The 60802 challenge of meeting time accuracy goals across ...
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McCall, Stanton with Woods, Weingartner, and SchlechterIEEE 802.1 TSN / 60802 Interim, September 2021 Available at http://www.ieee802.org/1/files/public/docs2021
McCall, Stanton with Woods, Weingartner, and SchlechterIEEE 802.1 TSN / 60802 Interim, September 2021 Available at http://www.ieee802.org/1/files/public/docs2021
The 60802 challenge of meeting time accuracy goals across long daisy-chains using 802.1ASโข-2020An analysis and a proposed path forward
David McCall, Kevin Stanton, Greg Schlechter (Intel)Jordan Woods, Tom Weingartner (Analog Devices)
September 2021 Interim of 802.1 TSN / 60802
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McCall, Stanton with Woods, Weingartner, and SchlechterIEEE 802.1 TSN / 60802 Interim, September 2021 Available at http://www.ieee802.org/1/files/public/docs2021
McCall, Stanton with Woods, Weingartner, and SchlechterIEEE 802.1 TSN / 60802 Interim, September 2021 Available at http://www.ieee802.org/1/files/public/docs2021
Updates
โข v2 โ v3โ Fixed minor typos and consistency issues.
โ Updated speaker notes to match revised slide content.
โ Changed top level references from Path Delay, ๐๐ท๐๐๐๐ฆ๐๐๐๐๐ and ๐๐ท๐๐๐๐ฆ๐๐๐๐๐๐ถ๐๐๐๐๐๐ก๐๐๐ to Link Delay, ๐๐๐๐๐ฟ๐๐๐๐ท๐๐๐๐ฆ๐๐๐๐๐ and ๐๐๐๐๐ฟ๐๐๐๐ท๐๐๐๐ฆ๐๐๐๐๐๐ถ๐๐๐๐๐๐ก๐๐๐ .
โข v1 โ v2โ In ๐๐ ๐ ๐๐๐๐๐ and ๐ ๐ ๐๐๐๐๐ sections, changed to reflect that pDelayResp, ๐ก3
& ๐ก4 are used to measure NRR (not pDelayReq, ๐ก1
& ๐ก2 )
โ In ๐๐ ๐ ๐๐๐๐๐ section, fixed calculation of NRR to represent ๐๐ ๐ ๐ด๐ต (was previously, erroneously, ๐๐ ๐ ๐ต๐ด). Also changed graphic to
make it clear that ๐๐ ๐ ๐ด๐ต is the ratio measured from A of its clock rate relative to B.
โ Fixed ๐๐ท๐๐๐๐ฆ๐๐๐๐๐๐๐ ๐ equation (should be /2)
โ Fixed ๐ ๐ ๐๐๐๐๐๐ท๐๐๐๐ก equations (should be ๐๐๐๐๐๐ท๐๐๐๐ก๐บ๐ ๐๐๐๐๐๐ท๐๐๐๐ก ๐ not ๐๐๐๐๐๐ท๐๐๐๐ก ๐ ๐๐๐๐๐๐ท๐๐๐๐ก๐บ๐)
โ Fixed discussion of ๐๐ ๐ ๐๐๐๐๐ in most recent time series simulationsโข NRR is measured across seven pDelay Intervals prior to taking the median of the past seven measurements. (Previously, only taking the median
was discussed.)
โ Completed formulae in Backup and added slides for relevant formulae at the end of each sectionโข Formulae in Backup include consideration of the units for each factor and secondary errors which will not be included in the initial model. Formulae
in the main presentation do not.
โ Changed some โpDelayโ references to โPath Delayโ where the latter is more appropriate (i.e. the reference isnโt to a pDelaymessage or used in a formula).
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McCall, Stanton with Woods, Weingartner, and SchlechterIEEE 802.1 TSN / 60802 Interim, September 2021 Available at http://www.ieee802.org/1/files/public/docs2021
McCall, Stanton with Woods, Weingartner, and SchlechterIEEE 802.1 TSN / 60802 Interim, September 2021 Available at http://www.ieee802.org/1/files/public/docs2021
Abstract
Industrial Automation Systems require microsecond-accurate time across long daisy-chains of devices using IEEE Std. 802.1ASโข-2020 as specified by IEEE/IEC 60802.
Simulated protocol and system parameters have thus far either been judged impractical or have failed to meet the time-accuracy requirement.
In this contribution we:
1. Summarize the assumptions, requirements and results to date
2. Challenge certain assumptions
3. Propose an approach that we believe could lead to a solution
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McCall, Stanton with Woods, Weingartner, and SchlechterIEEE 802.1 TSN / 60802 Interim, September 2021 Available at http://www.ieee802.org/1/files/public/docs2021
McCall, Stanton with Woods, Weingartner, and SchlechterIEEE 802.1 TSN / 60802 Interim, September 2021 Available at http://www.ieee802.org/1/files/public/docs2021
Some Requirements & Assumptions to Date
โข Requirements:โ Topology: 64-100 network hops (i.e. 2 endstations + 63-99 devices in a chain)
โ Time Accuracy: Not to exceed +/-1us with respect to the GM at any nodeโข Measured at each Working Clock, e.g. used by 802.1Qbv (i.e. GCL) and the Application Clock
โข Assumptions:โ Local Clock reference frequency (in each system) is an XO, i.e. a quartz crystal oscillator
โ The 802.1AS GM does not have better PPM stability than the other nodes
โ No averaging of the pDelay measurement is performed
โ Temperature gradients induce PPM gradients, resulting in time-duration-measurement errors
โ Various Ambient Temperature profiles (TAmbient) have been usedโข With various transfer functions of PPM = f(TCrystal)]
โ Zero thermal resistance between TAmbient and TCrystal (0oK/W)
Note: Assumptions that we recommend revisiting are marked with โโ
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McCall, Stanton with Woods, Weingartner, and SchlechterIEEE 802.1 TSN / 60802 Interim, September 2021 Available at http://www.ieee802.org/1/files/public/docs2021
McCall, Stanton with Woods, Weingartner, and SchlechterIEEE 802.1 TSN / 60802 Interim, September 2021 Available at http://www.ieee802.org/1/files/public/docs2021
Other Principles
โข Initial 60802 standard should not trigger new silicon design / spinsโ Requirements should be met by commodity switch and endpoint silicon today that
were designed to support IEEE Std. 802.1ASโข-2020
โข The solution should not unnecessarily limit the deployment of IEEE Std. 1588โข-2019 security mechanismsโ With todayโs 802.1AS-2020-capable silicon
โข Once viable 802.1AS-2020 parameters are chosen, the worst-case PPM/sec allowed per time-aware system should be derived
โข The function of PPM vs Tambient is a property of the electrical, mechanical and thermal design of the board / chassis / systemโ Thus the PPM / TAmbient requirement should be specified, but not the method by
which the PPM / TAmbient requirement must be met by the system design
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McCall, Stanton with Woods, Weingartner, and SchlechterIEEE 802.1 TSN / 60802 Interim, September 2021 Available at http://www.ieee802.org/1/files/public/docs2021
McCall, Stanton with Woods, Weingartner, and SchlechterIEEE 802.1 TSN / 60802 Interim, September 2021 Available at http://www.ieee802.org/1/files/public/docs2021
Viable Parameter Selections
โข Sync Interval
โข Sync-to-Follow-up delay
โข pDelay Interval
โข Timestamp Granularity
โข Rx/Tx Timestamp Asymmetry:
โข Residence Time (in addition to Sync-to-Follow-up delay)
A simple analytical Model is needed to more quickly choose scenarios for more detailed time-series simulationQuickly iterate a Model over minutes, then invest the >1 week for the detailed simulation
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McCall, Stanton with Woods, Weingartner, and SchlechterIEEE 802.1 TSN / 60802 Interim, September 2021 Available at http://www.ieee802.org/1/files/public/docs2021
McCall, Stanton with Woods, Weingartner, and SchlechterIEEE 802.1 TSN / 60802 Interim, September 2021 Available at http://www.ieee802.org/1/files/public/docs2021
Variable Time Error (vTE): Timestamp Model
Local Clock(f Hz)
PHY/MACRx
0 < e < Vr
PHY/MAC Tx
0 < e < Vt
Rx Timestamp Tx Timestamp
Va
lue
PTP Relay Instance
e: The time errorVr: Variable Rx delayVt: Variable Tx delay1/f: Timestamp granularity
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McCall, Stanton with Woods, Weingartner, and SchlechterIEEE 802.1 TSN / 60802 Interim, September 2021 Available at http://www.ieee802.org/1/files/public/docs2021
p. 8McCall, Stanton with Woods, Weingartner, and SchlechterIEEE 802.1 TSN Interim, Virtual, September 2021 Available at http://www.ieee802.org/1/files/public/docs2021
Motivation & Results (so far)
โข Develop a method of iterating fasterโข Input: key 802.1AS parametersโข Output: time sync accuracy over 64 and 100hops
โข Approach by modelling the errors and how they accumulateโข Requires an analysis of the errors first
โข Analysis of the errors has already suggested some assumptions inherent in the current time series simulation that should be revisited
โข This presentation walks through the error analysis and the most important insights it reveals
โข Goal is to introduce the overall approach share the insightsโข There are some areas that require further analysis; will present in a later session
โข Next steps:โข Build the modelโข Complete the additional analysis
This is a very graphical presentation, but the uploaded version includes extensive speaker notes (which you are reading now!).
There is a lot of information in this presentation. The main section Iโm about to go through is nearly over 100 slides, but many of them are builds, so we should get through them fairly quickly.
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McCall, Stanton with Woods, Weingartner, and SchlechterIEEE 802.1 TSN / 60802 Interim, September 2021 Available at http://www.ieee802.org/1/files/public/docs2021
p. 9McCall, Stanton with Woods, Weingartner, and SchlechterIEEE 802.1 TSN Interim, Virtual, September 2021 Available at http://www.ieee802.org/1/files/public/docs2021
Summary of Main Insights
โข meanLinkDelay should be an average of pDelay measurements.โข Should nearly eliminate meanLinkDelayerror
โข NRR should not be averaged.โข Provided (Timestamp Errors / pDelay Interval) is negligible in terms of ppm
โข There are significant benefits to the GM having greater ppm stability (lower ppm/s).
โข It is hard (impossible?) to eliminate Timestamp granularity and Dynamic Timestamp error effects from Residence Time
โข Need more work to see how far these elements can realistically be reduced
โข Synchronising pDelayReq to arrive just before Sync would minimise the effect Clock Drift induced errors it NRR and RR.
โข There may be opportunities to compensate for NRRerrorโข And therefore RRerror
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McCall, Stanton with Woods, Weingartner, and SchlechterIEEE 802.1 TSN / 60802 Interim, September 2021 Available at http://www.ieee802.org/1/files/public/docs2021
p. 10McCall, Stanton with Woods, Weingartner, and SchlechterIEEE 802.1 TSN Interim, Virtual, September 2021 Available at http://www.ieee802.org/1/files/public/docs2021
Link DelayResidence Time
Time Sync
GM 10
2 3 99End
Station100
97 98
Link DelayResidence Time
Link DelayResidence Time
Link DelayResidence Time
Link DelayResidence Time
Link DelayResidence Time
Link Delay
This is a simple view of how time sync works.
The basics are: measure Link Delays and Residence Times; add them to a Sync message as it is passed from the GM to the End Station.
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McCall, Stanton with Woods, Weingartner, and SchlechterIEEE 802.1 TSN / 60802 Interim, September 2021 Available at http://www.ieee802.org/1/files/public/docs2021
p. 11McCall, Stanton with Woods, Weingartner, and SchlechterIEEE 802.1 TSN Interim, Virtual, September 2021 Available at http://www.ieee802.org/1/files/public/docs2021
TIMECorrection Field
Rate Ratio
Link DelayResidence Time
NRR
Time Sync
GM 10
2 3 99End
Station100
97 98
Link DelayResidence Time
NRR
Link DelayResidence Time
NRR
Link DelayResidence Time
NRR
Link DelayResidence Time
NRR
Link DelayResidence Time
NRR
Link DelayNRR
TIMECorrection Field
Rate RatioTIME
TIMECorrection Field
Rate Ratio
TIMECorrection Field
Rate Ratio
TIMECorrection Field
Rate Ratio
TIMECorrection Field
Rate Ratio
TIMECorrection Field
Rate Ratio
Adding a bit more detail, this is how thatโs done in practice.
The Sync message contains the TIME of transmission from the GM, a Correction Field of the delay that has accumulated since it was transmitted, and a measure of the Rate Ratio, i.e. the ratio between the current clock speed and the GMโs clock speed.
