Unruh wrote: > Andy Yates <[email protected]> writes: > >> Unruh wrote: >>> 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. >>>> The reasons is that I would rather scale by adding strata - its a very >>>> big data center with thousands of clients and has several "zones" that >>>> are isolated. However some opinion is suggesting we run IRIG-B between >>>> the GPS receiver and a bunch of stratum 1 servers and clients access >>>> these directly. Much more expensive and any increase in accuracy from a >>>> client experience may be negligible. >>>> 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. >>> I think you need some specifications. What accuracy do you really need? >>> Without that you are simply wasting your, and everyones time and money. >>> Direct GPS connection can give you >>> 1 usec. Lan connection to those can give you 20usec average. Then each >>> stratum will loose about 20-50 usec (my experience). Do you need better >>> than msec > >> Hi Unruh > >> Totally agree about specification - trouble is the customers include >> third parties who don't issue specifications so we've been tasked to >> make it v.accurate, industry leading etc. however I'm not going to >> encourage blowing cash on diminishing returns. Thanks for the experience >> figures, this is exactly what I need. > > > Note also that it can depend on the network harware. When we ran 100Mb/s > switches, the delay time was astonishingly regular and repeatable. Now > that we have switched to Gb switches it is not. It much worse. There > seems to be a one way "quantization" It used to be that the round trip > time from the machines to the ntp stratum 1 source was consistantly > 140-160usec. Now it varies, with one level still at the 140usec, but > other round trip times of 280 or even longer (on the same machine). > And they seem to be one > way delays ( the worst kind). Ie, you really need to select the switches > between your stratum 1 sources and your machines carefully to get the > best behaviour. > > Interesting - I'll look for that during testing as we have a choice of several switches from 2 different vendors.
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