.sux to reply. ___ questions mailing list questions@lists.ntp.org http://lists.ntp.org/listinfo/questions