Itโs worth pointing out that each device is responsible for updating the Correction Field based on the incoming Link Delay plus itโs own Residence Time. Also a device measures the Neighbor Rate Ratio between itself and the upstream device. It then adds that to the incoming Rate Ratio. And finally uses the new Rate Radio to modify the measured Link Delay and the Residence Time prior modifying the Correction Field, and passing on everything to the next device. Rate Ratio is calculated and used for this step because the Correction Field is in terms of the GM clock, not the local clock โ which is used to measure Link Delay and Residence Time - and multiplying by the Rate Ratio accomplishes that translation.
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McCall, Stanton with Woods, Weingartner, and SchlechterIEEE 802.1 TSN / 60802 Interim, September 2021 Available at http://www.ieee802.org/1/files/public/docs2021
p. 12McCall, Stanton with Woods, Weingartner, and SchlechterIEEE 802.1 TSN Interim, Virtual, September 2021 Available at http://www.ieee802.org/1/files/public/docs2021
Time Sync โ Elements & Relationships
Time Sync
Link Delay Residence Time
Building up the logical elements and connection of this processโฆthe simple model looks like thisโฆ
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McCall, Stanton with Woods, Weingartner, and SchlechterIEEE 802.1 TSN / 60802 Interim, September 2021 Available at http://www.ieee802.org/1/files/public/docs2021
p. 13McCall, Stanton with Woods, Weingartner, and SchlechterIEEE 802.1 TSN Interim, Virtual, September 2021 Available at http://www.ieee802.org/1/files/public/docs2021
Time Sync โ Elements & Relationships
GM TIMEEnd Station
Time
Link Delay Residence Time
Corrections
Rate Ratio
Corrections
Rate Ratio
Accumulated Corrections
Rate Ratio (Accumulated NRRs)
As mentioned, the Time Sync is actually accomplished by accumulating the delays (i.e. the corrections) and measuring the rate ratio between the GM and the End Station.
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McCall, Stanton with Woods, Weingartner, and SchlechterIEEE 802.1 TSN / 60802 Interim, September 2021 Available at http://www.ieee802.org/1/files/public/docs2021
p. 14McCall, Stanton with Woods, Weingartner, and SchlechterIEEE 802.1 TSN Interim, Virtual, September 2021 Available at http://www.ieee802.org/1/files/public/docs2021
Time Sync โ Elements & Relationships
GM TIMEEnd Station
Time
Link Delay Residence Time
INCOMING OUTGOING
CorrectionAdjustment
Neighbor Rate Ratio
Correction
Rate Ratio
Correction
Rate Ratio
Rate Ratio
At each bridge there is a process for taking the incoming Correction Field and Rate Ratio and adding effects of the local pDelay (between the local and upstream device) and Residence Time.
Iโll be referring back to this diagram later, but when it comes to analysing the errors involved in this process, itโs helpful to take a slightly different viewโฆ
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p. 15McCall, Stanton with Woods, Weingartner, and SchlechterIEEE 802.1 TSN Interim, Virtual, September 2021 Available at http://www.ieee802.org/1/files/public/docs2021
Link Delay
Time Sync โ Elements & Relationships
Dynamic Time Sync Error
Timestamp Granularity Error
Residence Time
Dynamic Timestamp Error
Neighbor Rate Ratio Rate Ratio
Clock Drift
Link Delay Error Residence Time Error
Neighbor Rate Ratio Error
Rate RatioError
This is the start of the error analysis.
Timestamp Granularity Error, is related to the maximum resolution of Timestamps.
Dynamic Timestamp Error is in addition to Granularity and can be very implementation dependant.
Clock Drift is just the way that each device has itโs own local clock which is running a slightly different speedโฆand also changing that speed over time (mostly due to temperature changes; or at least those are the changes that have the greatest impact on our analysis).
All other errors are all rooted in these three.
Not all of the relationships need to be modelled. Figuring out which can be ignored is part of the analysis.
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p. 16McCall, Stanton with Woods, Weingartner, and SchlechterIEEE 802.1 TSN Interim, Virtual, September 2021 Available at http://www.ieee802.org/1/files/public/docs2021
Link Delay Error โTimestamp Granularity & Dynamic Time Stamp Error
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McCall, Stanton with Woods, Weingartner, and SchlechterIEEE 802.1 TSN / 60802 Interim, September 2021 Available at http://www.ieee802.org/1/files/public/docs2021
p. 17McCall, Stanton with Woods, Weingartner, and SchlechterIEEE 802.1 TSN Interim, Virtual, September 2021 Available at http://www.ieee802.org/1/files/public/docs2021
Time Sync โ Elements & Relationships
Dynamic Time Sync Error
Time Stamp Granularity
Link Delay Error Residence Time Error
Dynamic Time Stamp Error
Neighbor Rate Ratio Error
Rate RatioError
Clock Drift
Time Stamp Granularity
Link Delay Error
Dynamic Time Stamp Error
To be clear, weโre only looking at these three elements in this section. We arenโt looking at any of the others.
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p. 18McCall, Stanton with Woods, Weingartner, and SchlechterIEEE 802.1 TSN Interim, Virtual, September 2021 Available at http://www.ieee802.org/1/files/public/docs2021
A (e.g. 99)
B (e.g. 98)
t1
t2 t3
t4
(t2 โ t1) + (t4 โ t3)
2pDelay
t2 โ t1 = 14 ns t4 โ t3 = 14 ns
pDelayReq pDelayResp
=
This is the idealised viewโฆthe Platonic ideal of how pDelay is measured.
Note that โAโ is device 99. It initiates a pDelayReq to the upstream device (98)โฆwhich responds with a pDelayRespโฆafter the pDelay Response Time.
The default maximum for pDelay Response is 10ms, which is much longer than the pDelay which in this case is 14nsโฆindicating a cable length of 4.2mโฆbut for the moment Iโm going to model this shorter pDelay Response Time to make things legible and illustrate some principlesโฆ
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p. 19McCall, Stanton with Woods, Weingartner, and SchlechterIEEE 802.1 TSN Interim, Virtual, September 2021 Available at http://www.ieee802.org/1/files/public/docs2021
A
B
0 8 16 32 40 48 56 64 72 80
0 8 16 32 40 48 56 64 72 80
t1
t2 t3
t4
(t2 โ t1) + (t4 โ t3)
2= 14 ns
t2 โ t1 = 14 t4 โ t3 = 14
pDelayReq pDelayResp
24
24
Letโs put some numbers on the time line. Weโll use a resolution of 8ns, which is what youโd get with a clock speed of 125MHz.
Butโฆclocks A & B arenโt the sameโฆthey arenโt counting the same numbers at the same time.
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p. 20McCall, Stanton with Woods, Weingartner, and SchlechterIEEE 802.1 TSN Interim, Virtual, September 2021 Available at http://www.ieee802.org/1/files/public/docs2021
A
B
0 8 16 24 32 40 48 56 64 72 80
8000 8008 8016 8024 8032 8040 8048 8056 8064 8072 8080
t1
t2 t3
t4
(t4 โ t1) โ (t3 โ t2)
2= 14 ns
t4 โ t1 = 46
t3 โ t2 = 18
pDelayReq pDelayResp
Weโll model this by just adding a nice, round 8,000 to the lower line.
This requires changing the equation to subtract Bโs pDelay Response time (on the lower timeline) from the time between A sending the Request and receiving the Response. Itโs mathematically equivalentโฆand also allows compensation for differing clock speeds (via Neighbor Rate Ratioโฆwhich we are not considering in this section).
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p. 21McCall, Stanton with Woods, Weingartner, and SchlechterIEEE 802.1 TSN Interim, Virtual, September 2021 Available at http://www.ieee802.org/1/files/public/docs2021
A
B
0 8 16 24 32 40 48 56 64 72 80
t1 t4
= 16 ns(t4 โ t1) โ (t3 โ t2)
2
pDelayReq pDelayResp
t4 โ t1 = 48
t2 t3
8000 8008 8016 8024 8032 8040 8048 8056 8064 8072 8080
t3 โ t2 = 16
8ns is our Timestamp Granularityโฆwhich means the pDelay Req could arrive at any time between clock ticks and the Timestamp will actually reflect the next clock tick.
Iโm keeping things simple here by assuming the TX time is exactly on a clock tick (and measured as such)โฆ.but you can see the effect this has. The measurement of pDelay is 16ns, not 14ns.
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McCall, Stanton with Woods, Weingartner, and SchlechterIEEE 802.1 TSN / 60802 Interim, September 2021 Available at http://www.ieee802.org/1/files/public/docs2021
p. 22McCall, Stanton with Woods, Weingartner, and SchlechterIEEE 802.1 TSN Interim, Virtual, September 2021 Available at http://www.ieee802.org/1/files/public/docs2021
A
B
0 8 16 24 32 40 48 56 64 72 80
t1 t4
= 16 ns(t4 โ t1) โ (t3 โ t2)
2
pDelayReq pDelayResp
t4 โ t1 = 48
t2 t3
8000 8008 8016 8024 8032 8040 8048 8056 8064 8072 8080
t3 โ t2 = 16
Timelines A &B are independent of each other and move relative to each other due to different PPMs of Local Clocks.
Iโll deal with the movement relative to each other in a moment. For now, letโs just look at this measurement and the fact that the timelines are independent. This means the clock ticks donโt necessarily line up exactly. They mightโฆbut itโs more likely there will be an offset.
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McCall, Stanton with Woods, Weingartner, and SchlechterIEEE 802.1 TSN / 60802 Interim, September 2021 Available at http://www.ieee802.org/1/files/public/docs2021
p. 23McCall, Stanton with Woods, Weingartner, and SchlechterIEEE 802.1 TSN Interim, Virtual, September 2021 Available at http://www.ieee802.org/1/files/public/docs2021
A
B
0 8 16
t1
t2 t3
= 16 ns(t4 โ t1) โ (t3 โ t2)
2
8000 8008 8016 8024 8032 8040 8048 8056 8064 8072 8080
pDelayReq pDelayResp
t4 โ t1 = 48
t3 โ t2 = 16
24 32 40 48 56
t464 72 80
Offset: 0ns
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McCall, Stanton with Woods, Weingartner, and SchlechterIEEE 802.1 TSN / 60802 Interim, September 2021 Available at http://www.ieee802.org/1/files/public/docs2021
p. 24McCall, Stanton with Woods, Weingartner, and SchlechterIEEE 802.1 TSN Interim, Virtual, September 2021 Available at http://www.ieee802.org/1/files/public/docs2021
A
B
0 8 16
t1
t2 t3
= 16 ns(t4 โ t1) โ (t3 โ t2)
2
8000 8008 8016 8024 8032 8040 8048 8056 8064 8072 8080
pDelayReq pDelayResp
t4 โ t1 = 48
t3 โ t2 = 16
24 32 40 48 56
t464 72 80
Offset: 1ns
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McCall, Stanton with Woods, Weingartner, and SchlechterIEEE 802.1 TSN / 60802 Interim, September 2021 Available at http://www.ieee802.org/1/files/public/docs2021
p. 25McCall, Stanton with Woods, Weingartner, and SchlechterIEEE 802.1 TSN Interim, Virtual, September 2021 Available at http://www.ieee802.org/1/files/public/docs2021
A
B
0 8 16
t1
t2
= 16 ns(t4 โ t1) โ (t3 โ t2)
2
8008 8016 8024
pDelayReq pDelayResp
t4 โ t1 = 48
t3 โ t2 = 16
24 32 40 48 56
t464 72 80
t3
8032 8040 8048 8056 8064 8072 8080
Offset: 2ns
At an offset >2ns, the measurement is no longer 16nsโฆ
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McCall, Stanton with Woods, Weingartner, and SchlechterIEEE 802.1 TSN / 60802 Interim, September 2021 Available at http://www.ieee802.org/1/files/public/docs2021
p. 26McCall, Stanton with Woods, Weingartner, and SchlechterIEEE 802.1 TSN Interim, Virtual, September 2021 Available at http://www.ieee802.org/1/files/public/docs2021
A
B
0 8 16 24 32 40 48 56 64 72 80
t1 t4
= 20 ns(t4 โ t1) โ (t3 โ t2)
2
pDelayReq pDelayResp
t4 โ t1 = 56
t3 โ t2 = 16
8008 8016 8024 8032 8040 8048 8056 8064 8072 8080
t2 t3
Offset: 3ns
โฆitโs worse. 20ns.
This, by the way, is to be expected. With a Timestamp Granularity of 8ns, and a divide by 2 in the equation, we can expect a measurement granularity of 4ns.
