Andy Yates wrote: > Unruh wrote: >> Andy Yates <[email protected]> writes: >> >>> Hal Murray wrote: >>>> In article <[email protected]>, >>>> Andy Yates <[email protected]> writes: >>>>> Does anybody have any figures that shows the effect on accuracy of an >>>>> NTP v3 client using a stratum 1 server rather than a stratum 2 or 3 >>>>> server? It's all in a GE LAN based scenario, commercial stratum 1 >>>>> servers connected to GPS and stratum 2 and 3 servers are typically >>>>> dedicated Linux boxes. >>>>> However I'm been pressed to supply an SLA for accuracy. My argument is >>>>> that although you can get your stratum one server to synchronize to >>>>> microseconds of UTP, as soon as the client uses NTP v3 over the LAN, >>>>> even a GE LAN, then the accuracy degrades and putting well designed well >>>>> specified stratum between the boxes is not going to decrease accuracy >>>>> sufficiently to warrant purchasing many stratum one appliances. >>>> What sort of accuracy are you interested in? 1 ms? 10 ms? 100 ms? >>> Hi Hal >>> Its up to us to specify what we think the SLA should be - the guide is >>> "as accurate as possible"! >> That is of course completely idiotic. They do NOT want it as accurate as >> possible. That would cost them millions of dollars and is not in fact >> needed. What are they using the time for? what kind of machines are they >> ( Windows, linux, BSD, special home grown OS?) >> >> >>>> How stable is your temperature? (Both the room and the CPU load.) >> ntp is terrible if anything varies ( absurdly long settling times). >> >> >>> Temperature will be very stable, the DC is the very well specified and >>> scrupulously engineered - no cables blocking air flow etc. Generally >>> speaking the CPU is over specified. >>>> What is the load on the LAN between the clients and servers? >>>> (Delay is not a problem. Variation in delay is a problem.) >>> The NTP will be on a separate management LAN to the production traffic >>> so not subject to the variances that application load has on the network. >>>> I suggest you measure it. Start with your current system. >>>> >>>> Setup a box as a ntpd system and tell it to use several target boxes >>>> as servers and turn on logging. peerstats will tell you the difference >>>> between your local clock and the target system. >>>> >>> I'll look at the current NTP infrastructure however its completely >>> different to the new. The old has 3 geographically diverse GPS receivers >>> plus a GPS and radio source on the roof of the data centre and we use >>> network components to provide the intermediate strata between stratum >>> one and client - and have not really had many issues after almost 10 >>> years of use. However, requirements are changing and we will probably be >>> using dedicated stratum 2/3 servers as required. >> Does teh GPS have PPS ( puse per second) or are they just using the nmea >> time signal? Radio about as bad as GPS with just NMEA for timing. >> >> > Hi Unruh > > it uses IRIG-B but can use PPS amongst others - However I should have > said "as accurate as possible using a well designed NTP system". Our > stratum zero sources and stratum 1 sources will be v.accurate - its the > distribution via NTP that were trying to get our heads around.
How are you "distributing" NTP? On a LAN? On WAN? What are you using the time for? Do you have legal requirements for the accuracy of your timestamps? It's possible to get all the systems on a LAN to more or less agree on what time it is. +/- 20 milliseconds should be easy. +/- 10 milliseconds is achievable. The tighter the agreement, the more it's going to cost you! Add a WAN and it gets more difficult and more expensive. One of the keys to getting tight synchronization is a stable and accurate source of time. A GPS timing receiver can supply time of the required quality. Note that GPS receivers designed for navigation service are generally pooly suited to timing service, and vice versa. _______________________________________________ questions mailing list [email protected] https://lists.ntp.org/mailman/listinfo/questions
