[time-nuts] Changes in commercial GPS clocks over the decades

[Magnus Danielson]

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, the clock would generate a message to the connected elements that said “Don’t Use Me” (DUS).  This method is called, “Sync Status Messaging/SSM”, and also carry’s over to the latest packet timing designs so that subtending elements always know the pedigree of a timing/sync signals.

The International/European term is Do Not Use (DNU). They share SSM-code.

DNU is sent towards the source you pick your choice of synchronization
to avoid synchronization loop. It works locally between the boxes, but
one intermediary box and there is no loop-free guarantee in the system.

>
> It is notable that, while SSM provides a label as to the purported pedigree (stableness) of the timing signal, it is no guarantee the signal is actually as stable as reported, but generally it is truth.

The inability to avoid routing loops, at which time the frequency will
quickly drift away, can only be handled through either network planning
(blue/red network, trickle down direction, configuration to match) or
through off network intelligence to monitor and control. The later is
needed to ensure consistency of the first too.

In modern synchronization systems, more advanced routing protocols
occurs, with a higher degree of self-arrangement. For instance PTP use
the BMC algorithm, I do similar stuff that was independently developed.

>
> This has been my world for several decades so please let me know if this information is not useful to this board....so we don’t fill your inbox with unwanted ramblings of an old telephone guy.  :)

Well, there is some point to get an insight to how it worked, because
some of the gear we end up use as hobbyist have that experience. There
is odd frequencies such as 1544 kHz and 2048 kHz for normal time and
frequency world, but they make perfect sense in the PDH/SDH world. Back
in the day it was possible to build for the purpose robust networks.
That's hard to achieve now.

However, outside of the telco world, it is less meaningful except for study.

>
> The Fiber Guru
> (www.fiber.guru)
>
>> On Oct 31, 2020, at 10:42 AM, Magnus Danielson <magnus at rubidium.se> wrote:
>>
>> Hi,
>>
>>> On 2020-10-30 21:37, The Fiber Guru wrote:
>>> During my telco career I was responsible for Network Synchronization and
>>> witnessed several generations of clock designs.  Post-telco I now
>>> manufacture and sell Network synchronization systems.  Here are a few
>>> observations from legacy and modern topologies:
>>>
>>>
>>>
>>> 1. BITS clocks used to consume an entire 8 ft rack in a large central office
>> For those not in the Telco world. BITS is the US/ANSI term for the
>> international ETSI/ITU-T term SASE. In telco system you have a central
>> clock in the heart of the telephone station, so you route any
>> synchronization signal incoming to the station through the SASE/BITS
>> interface over to the SASE/BITS which then selected amongst available
>> sourced and configuration locked up it's OCXO or rubdium to that and
>> then distributed that out back. A modern SASE/BITS is a 6 U box.
>> Applicable standards are ITU-T G.781 for routing, ITU-T G.812 for clock
>> qualities and ANSI T1.101 for the specifics of the US system.
>>> 2. Legacy clocks easily cost $35k to $50k
>>>
>>> 3. The most critical part of clock installation is the antenna......this has
>>> never changed.  If you get this wrong the clock will flop around like a fish
>>> out of water
>> In traditional SASE/BITS installations, they never had a GPS antenna to
>> start with, the system was designed to have analog cesiums as clock
>> source, and frequency errors of 1E-11 was tolerated for the Primary
>> Reference Clock (PRC). The PRC is specified in ITU-T G.811. Now the
>> companies is competing in inventing harder and harder specs on PRCs
>> based on what they modern cesium technology can deliver, but with
>> marginal benefit to operators, which tend to follow the ITU-T a little
>> too much because that used to be a safe route and a recepy for working
>> solutions.
>>> 4. Most critical part of antenna installation is to have unobstructed view
>>> of the sky, but not be the highest electrical element (for lightning
>>> protection....come of protection).  Of equal importance is selecting the
>>> proper size antenna cable for the required distance (RG58 up to 100 ft,
>>> RG213 up to 300 ft, LMR400 up to 600 ft)
>
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