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McCall, Stanton with Woods, Weingartner, and SchlechterIEEE 802.1 TSN / 60802 Interim, September 2021 Available at http://www.ieee802.org/1/files/public/docs2021
p. 27McCall, Stanton with Woods, Weingartner, and SchlechterIEEE 802.1 TSN Interim, Virtual, September 2021 Available at http://www.ieee802.org/1/files/public/docs2021
A
B
0 8 16
t1
= 20 ns(t4 โ t1) โ (t3 โ t2)
2
pDelayReq pDelayResp
t4 โ t1 = 56
t3 โ t2=16
24 32 40 48 56 64 72 80
t4
8008 8016 8024 8032 8040 8048 8056 8064 8072 8080
t2 t3
Offset: 4ns
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McCall, Stanton with Woods, Weingartner, and SchlechterIEEE 802.1 TSN / 60802 Interim, September 2021 Available at http://www.ieee802.org/1/files/public/docs2021
p. 28McCall, Stanton with Woods, Weingartner, and SchlechterIEEE 802.1 TSN Interim, Virtual, September 2021 Available at http://www.ieee802.org/1/files/public/docs2021
A
B
0 8 16
t1
= 20 ns(t4 โ t1) โ (t3 โ t2)
2
pDelayReq pDelayResp
t4 โ t1 = 56
t3 โ t2 = 16
24 32 40 48 56 64 72 80
t4
8008 8016 8024 8032 8040 8048 8056 8064 8072 8080
t2 t3
Offset: 5ns
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McCall, Stanton with Woods, Weingartner, and SchlechterIEEE 802.1 TSN / 60802 Interim, September 2021 Available at http://www.ieee802.org/1/files/public/docs2021
p. 29McCall, Stanton with Woods, Weingartner, and SchlechterIEEE 802.1 TSN Interim, Virtual, September 2021 Available at http://www.ieee802.org/1/files/public/docs2021
A
B
0 8 16
t1
= 16 ns(t4 โ t1) โ (t3 โ t2)
2
pDelayReq pDelayResp
t4 โ t1 = 48
t3 โ t2 = 16
24 32 40 48 56
t464 72 80
8008 8016 8024 8032 8040 8048 8056 8064 8072 8080
t2 t3
Offset: 6ns
At 6ns the measurement gets back to 16ns.
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McCall, Stanton with Woods, Weingartner, and SchlechterIEEE 802.1 TSN / 60802 Interim, September 2021 Available at http://www.ieee802.org/1/files/public/docs2021
p. 30McCall, Stanton with Woods, Weingartner, and SchlechterIEEE 802.1 TSN Interim, Virtual, September 2021 Available at http://www.ieee802.org/1/files/public/docs2021
A
B
0 8 16
t1
= 16 ns(t4 โ t1) โ (t3 โ t2)
2
8096
pDelayReq pDelayResp
t4 โ t1 = 48
t3 โ t2 = 16
24 32 40 48 56
t464 72 80
8008 8016 8024 8032 8040 8048 8056 8064 8072 8080
t2 t3
Offset: 7ns
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McCall, Stanton with Woods, Weingartner, and SchlechterIEEE 802.1 TSN / 60802 Interim, September 2021 Available at http://www.ieee802.org/1/files/public/docs2021
p. 31McCall, Stanton with Woods, Weingartner, and SchlechterIEEE 802.1 TSN Interim, Virtual, September 2021 Available at http://www.ieee802.org/1/files/public/docs2021
A
B
0 8 16
t1
= 16 ns(t4 โ t1) โ (t3 โ t2)
2
8096
pDelayReq pDelayResp
t4 โ t1 = 48
t3 โ t2 = 16
24 32 40 48 56
t464 72 80
8008 8016 8024 8032 8040 8048 8056 8064 8072 8080
t2 t3
Offset: 0ns
And then weโre back to where we started, with an offset of 0ns.
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McCall, Stanton with Woods, Weingartner, and SchlechterIEEE 802.1 TSN / 60802 Interim, September 2021 Available at http://www.ieee802.org/1/files/public/docs2021
p. 32McCall, Stanton with Woods, Weingartner, and SchlechterIEEE 802.1 TSN Interim, Virtual, September 2021 Available at http://www.ieee802.org/1/files/public/docs2021
Offset / ns Measured pDelay / ns
0 โ 2 16
>2 โ 6 20
>6 โ <8 16
Average = 18 ns โ 14 ns
OK to average as, over multiple measurements, the timelines will move relative to each other.But on itโs own that doesnโt help.
Soโฆdoes this even work at all?
The good news is that yes, it does. There are two reasonsโฆ
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McCall, Stanton with Woods, Weingartner, and SchlechterIEEE 802.1 TSN / 60802 Interim, September 2021 Available at http://www.ieee802.org/1/files/public/docs2021
p. 33McCall, Stanton with Woods, Weingartner, and SchlechterIEEE 802.1 TSN Interim, Virtual, September 2021 Available at http://www.ieee802.org/1/files/public/docs2021
A
B
0 8 16 24 X+40 X+48 X+56 64X+32
t1
t2 t3
t4
= 16 ns(t4 โ t1) โ (t3 โ t2)
2
8000 8008 8016 8024 X+8040 X+8048 X+8056 X+8064X+8032
pDelayReq pDelayResp
t4 โ t1 = X+48
t3 โ t2 = X+16
X
X8032
32
Firstly, there is a relatively large amount of time between the left and right ends of the timeline due to the fact that pDelay Response Time is much longer than pDelayโฆa fact I previously ignored.
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McCall, Stanton with Woods, Weingartner, and SchlechterIEEE 802.1 TSN / 60802 Interim, September 2021 Available at http://www.ieee802.org/1/files/public/docs2021
p. 34McCall, Stanton with Woods, Weingartner, and SchlechterIEEE 802.1 TSN Interim, Virtual, September 2021 Available at http://www.ieee802.org/1/files/public/docs2021
A
B
0 8 16 24 X+40 X+48 X+56 64X+32
t1
t2
t4
= 16 ns(t4 โ t1) โ (t3 โ t2)
2
8000 8008 8016 8024
t3
X+8040 X+8048 X+8056 X+8064X+8032
pDelayReq pDelayResp
t4 โ t1 = X+48
t3 โ t2 = X+16
XA
XB8032
32
And secondly, that relatively long period of time is measured by two free running local clocks. Running at different speedsโฆand drifting slightly relative to each other.
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McCall, Stanton with Woods, Weingartner, and SchlechterIEEE 802.1 TSN / 60802 Interim, September 2021 Available at http://www.ieee802.org/1/files/public/docs2021
p. 35McCall, Stanton with Woods, Weingartner, and SchlechterIEEE 802.1 TSN Interim, Virtual, September 2021 Available at http://www.ieee802.org/1/files/public/docs2021
A
B
0 8 16 24 X+40 X+48 X+56 64X+32
t1 t4
= 16 ns(t4 โ t1) โ (t3 โ t2)
2
pDelayReq pDelayResp
t4 โ t1 = X+48
t3 โ t2 = X+16
XA
XBt2
8000 8008 8016 8024
t3
X+8040 X+8048 X+8056 X+8064X+80328032
32
Already know that timelines A & B move relative to each other.
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McCall, Stanton with Woods, Weingartner, and SchlechterIEEE 802.1 TSN / 60802 Interim, September 2021 Available at http://www.ieee802.org/1/files/public/docs2021
p. 36McCall, Stanton with Woods, Weingartner, and SchlechterIEEE 802.1 TSN Interim, Virtual, September 2021 Available at http://www.ieee802.org/1/files/public/docs2021
A
B
0 8 16 32
t1
t3
X+40 X+48 X+56 64X+32
t4
= 16 ns(t4 โ t1) โ (t3 โ t2)
2
X+8040 X+8048 X+8056 X+8064X+8032
pDelayReq pDelayResp
t4 โ t1 = X+48
t3 โ t2 = X+16
XA
XBt2
8000 8008 8016 8032 8040
40
Phase changes of Local Clocks betweenpDelayReq & pDelayResp mean the two ends of the timeline can be treated as independent.
Because the left and right ends of the timeline can effectively be decoupled from each otherโฆbecause there are different numbers of clock ticks in XA and XB (which is eventually compensated for by NRRโฆwhich we arenโt not currently considering right now)โฆyou can look at the measurement probabilisticallyโฆwhich looks like thisโฆ
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McCall, Stanton with Woods, Weingartner, and SchlechterIEEE 802.1 TSN / 60802 Interim, September 2021 Available at http://www.ieee802.org/1/files/public/docs2021
p. 37McCall, Stanton with Woods, Weingartner, and SchlechterIEEE 802.1 TSN Interim, Virtual, September 2021 Available at http://www.ieee802.org/1/files/public/docs2021
A
B
t1
t2 ยฑ4ns
t4 ยฑ4ns
= 14 ns ยฑ4 ns(t4 โ t1) โ (t3 โ t2)
2
pDelayReq pDelayResp
t4 โ t1
t3 โ t2
XA
XB t3TimestampGranularity
This shows only error due to Timestamp Granularityโฆ8nsโฆcentred on the 14ns, i.e. ยฑ4 ns
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McCall, Stanton with Woods, Weingartner, and SchlechterIEEE 802.1 TSN / 60802 Interim, September 2021 Available at http://www.ieee802.org/1/files/public/docs2021
p. 38McCall, Stanton with Woods, Weingartner, and SchlechterIEEE 802.1 TSN Interim, Virtual, September 2021 Available at http://www.ieee802.org/1/files/public/docs2021
A
B
t1 ยฑ4ns
t2 ยฑ4ns
t4 ยฑ4ns
= 14 ns ยฑ8 ns(t4 โ t1) โ (t3 โ t2)
2
pDelayReq pDelayResp
t4 โ t1
t3 โ t2
XA
XB t3 ยฑ4nsTimestampGranularity
Add TX Timestamp Granularity.
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McCall, Stanton with Woods, Weingartner, and SchlechterIEEE 802.1 TSN / 60802 Interim, September 2021 Available at http://www.ieee802.org/1/files/public/docs2021
p. 39McCall, Stanton with Woods, Weingartner, and SchlechterIEEE 802.1 TSN Interim, Virtual, September 2021 Available at http://www.ieee802.org/1/files/public/docs2021
A
B
t1 ยฑ4ns + ยฑ2ns
t2 ยฑ4ns + ยฑ2ns
t4 ยฑ4ns + ยฑ2ns
= 14 ns ยฑ12 ns(t4 โ t1) โ (t3 โ t2)
2
pDelayReq pDelayResp
t4 โ t1
t3 โ t2
XA
XB t3 ยฑ4ns + ยฑ2nsTimestampGranularity
Dynamic Timestamp Error
Add Dynamic Time Stamp Error, ยฑ2 ns.
Indications are that this error should be less than Timestamp granularity. It varies with implementation and Iโll let Kevin talk about that a bit more laterโฆbut for the moment letโs just model it as this probability.
If you add together all 8 uniform probabilitiesโฆ
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McCall, Stanton with Woods, Weingartner, and SchlechterIEEE 802.1 TSN / 60802 Interim, September 2021 Available at http://www.ieee802.org/1/files/public/docs2021
p. 40McCall, Stanton with Woods, Weingartner, and SchlechterIEEE 802.1 TSN Interim, Virtual, September 2021 Available at http://www.ieee802.org/1/files/public/docs2021
A
B
t1 ยฑ6ns
t2 ยฑ6ns
t4 ยฑ6ns
= 14 ns ยฑ12 ns(t4 โ t1) โ (t3 โ t2)
2
pDelayReq pDelayResp
t4 โ t1
t3 โ t2
XA
XB t3 ยฑ6ns
pDelay Error ยฑ12ns
Combined Timestamp Error
You get this.
At each TX and RX point, two uniform probabilities combine to give a triangular probabilityโฆand those combine to an overall bell curve probability.
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McCall, Stanton with Woods, Weingartner, and SchlechterIEEE 802.1 TSN / 60802 Interim, September 2021 Available at http://www.ieee802.org/1/files/public/docs2021
p. 41McCall, Stanton with Woods, Weingartner, and SchlechterIEEE 802.1 TSN Interim, Virtual, September 2021 Available at http://www.ieee802.org/1/files/public/docs2021
A
B
t1
t2 t3
t4
= 14 ns (on average)(t4 โ t1) โ (t3 โ t2)
2
pDelayReq pDelayResp
t4 โ t1 = X+28 (on average)
t3 โ t2 = X (on average)
XA
XB
Which can be averaged out.
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McCall, Stanton with Woods, Weingartner, and SchlechterIEEE 802.1 TSN / 60802 Interim, September 2021 Available at http://www.ieee802.org/1/files/public/docs2021
p. 42McCall, Stanton with Woods, Weingartner, and SchlechterIEEE 802.1 TSN Interim, Virtual, September 2021 Available at http://www.ieee802.org/1/files/public/docs2021
Link Delay Error โ Observations
โข Averaging of pDelay measurements should be mandatoryโข Delivers meanLinkDelay with much lower error
โข Helpful if implementations exhibit error probability distributions that can be averaged out to zero
โข For Ethernetโฆโข The goal is to estimate a relatively fixed quantityโข Very low bandwidth lowโpass filter is appropriate due to stability of pDelay. (IIR?)โข Some dedicated startup behaviour may be appropriate to arrive quickly at a useful average
โข For wirelessโฆโข More complex due to multiโpath and potential movement of devices, butโฆโข Unlikely to have long chains of devices, or at least long chains of mobile devices.โข Out of scope for the current discussion.
