Thanks for the insight into these matters. I found the terms "traceability" and "traceable" were used on occasion (perhaps incorrectly) when discussing network clock sources in an enterprise networking context.
I also wondered a bit about the stratum 4 designation but I suppose it did (and probably still does ?) provide a standard for equipment vendors to meet that was of some use in certain private network scenarios ? Ie a customer had two phone systems with only analog connections to the public network but had a private T1 connecting them together. Presumably in that context a specification such as Stratum 4 made some sense. Best regards Mark Spencer [email protected] 604 762 4099 > On Nov 1, 2020, at 1:29 PM, The Fiber Guru <[email protected]> wrote: > > This is a good example of the difference between metrology and operational > terminology. Yes, not traceable in the strict sense of a metrologist, but in > this instance, “traceability” implies the reported Stratum level of the > delivered clock signal. For example, if I receive a clock signal with label > of PRS, it is believed to originate from a clock that is using GPS to > discipline the oscillator. While that is no guarantee of its stability, it > is at least an indicator to use when decisioning where the network element > will derive synchronization. > > When visiting NIST, they always tend to treat any clock source as inherently > unstable. This can be concerning until you understand they are thinking it > terms of 23 places or more behind the decimal point, where as a network > operators we are happy with magnitudes less than that. All a degree of > perspective. > > db > >> On Nov 1, 2020, at 9:57 AM, Magnus Danielson <[email protected]> wrote: >> >> Hi, >> >>> On 2020-10-31 17:29, The Fiber Guru wrote: >>> Yes, prior to use of GPS to discipline the Local oscillator, telecom timing >>> was a “trickle down” topology where a cesium source in Kansas City was >>> distributed across the continent. >> In Europe for instance, we chose other towns than Kansas City, but ah well. >>> The cesium was the gold standard and as the timing signals cascaded to >>> distant geographical regions, it was obviously less pure, so if cesium was >>> at Stratum 1 (stability), the next element in line that mediated time of >>> downstream was at Stratum 2. As it arrived at your local telco central >>> office, it was at Stratum 4 (ok, but useless in today sadly networks). >>> >>> Enter GPS and instantly every local central office could achieve Stratum 1 >>> traceability, and if GPS was lost the best Rubidium's could holdover at >>> Stratum 2e (slightly better that Stratum 2). If the clock had OCXO, >>> holdover was Stratum 3 or 3e depending on the quality of the oscillator. >> >> No, that's not traceability. Use of that term for meaning locked to is >> strongly discouraged. Traceability is about unbroken chain of >> calibrations to SI units and the paper trail that goes with it. You do >> not achieve that here. >> >> Stratum 1, 2, 3 and 4 is ANSI T1.101 terminology, but not used in >> international standards. For instance, there is no real equivalence of >> Stratum 4 in the international standards, because it makes no real sense >> to synchronize to the line card oscillator. >> >>> Stratum levels are reported to most network elements by embedding the >>> Stratum value message in the Facility Data Link of an ESF T1 signal. The >>> network element would read the Stratum level from the incoming timing >>> signal to determine if it should lock to the signal, or fall back to its >>> internal clock, usually an ocxo at Stratum 3. >> >> This is the SSM code. >> >> The routing protocol was limited to source quality. It created great >> pains since routing-wise it is far from sufficient. It required separate >> monitoring and reconfiguration tool or very strict planning to be >> "safe". Good description for routing is found in ITU-T G.781. ETSI has a >> good overview/teaching document for which the ref number now slipped my >> mind. >> >>> >>> If the master GPS clock suffered loss of GPS and the backup oscillator >>> deteriorated to a low stability, t > > > _______________________________________________ > time-nuts mailing list -- [email protected] > To unsubscribe, go to > http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com > and follow the instructions there. _______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go to http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com and follow the instructions there.