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McCall, Stanton with Woods, Weingartner, and SchlechterIEEE 802.1 TSN / 60802 Interim, September 2021 Available at http://www.ieee802.org/1/files/public/docs2021
p. 43McCall, Stanton with Woods, Weingartner, and SchlechterIEEE 802.1 TSN Interim, Virtual, September 2021 Available at http://www.ieee802.org/1/files/public/docs2021
Link Delay & Timestamp Errors โCurrent Simulation
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McCall, Stanton with Woods, Weingartner, and SchlechterIEEE 802.1 TSN / 60802 Interim, September 2021 Available at http://www.ieee802.org/1/files/public/docs2021
p. 44McCall, Stanton with Woods, Weingartner, and SchlechterIEEE 802.1 TSN Interim, Virtual, September 2021 Available at http://www.ieee802.org/1/files/public/docs2021
A
B
t1 ยฑ4ns + ยฑ2ns
t2 ยฑ4ns + ยฑ2ns
t4 ยฑ4ns + ยฑ2ns
= 14ns ยฑ12ns(t4 โ t1) โ (t3 โ t2)
2
pDelayReq pDelayResp
t4 โ t1
t3 โ t2
XA
XB t3 ยฑ4ns + ยฑ2nsTimestampGranularity
Dynamic Timestamp Error
This is the model we just saw. The simulation does not do this. Instead it doesโฆ
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McCall, Stanton with Woods, Weingartner, and SchlechterIEEE 802.1 TSN / 60802 Interim, September 2021 Available at http://www.ieee802.org/1/files/public/docs2021
p. 45McCall, Stanton with Woods, Weingartner, and SchlechterIEEE 802.1 TSN Interim, Virtual, September 2021 Available at http://www.ieee802.org/1/files/public/docs2021
A
B
t1 ยฑ4ns + ยฑ8ns
t2 ยฑ4ns + ยฑ8ns
t4 ยฑ4ns + ยฑ8ns
= 14ns ยฑ24ns(t4 โ t1) โ (t3 โ t2)
2
pDelayReq pDelayResp
t4 โ t1
t3 โ t2
XA
XB t3 ยฑ4ns + ยฑ8nsTimestampGranularity
Dynamic Timestamp Error
This.
Dynamic Timestamp Error is ยฑ8nsโฆbut itโs either +8 or -8 with a 50:50 change of either for each and every timestamp. Thatโs represented by the two vertical linesโฆone at +8, one at -8.
Working out the probabilitiesโฆ
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McCall, Stanton with Woods, Weingartner, and SchlechterIEEE 802.1 TSN / 60802 Interim, September 2021 Available at http://www.ieee802.org/1/files/public/docs2021
p. 46McCall, Stanton with Woods, Weingartner, and SchlechterIEEE 802.1 TSN Interim, Virtual, September 2021 Available at http://www.ieee802.org/1/files/public/docs2021
A
B
t1 ยฑ12ns
t2 ยฑ12ns
t4 ยฑ12ns
= 14ns ยฑ24ns(t4 โ t1) โ (t3 โ t2)
2
pDelayReq pDelayResp
t4 โ t1
t3 โ t2
XA
XB t3 ยฑ12nsCombined Timestamp Error
pDelay Error ยฑ24ns
1
6
1
4 4
This is Case 1 (and most other cases) in 60802-garner-new-simulation-results-new-freq-stab-model-0421-v02.pdf, i.e. the most recent set of time series simulations.
And there is no averaging of pDelay in the current simulation. The latest pDelaymeasurement is passed directly into the Correction Field.
Ohโฆand if you are wondering if this could result in a negative pDelay, i.e. the simulation of time travelโฆyes, it could. But not with the current simulationโs pDelay value of 150ns.
There were also three other simulations of interest in that same run of simulations.
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McCall, Stanton with Woods, Weingartner, and SchlechterIEEE 802.1 TSN / 60802 Interim, September 2021 Available at http://www.ieee802.org/1/files/public/docs2021
p. 47McCall, Stanton with Woods, Weingartner, and SchlechterIEEE 802.1 TSN Interim, Virtual, September 2021 Available at http://www.ieee802.org/1/files/public/docs2021
A
B
t1
t2
t4
= 14ns(t4 โ t1) โ (t3 โ t2)
2
pDelayReq pDelayResp
t4 โ t1
t3 โ t2
XA
XB t3Combined
Timestamp Error
pDelay Error 0ns
Case 13 in 60802-garner-new-simulation-results-new-freq-stab-model-0421-v02.pdf
This turned both errors to zero. Which, for Link Delay is the equivalent of what averaging pDelay measurements should accomplish.
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McCall, Stanton with Woods, Weingartner, and SchlechterIEEE 802.1 TSN / 60802 Interim, September 2021 Available at http://www.ieee802.org/1/files/public/docs2021
p. 48McCall, Stanton with Woods, Weingartner, and SchlechterIEEE 802.1 TSN Interim, Virtual, September 2021 Available at http://www.ieee802.org/1/files/public/docs2021
A
B
= 14ns ยฑ16ns(t4 โ t1) โ (t3 โ t2)
2
pDelayReq pDelayResp
t4 โ t1
t3 โ t2
XA
XBCombined
Timestamp Error
pDelay Error ยฑ16ns
t1 ยฑ8ns
t2 ยฑ8ns
t4 ยฑ8ns
t3 ยฑ8ns
1 6 14 4
Case 14 in 60802-garner-new-simulation-results-new-freq-stab-model-0421-v02.pdf
This removed Timestamp Granularity Errorโฆbut kept Dynamic Timestamp Error.
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McCall, Stanton with Woods, Weingartner, and SchlechterIEEE 802.1 TSN / 60802 Interim, September 2021 Available at http://www.ieee802.org/1/files/public/docs2021
p. 49McCall, Stanton with Woods, Weingartner, and SchlechterIEEE 802.1 TSN Interim, Virtual, September 2021 Available at http://www.ieee802.org/1/files/public/docs2021
A
B
t1 ยฑ4ns
t2 ยฑ4ns
t4 ยฑ4ns
= 14ns ยฑ8ns(t4 โ t1) โ (t3 โ t2)
2
pDelayReq pDelayResp
t4 โ t1
t3 โ t2
XA
XB t3 ยฑ4nsCombined Timestamp Error
pDelay Error ยฑ8ns
Case 15 in 60802-garner-new-simulation-results-new-freq-stab-model-0421-v02.pdf
And this did the oppositeโฆkept Timestamp Granularity Error but removed Dynamic Timestamp Error.
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McCall, Stanton with Woods, Weingartner, and SchlechterIEEE 802.1 TSN / 60802 Interim, September 2021 Available at http://www.ieee802.org/1/files/public/docs2021
p. 50McCall, Stanton with Woods, Weingartner, and SchlechterIEEE 802.1 TSN Interim, Virtual, September 2021 Available at http://www.ieee802.org/1/files/public/docs2021
CaseTimestamp
Granularity / nsDynamic
Timestamp Error
1 ยฑ4 ยฑ8
14 0 ยฑ8
15 ยฑ4 0
13 0 0
pDelay Error ยฑ24ns
pDelay Error ยฑ16ns
pDelay Error ยฑ8ns
pDelay Error 0ns
Comparison of Analysis & Latest Simulations
1855 2841
496 733
27 32
96%
26%
1.4%
105%
27%
1.2%
1924 2711
max|dTE| 64 Hops / ns max|dTE| 100 Hops / ns
Note that the simulation removed the effects of Timestamp Granularity and Dynamic Timestamp Error from both Link Delay and Residence Time.
While Link Delay could get close to this via averagingโฆyou canโt do that with Residence Time. So youโll get some big benefits from averagingโฆbut not quite as much as shown here.
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McCall, Stanton with Woods, Weingartner, and SchlechterIEEE 802.1 TSN / 60802 Interim, September 2021 Available at http://www.ieee802.org/1/files/public/docs2021
p. 51McCall, Stanton with Woods, Weingartner, and SchlechterIEEE 802.1 TSN Interim, Virtual, September 2021 Available at http://www.ieee802.org/1/files/public/docs2021
Modelling Link Delay Error โ Timestamp Granularity & Dynamic Timestamp Errorโข Model linear error with uniform distribution for both factors.
โข Allow separate RX & TX values for both, i.e. four factorsโฆ
โข Add โError Correction Factorโ to account for averaging.โข One value to cover all errors from all four factors.
โข 25% Effec ve โ Removes 25% of the error.
Timestamp Granularity Dynamic Time Stamp Error
RX
TX
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McCall, Stanton with Woods, Weingartner, and SchlechterIEEE 802.1 TSN / 60802 Interim, September 2021 Available at http://www.ieee802.org/1/files/public/docs2021
p. 52McCall, Stanton with Woods, Weingartner, and SchlechterIEEE 802.1 TSN Interim, Virtual, September 2021 Available at http://www.ieee802.org/1/files/public/docs2021
Formulae โ pDelayerrorTS
๐๐๐๐๐ฟ๐๐๐๐ท๐๐๐๐ฆ๐๐๐๐๐ 1 ๐๐๐๐๐ณ๐๐๐๐ซ๐๐๐๐๐๐๐๐๐๐ช๐๐๐๐๐๐๐๐๐ ๐๐ท๐๐๐๐ฆ๐๐๐๐๐๐๐ ๐๐ท๐๐๐๐ฆ๐๐๐๐๐๐๐ ๐
๐๐ท๐๐๐๐ฆ๐๐๐๐๐๐๐
๐ก4๐๐๐๐๐๐๐ ๐ก1๐๐๐๐๐๐๐ ๐ก3๐๐๐๐๐๐๐ ๐ก2๐๐๐๐๐๐๐
2
๐ก1๐๐๐๐๐๐๐ ๐ป๐บ๐ฎ๐ฌ๐ป๐ฟ ๐ซ๐ป๐บ๐ฌ๐ป๐ฟ ๐ก2๐๐๐๐๐๐๐ ๐ป๐บ๐ฎ๐ฌ๐น๐ฟ ๐ซ๐ป๐บ๐ฌ๐น๐ฟ
๐ก3๐๐๐๐๐๐๐ ๐ป๐บ๐ฎ๐ฌ๐ป๐ฟ ๐ซ๐ป๐บ๐ฌ๐ป๐ฟ ๐ก4๐๐๐๐๐๐๐ ๐ป๐บ๐ฎ๐ฌ๐น๐ฟ ๐ซ๐ป๐บ๐ฌ๐น๐ฟ
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McCall, Stanton with Woods, Weingartner, and SchlechterIEEE 802.1 TSN / 60802 Interim, September 2021 Available at http://www.ieee802.org/1/files/public/docs2021
p. 53McCall, Stanton with Woods, Weingartner, and SchlechterIEEE 802.1 TSN Interim, Virtual, September 2021 Available at http://www.ieee802.org/1/files/public/docs2021
Neighbor Rate Ratio Error
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McCall, Stanton with Woods, Weingartner, and SchlechterIEEE 802.1 TSN / 60802 Interim, September 2021 Available at http://www.ieee802.org/1/files/public/docs2021
p. 54McCall, Stanton with Woods, Weingartner, and SchlechterIEEE 802.1 TSN Interim, Virtual, September 2021 Available at http://www.ieee802.org/1/files/public/docs2021
Time Sync โ Elements & Relationships
Dynamic Time Sync Error
Time Stamp Granularity
Link Delay Error Residence Time Error
Dynamic Time Stamp Error
Neighbor Rate Ratio Error
Rate RatioError
Clock Drift
Time Stamp Granularity
Dynamic Time Stamp Error
Neighbor Rate Ratio Error
Clock Drift
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McCall, Stanton with Woods, Weingartner, and SchlechterIEEE 802.1 TSN / 60802 Interim, September 2021 Available at http://www.ieee802.org/1/files/public/docs2021
p. 55McCall, Stanton with Woods, Weingartner, and SchlechterIEEE 802.1 TSN Interim, Virtual, September 2021 Available at http://www.ieee802.org/1/files/public/docs2021
B (e.g. 98)
A (e.g. 99)
t3
t4
pDelayResp
t3' โ t3
t4' โ t4
YA
YB
pDelayRespโ
t3'
t4'
=NRRAB @ t4' (t3' โ t3)
(t4' โ t4)
NRRAB
The Platonic idealโฆ
Note that A uses pDelayResp messages from B to A measure the Rate Ratio between the two (NRRAB). These are the same pDelayResp messages that are uses to measure Link Delay between A and B. (Iโm just not showing the pDelayReqmessages). Also note that the intent here is to measure NRR at t4.
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McCall, Stanton with Woods, Weingartner, and SchlechterIEEE 802.1 TSN / 60802 Interim, September 2021 Available at http://www.ieee802.org/1/files/public/docs2021
p. 56McCall, Stanton with Woods, Weingartner, and SchlechterIEEE 802.1 TSN Interim, Virtual, September 2021 Available at http://www.ieee802.org/1/files/public/docs2021
B
A
t3
t4
t3' โ t3
t4' โ t4
1s
1s
t3'
t4'
localClkB125MHz + 10ppm
localClkB125MHz + 10ppm
localClkA125MHz โ 10ppm
localClkA125MHz โ 10ppm
874,998,750
1,875,001,250
1,000,000,000
2,000,000,000
=NRRAB @ t4' (t3' โ t3)
(t4' โ t4)=
124,998,750
125,001,250โ โ0.002% = โ20 ppm
pDelayResp pDelayRespโ
NRRAB
Put some realistic number on itโฆ
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McCall, Stanton with Woods, Weingartner, and SchlechterIEEE 802.1 TSN / 60802 Interim, September 2021 Available at http://www.ieee802.org/1/files/public/docs2021
p. 57McCall, Stanton with Woods, Weingartner, and SchlechterIEEE 802.1 TSN Interim, Virtual, September 2021 Available at http://www.ieee802.org/1/files/public/docs2021
t4 ยฑ6ns
t4 ยฑ6ns
B
A
t3' โ t3
t4' โ t4
1s
1s
localClkB125MHz + 10ppm
localClkB125MHz + 10ppm
localClkA125MHz โ 10ppm
localClkA125MHz โ 10ppm
874,998,750
1,875,001,250
1,000,000,000
2,000,000,000
t3 ยฑ6ns
t4 ยฑ6ns
=NRRAB @ t4' (t3' โ t3)
(t4' โ t4)=
124,998,750
125,001,250โ โ0.002% = โ20 ppm
pDelayResp pDelayRespโ
NRRAB
These are the errors that we could model.
Timestamp Granularity Error & Dynamic Timestamp Error (shown combined)
Clock Drift (not the +10ppm and -10ppmโฆbut the fact that those ppm values are changing over time, i.e. at the left end of the timeline, the points they arenโt+10ppm and -10ppm)
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McCall, Stanton with Woods, Weingartner, and SchlechterIEEE 802.1 TSN / 60802 Interim, September 2021 Available at http://www.ieee802.org/1/files/public/docs2021
p. 58McCall, Stanton with Woods, Weingartner, and SchlechterIEEE 802.1 TSN Interim, Virtual, September 2021 Available at http://www.ieee802.org/1/files/public/docs2021
What factors to model?Factor YES / NO
Time Stamp Granularity& Dynamic Time Stamp Error
NOยฑ20ns over 1s = ยฑ0.02ppmWill be swamped by other errors.
Drift Rate of Local Clocks A & B (ppm/s)(Difference between Local Clock A ppm/s andLocal Clock B ppm/s at start)
YES
For most recent temp profile, max possible difference between two clocksโ rate of change of ppm is ยฑ1.2ppm/s.ยฑ1.2ppm/s over 1s = ยฑ1.2ppmMay be a significant contributor to overall dynamic time sync error.
Rate of change of Drift Rate of Local Clocks A&B(Difference between Local Clocksโ ppm/s at start & finish.)
NO
For most recent temp profile, max rate of change of ppm/s (i.e.ppm/s per second) is ~ยฑ0.006ppm/s2.Rounding up to ยฑ0.01ppm/s2, over 1s (or 5s) the change is not significant.(Note: ignores discontinuities at start and end of temperature ramp, which are not realistic.)
It turns out that the Timestamp errors will be small relative to other errors and can be ignored.
The same applies to the rate of change of clock drift (in ppm/s2), which is low enough that, over the periods of time we are interested in, clock drift can be treated as constant, i.e. linear.
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McCall, Stanton with Woods, Weingartner, and SchlechterIEEE 802.1 TSN / 60802 Interim, September 2021 Available at http://www.ieee802.org/1/files/public/docs2021
p. 59McCall, Stanton with Woods, Weingartner, and SchlechterIEEE 802.1 TSN Interim, Virtual, September 2021 Available at http://www.ieee802.org/1/files/public/docs2021
Sources of Dynamic Time Sync Error
Dynamic Time Sync Error
Time Stamp Granularity
Link Delay Error Residence Time Error
Dynamic Time Stamp Error
Neighbor Rate Ratio Error
Rate RatioError
Clock Drift
Neighbor Rate Ratio Error
Clock Drift
So, we only need to look at Clock Driftโฆ
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McCall, Stanton with Woods, Weingartner, and SchlechterIEEE 802.1 TSN / 60802 Interim, September 2021 Available at http://www.ieee802.org/1/files/public/docs2021
p. 60McCall, Stanton with Woods, Weingartner, and SchlechterIEEE 802.1 TSN Interim, Virtual, September 2021 Available at http://www.ieee802.org/1/files/public/docs2021
B
A
1s
1s
localClkB125MHz + 10ppm
localClkB125MHz + 10ppm
localClkA125MHz โ 10ppm
localClkA125MHz โ 10ppm
874,998,750
1,875,001,250
1,000,000,000
2,000,000,000
=NRRAB @ t4' (t3' โ t3)
(t4' โ t4)=
124,998,750
125,001,250โ โ0.002% = โ20 ppm
pDelayResp pDelayRespโ
NRRAB
t4 ยฑ6ns
t4 ยฑ6ns
t3' โ t3
t4' โ t4
t3 ยฑ6ns
t4 ยฑ6ns
Which means we can remove the Timestamp Errors and also modify the diagram to remove the 1s discontinuity in the middle so that weโre looking at the appropriate scaleโฆ
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McCall, Stanton with Woods, Weingartner, and SchlechterIEEE 802.1 TSN / 60802 Interim, September 2021 Available at http://www.ieee802.org/1/files/public/docs2021
p. 61McCall, Stanton with Woods, Weingartner, and SchlechterIEEE 802.1 TSN Interim, Virtual, September 2021 Available at http://www.ieee802.org/1/files/public/docs2021
B
A
t3' โ t3
t4' โ t4
1s
localClkB125MHz + 10ppm
localClkB125MHz + 10ppm
localClkA125MHz โ 10ppm
localClkA125MHz โ 10ppm
874,998,750
1,875,001,250
1,000,000,000
2,000,000,000
t3
t4
t3'
t4'
=NRRAB @ t4' (t3' โ t3)
(t4' โ t4)=
124,998,750
125,001,250โ โ0.002% = โ20 ppm
pDelayResp pDelayRespโ
NRRAB
Then squeeze things together to make a bit of roomโฆ
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McCall, Stanton with Woods, Weingartner, and SchlechterIEEE 802.1 TSN / 60802 Interim, September 2021 Available at http://www.ieee802.org/1/files/public/docs2021
p. 62McCall, Stanton with Woods, Weingartner, and SchlechterIEEE 802.1 TSN Interim, Virtual, September 2021 Available at http://www.ieee802.org/1/files/public/docs2021
B
A
t3' โ t3
t4' โ t4
1s
localClkB125MHz + 10ppm
localClkB125MHz + 10ppm
localClkA125MHz โ 10ppm
localClkA125MHz โ 10ppm
874,998,750
1,875,001,250
1,000,000,000
2,000,000,000
t3
t4
t3'
t4'
=NRRAB @ t4' (t3' โ t3)
(t4' โ t4)=
124,998,750
125,001,250โ โ0.002% = โ20 ppm
pDelayResp pDelayRespโ
NRRAB
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McCall, Stanton with Woods, Weingartner, and SchlechterIEEE 802.1 TSN / 60802 Interim, September 2021 Available at http://www.ieee802.org/1/files/public/docs2021
p. 63McCall, Stanton with Woods, Weingartner, and SchlechterIEEE 802.1 TSN Interim, Virtual, September 2021 Available at http://www.ieee802.org/1/files/public/docs2021
B
A
1s
localClkB125MHz + 10.6ppm
localClkB125MHz + 10ppm
localClkA125MHz โ 10ppm
localClkA125MHz โ 10ppm
874,998,750
1,875,001,250
1,000,000,000
2,000,000,000
0.6
0
ppmDriftB
โ0.6ppm/s
=NRRAB @ t4' (t3' โ t3)
(t4' โ t4)=
124,998,750
125,001,250โ โ0.002% = โ20 ppm
pDelayResp pDelayRespโ
NRRAB
t3' โ t3
t4' โ t4
t3
t4
t3'
t4'
Firstโฆadd a -0.6ppm drift on Bโs local clock.
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McCall, Stanton with Woods, Weingartner, and SchlechterIEEE 802.1 TSN / 60802 Interim, September 2021 Available at http://www.ieee802.org/1/files/public/docs2021
p. 64McCall, Stanton with Woods, Weingartner, and SchlechterIEEE 802.1 TSN Interim, Virtual, September 2021 Available at http://www.ieee802.org/1/files/public/docs2021
B
A
1s
localClkB125MHz + 10.6ppm
localClkB125MHz + 10ppm
localClkA125MHz โ 10ppm
localClkA125MHz โ 10ppm
874,998,713
1,875,001,250
1,000,000,000
2,000,000,000
0.6
0
ppmDriftB
Effective Measurement Point
โ0.6ppm/s
=NRRAB @ t4' (t3' โ t3)
(t4' โ t4)=
124,998,750
125,001,288โ โ0.00203% = โ20.3 ppm
pDelayResp pDelayRespโ
NRRAB
t3' โ t3
t4' โ t4
t3
t4
t3'
t4'
This means that Bโs local clock was running faster at the left of the timeline than at the rightโฆspecifically 0.6ppm faster.
Which means that there were more than 125,001,250 clock ticks between t3 and t3โโฆwhich means the fraction is smallerโฆby 0.3ppm.
Only half of the ppm drift shows up in the NRR because you need to integrate under the curveโฆor rather line, because we can treat clock drift as linear. Another way of thinking about this is that weโre effectively measuring NRR half way between the current and the previous pDelayResp.
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McCall, Stanton with Woods, Weingartner, and SchlechterIEEE 802.1 TSN / 60802 Interim, September 2021 Available at http://www.ieee802.org/1/files/public/docs2021
p. 65McCall, Stanton with Woods, Weingartner, and SchlechterIEEE 802.1 TSN Interim, Virtual, September 2021 Available at http://www.ieee802.org/1/files/public/docs2021
B
A
1s
localClkB125MHz + 10.6ppm
localClkB125MHz + 10ppm
localClkA125MHz โ 10.6ppm
localClkA125MHz โ 10ppm
874,998,713
1,875,001,288
1,000,000,000
2,000,000,000
0.6
0
ppmDriftB
0
โ0.6
ppmDriftA
Effective Measurement Point
Effective Measurement Point
โ0.6ppm/s
+0.6ppm/s
=NRRAB @ t4' (t3' โ t3)
(t4' โ t4)=
124,998,712
125,001,288โ โ0.00206% = โ20.6 ppm
pDelayResp pDelayRespโ
NRRAB
t3' โ t3
t4' โ t4
t3
t4
t3'
t4'
Adding in a positive clock drift to Aโs local clock has a similar effect.
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p. 66McCall, Stanton with Woods, Weingartner, and SchlechterIEEE 802.1 TSN Interim, Virtual, September 2021 Available at http://www.ieee802.org/1/files/public/docs2021
B
A
1s
localClkBdriftRateB
localClkB
localClkAdriftRateA
localClkA
=
mNRRerror (ppm)
2
(driftRateB โ driftRateA)*pDelayInterval
=2
(driftRateB โ driftRateA)*(t2' โ t2)
(Difference between measured and actual Neighbor Rate Ratio @ t1)
(For simulation)
pDelayResp pDelayRespโ
NRRAB
t3' โ t3
t4' โ t4
t3
t4
t3'
t4'
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p. 67McCall, Stanton with Woods, Weingartner, and SchlechterIEEE 802.1 TSN Interim, Virtual, September 2021 Available at http://www.ieee802.org/1/files/public/docs2021
Neighbor Rate Ratio Error โ Observations
โข There is a difference between actual NRR and measured NRR (mNRR)โข When clock rates are stable mNRR is very accurate
โข Almost no difference between real NRR and mNRRโข No large impact from timestamp granularity or errorsโข No benefit from averaging mNNR (or taking a median of previous values) if (Timestamp Errors / pDelay Interval) is negligible in terms of ppm (it actually makes things worse)
โข When clock rates are drifting mNRR may have significant errorsโข Significant difference between real NRR and mNRRโข Minimised when clock rates of neighboring devices drift in the same directionโข Maximised when clock rates of neighboring devices drift in different directions
โข PPM vs. Temp curve for XO means the latter could be more common than expected
โข There is the possibility of compensating for Neighbor Rate Ratio Error
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Modelling Neighbor Rate Ratio โ 1
โข Use formula from earlier to model NRRerror at pDelayReqโฆ
โข Model driftRate as stable (0ppm) or varying ยฑ0.6ppm (linear uniform distribution)
โข X% stable; 100โX% varying
โข Add โError Correction Factorโ to account for modelling of drift (NRRdrift), correction of NRR (cNRR), and correction of other calculations via use of aNRR
โข One value to cover all errors related to NRRerror and drift.โข 25% Effec ve โ Removes 25% of the error.
2
(driftRateB โ driftRateA)*pDelayIntervalmNNRerror =
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Modelling Neighbor Rate Ratio โ 2
โข Actual error when NRR is used (NRRerror) will be larger than mNRRerror
if time has passed since measurement, allowing clocks to drift further apart. Relevant for Residence Time Error.
โข Model time Since mNRR as uniform distribution with min=0, max=pDelay.
โข Apply same NRRerror correction factor to second termโข 25% Effec ve โ Removes 25% of the error.
NNRerror = mNRRerror + (driftRateB โ driftRateA) * time Since mNRR
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Current Simulation
โข Mean pDelay Interval: 31.25ms
โข Would mean mNRRerror = ยฑ0.009375ppm with no average / median calculations, butโฆ
โข NRR is measured based on t3 & t4 from most recent pDelayResp messageโฆand t3 & t4 from the pDelayResp 7 pDelay Intervals prior. The median of the most recent measurement and the 6 prior measurements is then used as mNRR.
โข Using t3 & t4 from the pDelayResp 7 pDelay Intervals prior pushes back the effective measurement point to 7/2, i.e. 3.5 pDelay Intervals prior to mNRR (vs. 0.5 pDelay Intervals prior, i.e. 7x worse).
โข As rate of change of clock drift is (ppm/s2) is low, clock drift can be treated as linear, so the median will be the NRR from 3 pDelay intervals prior, i.e. the effective measurement point is an additional 3 pDelay Intervals prior.
โข Worst case mNRRerror due to drift is therefore 13x worse: ยฑ0.12ppm
โข pDelay Interval is short enough that Timestamp errors have an appreciable effect
โข ยฑ24ns over 31.25ms is ยฑ0.77ppm
โข Worst Case mNRRerror: ยฑ0.89ppm
โข Effect of Clock drift on calculations made between NRR measurements is low due to low pDelay Interval
โข ยฑ1.2ppm over 31.25ms is ยฑ0.0375ns
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Formulae โNRRerror
๐๐๐ ๐ ๐๐๐๐๐ ๐ ๐ ๐๐๐๐๐น๐๐๐๐ฌ๐๐๐๐๐๐๐๐๐๐๐๐๐๐
๐๐ซ๐๐๐๐๐ฐ๐๐๐๐๐๐๐2
๐๐๐๐๐๐ซ๐๐๐๐ ๐ 1 ๐๐๐๐๐๐ซ๐๐๐๐ ๐
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Link Delay Error โNeighbor Rate Ratio Error
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Time Sync โ Elements & Relationships
Dynamic Time Sync Error
Time Stamp Granularity
Residence Time Error
Dynamic Time Stamp Error
Neighbor Rate Ratio Error
Rate RatioError
Clock Drift
Link Delay ErrorLink Delay Error
Neighbor Rate Ratio Error
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A
B
t1
t2 t3
t4
(t4 โ t1) โ mNRR(t3 โ t2)
2
pDelayReq pDelayResp
t4 โ t1
t3 โ t2
XA
XB
pDelay =
localClkA localClkA
localClkB localClkB
mNRRerror(t3 โ t2)
2pDelayerrorNRR (ns) =
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WORST CASE:
= 3ns
Conclusion: Model pDelayErrorNNR
NRRerror(t3 โ t2)
2pDelayerrorNRR (ns) =
10ms * (0.6ppm)
2pDelayerrorNRR (ns) =
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Formulae โ pDelayerrorNNR
๐๐ท๐๐๐๐ฆ๐๐๐๐๐๐๐ ๐ ๐๐๐ ๐ ๐๐๐๐๐
๐๐ซ๐๐๐๐๐น๐๐๐๐๐๐๐2
๐๐๐๐๐ฟ๐๐๐๐ท๐๐๐๐ฆ๐๐๐๐๐ 1 ๐๐๐๐๐ณ๐๐๐๐ซ๐๐๐๐๐๐๐๐๐๐ช๐๐๐๐๐๐๐๐๐ ๐๐ท๐๐๐๐ฆ๐๐๐๐๐๐๐ ๐๐ท๐๐๐๐ฆ๐๐๐๐๐๐๐ ๐
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Rate Ratio Error
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Time Sync โ Elements & Relationships
Dynamic Time Sync Error
Time Stamp Granularity
Residence Time Error
Dynamic Time Stamp Error
Neighbor Rate Ratio Error
Rate RatioError
Clock Drift
Link Delay Error
Neighbor Rate Ratio Error
Rate RatioError
Clock Drift
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RateRation = RateRationโ1 * NRR
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1 2 3GM
Start with a โsimpleโ analysis.
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1 2 3
125MHz +5 ppmDrift = 0 ppm/sNRR32 = +15 ppmmNRR32 = +15 ppm NRRerror32 = โ0.3 ppm
125MHz โ10 ppmDrift = 0 ppm/sNRR21 = โ20 ppm
mNRR21 = โ20.3 ppmNRRerror21 = โ0.3 ppm
RR = 10 ppmmRR1 = +10.3 ppmRRerror1 = +0.3 ppm
RR = โ10 ppmmRR2 = โ10 ppmRRerror2 = 0 ppm
RR = +5 ppmmRR3 = +5 ppmRRerror3 = 0 ppm
GM
Drift = 0 ppm/s 125MHz +10 ppmDrift = โ0.6 ppm/sNRR1GM = +10 ppm
mNRR1GM = +10.3 ppmNRRerror1GM = +0.3 ppm
RRerrorRange ppm
+0.3
0
Simple in thatโฆif you feed NRR directly into RR, and donโt account for time between measuring mNRR and using mNRR, i.e. applying it to Rate Ratio.
For pDelay Interval = 1s
โฆwhich is the 802.1AS-2020 default value, and the value used in this presentation when looking at NRR error.
See earlier in the presentation for full analysis.
The RRerror at 1 is fully removed at 2. This makes sense as RR is trying to calculate the ratio of Local Clock to GM clock. If everything is working correctly, the error will depend on the drift between the twoโฆnot the drift of clocks in the transmission path.
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1 2 3GM RRerrorRange ppm
+0.3
โ0.3125MHz +5 ppmDrift = 0 ppm/sNRR32 = +15 ppm
mNRR32 = +15.6 ppm NRRerror32 = +0.3 ppm
125MHz โ10 ppmDrift = +0.6 ppm/sNRR21 = โ20 ppm
mNRR21 = โ20.6 ppmNRRerror21 = โ0.6 ppm
RR = 10 ppmmRR1 = +10.3 ppmRRerror1 = +0.3 ppm
RR = โ10 ppmmRR2 = โ10.3 ppmRRerror2 = โ0.3 ppm
RR = +5 ppmmRR3 = +5 ppmRRerror3 = 0 ppm
Drift = 0 ppm/s 125MHz +10 ppmDrift = โ0.6 ppm/sNRR1GM = +10 ppm
mNRR1GM = +10.3 ppmNRRerror1GM = +0.3 ppm
This continues to line up as we add Clock Drift to 2โฆ
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p. 83McCall, Stanton with Woods, Weingartner, and SchlechterIEEE 802.1 TSN Interim, Virtual, September 2021 Available at http://www.ieee802.org/1/files/public/docs2021
1 2 3GM RRerrorRange ppm
+0.3
โ0.3125MHz +5 ppmDrift = +0.6 ppm/sNRR32 = +15 ppmmNRR32 = +15 ppm NRRerror32 = +0 ppm
125MHz โ10 ppmDrift = +0.6 ppm/sNRR21 = โ20 ppm
mNRR21 = โ20.6 ppmNRRerror21 = โ0.6 ppm
RR = 10 ppmmRR1 = +10.3 ppmRRerror1 = +0.3 ppm
RR = โ10 ppmmRR2 = โ10.3 ppmRRerror2 = โ0.3 ppm
RR = +5 ppmmRR3 = +5 ppm
RRerror3 = โ0.3 ppm
Drift = 0 ppm/s 125MHz +10 ppmDrift = โ0.6 ppm/sNRR1GM = +10 ppm
mNRR1GM = +10.3 ppmNRRerror1GM = +0.3 ppm
โฆand 3.
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1 2 3GM RRerrorRange ppm
+0.6
0
125MHz +5 ppmDrift = +0.6 ppm/sNRR32 = +15 ppmmNRR32 = +15 ppm NRRerror32 = 0 ppm
125MHz โ10 ppmDrift = +0.6 ppm/sNRR21 = โ20 ppm
mNRR21 = โ20.6 ppmNRRerror21 = โ0.6 ppm
RR = 10 ppmmRR1 = +10.6 ppmRRerror1 = +0.6 ppm
RR = โ10 ppmmRR2 = โ10.3 ppmRRerror2 = 0 ppm
RR = +5 ppmmRR3 = +5 ppmRRerror3 = 0 ppm
Drift = +0.6 ppm/s 125MHz +10 ppmDrift = โ0.6 ppm/sNRR1GM = +10 ppm
mNRR1GM = +10.6 ppmNRRerror1GM = +0.6 ppm
Adding Clock Drift to the GM is very different because it affects the Rate Ratio Error at every single downstream device.
Worse, it pushes them all in one direction, which could have further implicationsโฆwhen you start adding in the next layer of Rate Ratio Errorโฆ
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However, there is an additional source of error.
(driftRateGM โ driftRatelocal) * pDelayIntervalRRerror (ppm) =
2
Note: Effect of GM clock drift skews RRerror for all downstream instances in the same direction.This will have implications for how errors accumulate.
Therefore, model GM clock separately, i.e. separate ppm/s range vs. other clocks in the sysem.(A more accurate GM clock could address up to 50% of basic Rate Ratio related errors in the system.
Note that the latest time series simulations do not include any GM clock error.)
RateRatio is used during Time Sync processing.
RateRatio is effectively measured at the same time as NRR. There is a time delay between measurement and useโฆwhich is different at each node.
(NRR is measured as part of pDelay exchange.)
?
Which looks a lot like the NRRerror equation, but with driftRateGM instead of driftRatenโ1
The โsimpleโ analysis suggest that the equation at the top of the screen is sufficient. But the time delay means we need to dig deeperโฆ
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Time Sync โ Elements & Relationships
GM TIMEEnd Station
Time
pDelay Residence Time
INCOMING OUTGOING
CorrectionAdjustment
Neighbor Rate Ratio
Correction
Rate Ratio
Correction
Rate Ratio
Rate Ratio
pDelay
CorrectionAdjustment
Correction
Rate Ratio
Neighbor Rate Ratio
Correction
Rate Ratio
Rate Ratio
CorrectionAdjustment
Link Delay
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1 2 3GM
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1
2
3
GM
pDelayResp Sync
pDelayResp Sync
SyncpDelayResp
pDelayRsp Sync
t3โpDelayGM1
t4โpDelayGM1
t1โsyncGM1
t2โsyncGM1
t1โsync12
t2โsync12
t1โsync23
t2โsync23
t3โpDelay12
t4โpDelay12
t3โpDelay23
t4โpDelay23
t1โsync23
A more complex analysis includes different periods between pDelay and Sync messages. The minimum is close to zero (at scales of tens of hundreds of milliseconds) while the maximum is the pDelay Interval.
Note that in this graphic Iโve used an unrealistically long Residence Time to make it visible at this scale. The error analysis is unaffected.
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p. 89McCall, Stanton with Woods, Weingartner, and SchlechterIEEE 802.1 TSN Interim, Virtual, September 2021 Available at http://www.ieee802.org/1/files/public/docs2021
Sync
t3โpDelayGM1
t4โpDelayGM1
mNRRGM1
Sync
1
2
3
GM
Sync
t1โsyncGM1
t2โsyncGM1
t1โsync12
t2โsync12
t1โsync23
t2โsync23
t3โpDelay12
t4โpDelay12mNRR12
t3โpDelay23
t4โpDelay23mNRR23
Sync
t1โsync23
mNRRGM1 addedto RR & used for Correction Field Adjustment
mNRR12 addedto RR & used for Correction Field Adjustment
mNRR23 addedto RR & used for Correction Field Adjustment
pDelayResp
pDelayResp
pDelayResp
pDelayRsp
NRR is measured at last pDelayResp message (mNRR)โฆbut added to Rate Ratio at end of Residence Time (Sync message TX). This means that the amount of adjustment to Rate Ratio (i.e. NRR) is effectively measured at mNRR.
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p. 90McCall, Stanton with Woods, Weingartner, and SchlechterIEEE 802.1 TSN Interim, Virtual, September 2021 Available at http://www.ieee802.org/1/files/public/docs2021
t3โpDelayGM1
t4โpDelayGM1
mNRR1GM = +10.6 ppmmRR1 = +10.6 ppm
1
2
3
0.5s
GMt1โsyncGM1
t2โsyncGM1
t1โsync12
t2โsync12
t1โsync23
t2โsync23
t3โpDelay12
t4โpDelay12
t3โpDelay23
t4โpDelay23
t1โsync23
125MHz 0 ppmDrift = 0 ppm/s
125MHz +10 ppmDrift = โ0.6 ppm/s
125MHz โ10 ppmDrift = 0 ppm/s
125MHz +5 ppmDrift = 0 ppm/s
RR1
+10 ppm
RRerror1 = +0.6 ppm
RRerrorRange ppm
+0.6
0
ppmDrift1GMEffective Measurement
Point
pDelayResp
pDelayResp
pDelayResp
pDelayRsp
Sync
Sync
Sync
Sync
Looking at the first stepโฆ
mNRR is effectively measured pDelay Interval / 2 prior to the incoming pDelaymessage. When pDelay Interval is 1 second, thatโs 0.5s prior to the mNRRmeasurement, i.e. it has an error of +0.3ppm.
But that is 0.5s prior to when the measurement is actually usedโฆby which time there is an additional 0.5s of drift leading to an additional +0.3ppm of error, for a total of +0.6ppm.
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p. 91McCall, Stanton with Woods, Weingartner, and SchlechterIEEE 802.1 TSN Interim, Virtual, September 2021 Available at http://www.ieee802.org/1/files/public/docs2021
0
timeSync
t3โpDelayGM1
t4โpDelayGM1
mNRR1GM = +10.6 ppmmRR1 = +10.6 ppm
timeSync
1
2
3
0.5s
GM
timeSync
t1โsyncGM1
t2โsyncGM1
t1โsync12
t2โsync12
t1โsync23
t2โsync23
t3โpDelay12
t4โpDelay12mNRR21 = โ20.9 ppmmRR2 = โ10.3 ppm
t3โpDelay23
timeSync
t1โsync34
1s
125MHz 0 ppmDrift = 0 ppm/s
125MHz +10 ppmDrift = โ0.6 ppm/s
125MHz โ10 ppmDrift = 0 ppm/s
125MHz +5 ppmDrift = 0 ppm/s
RR1
+10 ppm
RR2
โ10 ppm
RRerror1 = +0.6 ppm
RRerror2 = โ0.3 ppm
RRerrorRange ppm
+0.6
ppmDrift21 Effective Measurement Point
โ0.3
t4โpDelay23
pDelayResp
pDelayResp
pDelayResp
pDelayRsp
In the second step, the delay between measuring NRR and using the measurement is at itโs maximum, i.e. a pDelay Interval of 1 secondโฆwhich means the effective measurement point is 1.5 seconds prior to when the value is used.
The NRR error at the point when itโs added to the Rate Ratio is therefore off by -0.9ppmโฆwhich doesnโt just cancel out the +0.6ppm error from the previous step, it overshoots by 0.3ppm.
Note that the calculations shown above do not explicitly include the effect of clock drift during Residence Time. However, this does not affect the error analysis.
โข Clock1 will drift slightly (slower, since its drift rate is -0.6ppm) between t2-sync12
(where itโs marked as +10ppm) and t1-sync23 where itโs implicitly used in the calculation of RR2 butโฆ
โข Clock drift over a typical Residence Time (default 802.1AS maximum is 10ms) is not significant, e.g. ยฑ1.2ppm over 10ms is 0.012ppm. And even this can be ignored becauseโฆ
โข NRRerror and RRerror are driven by a clockโs drift rate, not itโs absolute value, and the rate of change of clock drift (ppm/s2) is low enough that, over the time periods of interest, clock drift can be considered linear. (See meanLinkDelayerror
for details.)
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p. 92McCall, Stanton with Woods, Weingartner, and SchlechterIEEE 802.1 TSN Interim, Virtual, September 2021 Available at http://www.ieee802.org/1/files/public/docs2021
timeSync
t3โpDelayGM1
t4โpDelayGM1
mNRR1GM = +10.6 ppmmRR1 = +10.6 ppm
timeSync
1
2
3
0.5s
GM
timeSync
t1โsyncGM1
t2โsyncGM1
t1โsync12
t2โsync12
t1โsync23
t2โsync23
t3โpDelay12
t4โpDelay12mNRR21 = โ20.9 ppmmRR2 = โ10.3 ppm
t3โpDelay23
t4โpDelay23mNRR32 = +15 ppmmRR3 = +4.7 ppmtimeSync
t1โsync34
1s
0.25s
125MHz 0 ppmDrift = 0 ppm/s
125MHz +10 ppmDrift = โ0.6 ppm/s
125MHz โ10 ppmDrift = 0 ppm/s
125MHz +5 ppmDrift = 0 ppm/s
RR1
+10 ppm
RR2
โ10 ppm
RR3
+5 ppm
RRerror1 = +0.6 ppm
RRerror3 = โ0.3 ppm
RRerror2 = โ0.3 ppm
RRerrorRange ppm
+0.6
โ0.3
pDelayResp
pDelayResp
pDelayResp
pDelayRsp
There is no relative drift between 2 and 3, so there is no error in the NRR measurementโฆwhich means there is still nothing to cancel out the 0.3ppm error in Rate Ratio.
The basic mNRR errors from the previous section (the simple analysis) cancel out, because pDelay interval is constant (or close enough; if you want to be really accurate, youโd need to model variation in pDelay interval, but for the purpose of this error model, weโll assume there is no variation).
But the errors due to drift can remain in the system. This is the same inputs as for the simple analysisโฆbut with added delay. In the simple analysis the RRerrorRange
was ยฑ0.3ppm. In this case, itโs already higher. And it can get worse.
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p. 93McCall, Stanton with Woods, Weingartner, and SchlechterIEEE 802.1 TSN Interim, Virtual, September 2021 Available at http://www.ieee802.org/1/files/public/docs2021
timeSync
t3โpDelayGM1
t4โpDelayGM1
mNRR1GM = +10.6 ppmmRR1 = +10.6 ppm
timeSync
1
2
3
0.5s
GM
timeSync
t1โsyncGM1
t2โsyncGM1
t1โsync12
t2โsync12
t1โsync23
t2โsync23
t3โpDelay12
t4โpDelay12mNRR21 = โ21.8 ppmmRR2 = โ11.2 ppm
t3โpDelay23
t4โpDelay23mNRR32 = +15.45 ppmmRR3 = +0.45 ppmtimeSync
t1โsync23
1s
0.25s
125MHz 0 ppmDrift = 0 ppm/s
125MHz +10 ppmDrift = โ0.6 ppm/s
125MHz โ10 ppmDrift = +0.6 ppm/s
125MHz +5 ppmDrift = 0 ppm/s
RR1
+10 ppm
RR2
โ10 ppm
RR3
+5 ppm
RRerror1 = +0.6 ppm
RRerror3 = โ0.75 ppm
RRerror2 = โ1.2 ppm
RRerrorRange ppm
+0.6
โ1.2
pDelayResp
pDelayResp
pDelayResp
pDelayRsp
Adding a +0.6ppm/s drift to 2โs clock means the mNRR is off by -1.2pmm (the difference between 2 and 1โs clocks) multiplied by 1.5s, which is the time from the effective measurement point (pDelay Interval / 2, i.e. 0.5s) plus the delay between mNRR and when itโs used (a full pDelay Interval in this case, i.e. 1s).
-1.2ppm x 1.5s = -1.8ppm, so mNRR21 isnโt -20ppm, or -20.6ppm, itโs -21.8ppm.
On the previous slide mRR2 corrected mRR1 (of +0.3ppm) and overshot by 0.3ppm to end up at -0.3ppm. In this case, with 2โs clock also driftingโฆin the opposite direction to 1โฆand such a long delay between mNRR21 and the Sync messageโฆthe overshoot is now an additional 0.9ppm to end up at -1.2ppm.
Some of that is offset over the next hop by the difference in drift rate between 3 and 2. In the simple analysis of this scenario, RRerror2 was -0.3ppm and all of the error (i.e. 0.3ppm) was offset when moving on to RRerror3. In this scenario 0.45ppm is offsetโฆwhich makes sense since the effective measurement point is 0.75s prior to when the measurement is used. But that still leaves -0.75ppm remaining, which will be passed down the chain of devices and, if every subsequent device has zero drift, will remain at the End Station.
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p. 94McCall, Stanton with Woods, Weingartner, and SchlechterIEEE 802.1 TSN Interim, Virtual, September 2021 Available at http://www.ieee802.org/1/files/public/docs2021
timeSync
t3โpDelayGM1
t4โpDelayGM1
mNRR1GM = +10.6 ppmmRR1 = +10.6 ppm
timeSync
1
2
3
0.5s
GM
timeSync
t1โsyncGM1
t2โsyncGM1
t1โsync12
t2โsync12
t1โsync23
t2โsync23
t3โpDelay12
t4โpDelay12mNRR21 = โ21.8 ppmmRR2 = โ11.2 ppm
t3โpDelay23
t4โpDelay23mNRR32 = +15 ppmmRR3 = +3.8 ppmtimeSync
t1โsync34
1s
0.25s
125MHz 0 ppmDrift = 0 ppm/s
125MHz +10 ppmDrift = โ0.6 ppm/s
125MHz โ10 ppmDrift = +0.6 ppm/s
125MHz +5 ppmDrift = +0.6 ppm/s
RR1
+10 ppm
RR2
โ10 ppm
RR3
+5 ppm
RRerror1 = +0.6 ppm
RRerror3 = โ1.2 ppm
RRerror2 = โ1.2 ppm
RRerrorRange ppm
โ1.2
+0.6
pDelayResp
pDelayResp
pDelayResp
pDelayRsp
Adding the +0.6pp drift to device 3 (as was done in the simple analysis) has the expected effect. There is no relative drift between 3 and 2, so NRRerror is zero and RRerror3 remains the same as RRerror2.
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p. 95McCall, Stanton with Woods, Weingartner, and SchlechterIEEE 802.1 TSN Interim, Virtual, September 2021 Available at http://www.ieee802.org/1/files/public/docs2021
timeSync
t3โpDelayGM1
t4โpDelayGM1
mNRR1GM = +11.2 ppmmRR1 = +11.2 ppm
timeSync
1
2
3
0.5s
GM
timeSync
t1โsyncGM1
t2โsyncGM1
t1โsync12
t2โsync12
t1โsync23
t2โsync23
t3โpDelay12
t4โpDelay12mNRR21 = โ21.8 ppmmRR2 = โ10.6 ppm
t3โpDelay23
t4โpDelay23mNRR32 = +15 ppmmRR3 = +4.4 ppmtimeSync
t1โsync34
1s
0.25s
125MHz 0 ppmDrift = +0.6 ppm/s
125MHz +10 ppmDrift = โ0.6 ppm/s
125MHz โ10 ppmDrift = +0.6 ppm/s
125MHz +5 ppmDrift = +0.6 ppm/s
RR1
+10 ppm
RR2
โ10 ppm
RR3
+5 ppm
RRerror1 = +1.2 ppm
RRerror3 = โ0.6 ppm
RRerror2 = โ0.6 ppm
RRerrorRange ppm
โ0.6
+1.2
pDelayResp
pDelayResp
pDelayResp
pDelayRsp
Note that a GM Clock Drift has a similar effect as in the simple analysis: it pushes every subsequent Rate Ratio measurement off in one direction, but in this case the effect is amplified by the initial delay between mNRR1GM.
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p. 96McCall, Stanton with Woods, Weingartner, and SchlechterIEEE 802.1 TSN Interim, Virtual, September 2021 Available at http://www.ieee802.org/1/files/public/docs2021
Observations
โข Effect of RRerror can be significant
โข See section on effect of RRerror Residence Time Error for details
โข RRerror due to mNRRerror cancels out at the next node
โข RRerror due to additional Clock Drift between when NRR is measured (mNRR) and then used as part of Time Sync does not necessarily cancel out.
โข Can accumulate to some degree. Should average to zero, unlessโฆ
โข GM clock drift affects every device in the chain, and in the same direction, i.e. affects the average
โข Less time between pDelay message (used to measure NRR) and TimeSync is better
โข Lower pDelay Interval is one method
โข If pDelay Interval is < Sync Interval many pDelay messages are โwastedโ
โข Setting pDelay to a multiple of Sync Interval and synchronising to send pDelay just before Sync would be more efficient
โข Anything that can be done to compensate for NRRerror will flow through to RRerror
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Modelling Rate Ratio Error
โข Model two aspectsโฆโข Error from mNNRerror
โข Errors out along the chain, nodeโtoโnodeโtoโnode.
โข RRerrorNRR never deviates by more than mNRRerrorRangeโข See earlier for equation
โข Error from drift between time of mNNR measurement and Time Syncโข Should average out eventuallyโฆbut with potentially large variances as the errors wonโt cancel out nodeโtoโnodeโtoโnode.
โข RRerrorDrift may vary by a lot more than mNRRerrorRangeโข Need more work to model and quantify this.
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Formulae โRRerror
๐ ๐ ๐๐๐๐๐ ๐ ๐ ๐๐๐๐๐๐๐ ๐ ๐ ๐ ๐๐๐๐๐๐ท๐๐๐๐ก
๐๐๐๐๐ฆ๐๐๐ ๐ _๐๐ฆ๐๐~๐ ๐๐๐ 0, ๐๐๐ฅ ๐๐ซ๐๐๐๐๐ฐ๐๐๐๐๐๐๐ 1 ๐๐ซ๐๐๐๐๐น๐๐๐๐บ๐๐๐๐๐๐๐๐๐๐๐๐๐
๐ ๐ ๐๐๐๐๐๐๐ ๐ ๐ ๐๐๐๐๐น๐๐๐๐ฌ๐๐๐๐๐๐๐๐๐๐๐๐๐๐
๐๐ซ๐๐๐๐๐ฐ๐๐๐๐๐๐๐2
๐๐๐๐๐๐ซ๐๐๐๐๐ฎ๐ด ๐๐๐๐๐๐ซ๐๐๐๐
๐ ๐ ๐๐๐๐๐๐ท๐๐๐๐ก ๐ ๐ ๐ ๐๐๐๐๐๐ท๐๐๐๐ก ๐ 1 ๐ ๐๐๐๐๐น๐๐๐๐ฌ๐๐๐๐๐๐๐๐๐๐๐๐๐๐ ๐๐๐๐๐ฆ๐๐๐ ๐ _๐๐ฆ๐๐ ๐๐๐๐๐๐ซ๐๐๐๐๐ฎ๐ด ๐๐๐๐๐๐ซ๐๐๐๐ ๐
๐๐๐๐๐ฆ๐๐๐ ๐ _๐๐ฆ๐๐ is modelled as a uniform continuous distribution withminimum = 0 and a maximum = ๐๐ซ๐๐๐๐๐ฐ๐๐๐๐๐๐๐ 1 ๐๐ซ๐๐๐๐๐น๐๐๐๐บ๐๐๐๐๐๐๐๐๐๐๐๐๐ .
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pDelay Error โ Rate Ratio Error
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Time Sync โ Elements & Relationships
Dynamic Time Sync Error
Time Stamp Granularity
Link Delay Error Residence Time Error
Dynamic Time Stamp Error
Neighbor Rate Ratio Error
Rate RatioError
Clock Drift
Link Delay Error
Rate RatioError
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pDelayErrorRR (ns) = pDelay * RRError
pDelayErrorRR (ns) = 150ns * (ยฑ10 ppm)
= 0.0015ns (i.e. 1.5ps)
Conclusion: Do Not Model pDelayErrorRR
WORST CASE: RRError (ppm) = ยฑ10
150ns pDelay โ 45m
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Residence Time Error โTimestamp Errors& Rate Ratio Error
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Time Sync โ Elements & Relationships
Dynamic Time Sync Error
Time Stamp Granularity
Link Delay Error Residence Time Error
Dynamic Time Stamp Error
Neighbor Rate Ratio Error
Rate RatioError
Clock Drift
Time Stamp Granularity
Residence Time Error
Dynamic Time Stamp Error
Rate RatioError
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t1out
t2inA
Sync
t1out โ t2in
XB
Sync
localClkA localClkA
residenceTime = RateRatio (t1out โ t2in)
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p. 105McCall, Stanton with Woods, Weingartner, and SchlechterIEEE 802.1 TSN Interim, Virtual, September 2021 Available at http://www.ieee802.org/1/files/public/docs2021
B
CU
t1out ยฑ6ns
t2in ยฑ6nsA
Sync
t1out โ t2in
XB
Sync
localClkA localClkA
residenceTime = RateRatio (t1out โ t2in)
residenceTimeerror = t2inerror + t1outerror + RRerror (t1out โ t2in)
localClkA localClkA
GM
Sync
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p. 106McCall, Stanton with Woods, Weingartner, and SchlechterIEEE 802.1 TSN Interim, Virtual, September 2021 Available at http://www.ieee802.org/1/files/public/docs2021
residenceTimeErrorRR (ns) = (t1out โ t2in) * RRError
WORST CASE:
Conclusion: Model residenceTimeErrorRR
Conclusion: Model residenceTimeerrort2in and residenceTimeerrort2out
residenceTimeErrorRR (ns) = (t1out โ t2in) * RRError
= (10ms) * 10ppm
= 100ns
RRError (ppm) = ยฑ 10
150ns pDelay โ 45m
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p. 107McCall, Stanton with Woods, Weingartner, and SchlechterIEEE 802.1 TSN Interim, Virtual, September 2021 Available at http://www.ieee802.org/1/files/public/docs2021
Formulae โ residenceTimeerror
๐๐๐ ๐๐๐๐๐๐๐๐๐๐๐๐๐๐๐ ๐๐๐ ๐๐๐๐๐๐๐๐๐๐๐๐๐๐๐๐๐ ๐๐๐ ๐๐๐๐๐๐๐๐๐๐๐๐๐๐๐๐ ๐
๐๐๐ ๐๐๐๐๐๐๐๐๐๐๐๐๐๐๐๐๐ ๐ก1๐๐๐๐๐๐ ๐ก2๐๐๐๐๐๐
๐ก2๐๐๐๐๐๐ ๐ป๐บ๐ฎ๐ฌ๐น๐ฟ ๐ซ๐ป๐บ๐ฌ๐น๐ฟ ๐ก1๐๐๐๐๐๐ ๐ป๐บ๐ฎ๐ฌ๐ป๐ฟ ๐ซ๐ป๐บ๐ฌ๐ป๐ฟ
๐๐๐ ๐๐๐๐๐๐๐๐๐๐๐๐๐๐๐๐ ๐ ๐ ๐ ๐๐๐๐๐ ๐๐๐๐๐ ๐๐๐๐๐ป๐๐๐
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p. 108McCall, Stanton with Woods, Weingartner, and SchlechterIEEE 802.1 TSN Interim, Virtual, September 2021 Available at http://www.ieee802.org/1/files/public/docs2021
Time Sync โ Errors to Model
Dynamic Time Sync Error
Time Stamp Granularity
pDelay Residence Time
Dynamic Time Stamp Error
Neighbor Rate Ratio Rate Ratio
Clock Drift(Separate GM Model)
Link Delay Error Residence Time Error
Neighbor Rate Ratio Error
Rate RatioError
See Backup for Summary of Probabilities & Formulae
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p. 109McCall, Stanton with Woods, Weingartner, and SchlechterIEEE 802.1 TSN Interim, Virtual, September 2021 Available at http://www.ieee802.org/1/files/public/docs2021
Summary of Main Insights
โข meanLinkDelay should be an average of pDelay measurements.โข Should nearly eliminate meanLinkDelayerror
โข NRR should not be averaged.โข Provided (Timestamp Errors / pDelay Interval) is negligible in terms of ppm
โข There are significant benefits to the GM having greater ppm stability (lower ppm/s).
โข It is hard (impossible?) to eliminate Timestamp granularity and Dynamic Timestamp error effects from Residence Time
โข Need more work to see how far these elements can realistically be reduced
โข Synchronising pDelayReq to arrive just before Sync would minimise the effect of Clock Drift.
โข There may be opportunities to compensate for NRRerrorโข And therefore RRerror
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McCall, Stanton with Woods, Weingartner, and SchlechterIEEE 802.1 TSN / 60802 Interim, September 2021 Available at http://www.ieee802.org/1/files/public/docs2021
McCall, Stanton with Woods, Weingartner, and SchlechterIEEE 802.1 TSN / 60802 Interim, September 2021 Available at http://www.ieee802.org/1/files/public/docs2021
Recommended Next Steps
โข Complete the Model
โข Validate the Model using existing simulation(s) from Geoff
โข Report back with a specific proposal containing 802.1AS parameters (e.g. rates etc.) and other recommendations that would meet the time accuracy goals for JP 60802 review
p. 110
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p. 111McCall, Stanton with Woods, Weingartner, and SchlechterIEEE 802.1 TSN Interim, Virtual, September 2021 Available at http://www.ieee802.org/1/files/public/docs2021
Backup
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p. 112McCall, Stanton with Woods, Weingartner, and SchlechterIEEE 802.1 TSN Interim, Virtual, September 2021 Available at http://www.ieee802.org/1/files/public/docs2021
Input Errors
Error DistributionDefault Min
Default Max Equation Unit
clockDriftGMClockDriftGMmin
& ClockDriftGMmax
X% Stable (zero)(100โX)% Uniform
โ0.6 +0.6 ๐๐๐๐๐๐ท๐๐๐๐ก๐บ๐~๐ ๐๐๐๐๐๐ท๐๐๐๐ก๐บ๐๐๐๐, ๐๐๐๐๐๐ท๐๐๐๐ก๐บ๐๐๐๐ฅ ppm/s
clockDriftX% Stable (zero)(100โX)% Uniform
โ0.6 +0.6 ๐๐๐๐๐๐ท๐๐๐๐ก~๐ ๐๐๐๐๐๐ท๐๐๐๐ก, ๐๐๐๐๐๐ท๐๐๐๐ก ppm/s
TSGETX Uniform โ4 +4 ๐๐๐บ๐ธ๐๐~๐ ๐๐๐บ๐ธ๐๐, ๐๐๐บ๐ธ๐๐ ns
TSGERX Uniform โ4 +4 ๐๐๐บ๐ธ๐ ๐~๐ ๐๐๐บ๐ธ๐ ๐, ๐๐๐บ๐ธ๐ ๐ ns
DTSETX Uniform โ2 +2 ๐ท๐๐๐ธ๐๐~๐ ๐ท๐๐๐ธ๐๐, ๐ท๐๐๐ธ๐๐ ns
DTSERX Uniform โ1 +1 ๐ท๐๐๐ธ๐ ๐~๐ ๐ท๐๐๐ธ๐ ๐, ๐ท๐๐๐ธ๐ ๐ ns
Formulae below take into account the unit of the input value.Formulae in the main presentation do not.
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p. 113McCall, Stanton with Woods, Weingartner, and SchlechterIEEE 802.1 TSN Interim, Virtual, September 2021 Available at http://www.ieee802.org/1/files/public/docs2021
Other Variable Inputs
Error Default Value Unit
pDelay 150 ns
pDelayInterval 1,000 ms
pDelayResponse 10 ms
residenceTime 10 ms
Formulae below take into account the unit of the input value.Formulae in the main presentation do not.
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p. 114McCall, Stanton with Woods, Weingartner, and SchlechterIEEE 802.1 TSN Interim, Virtual, September 2021 Available at http://www.ieee802.org/1/files/public/docs2021
Correction Factor Inputs
Error Default Value Unit
meanLinkDelayerrorTScorrection 0 %
driftRateErrorcorrection 0 %
pDelayRespSynccorrection 0 %
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p. 115McCall, Stanton with Woods, Weingartner, and SchlechterIEEE 802.1 TSN Interim, Virtual, September 2021 Available at http://www.ieee802.org/1/files/public/docs2021
Formulae โ Top Level
๐ท๐๐ธ ๐ฅ ๐๐๐๐๐ฟ๐๐๐๐ท๐๐๐๐ฆ๐๐๐๐๐ ๐ ๐๐๐ ๐๐๐๐๐๐๐๐๐๐๐๐๐๐๐ ๐
๐๐๐ ๐๐๐๐๐๐๐๐๐๐๐๐๐๐๐ ๐๐๐ ๐๐๐๐๐๐๐๐๐๐๐๐๐๐๐๐๐ ๐๐๐ ๐๐๐๐๐๐๐๐๐๐๐๐๐๐๐๐ ๐
The initial model will not include factors shown in grey.x is number of hops.
ns
ns
ns
๐๐๐๐๐ฟ๐๐๐๐ท๐๐๐๐ฆ๐๐๐๐๐ 1 ๐๐๐๐๐ณ๐๐๐๐ซ๐๐๐๐๐๐๐๐๐๐ช๐๐๐๐๐๐๐๐๐ ๐๐ท๐๐๐๐ฆ๐๐๐๐๐๐๐ ๐๐ท๐๐๐๐ฆ๐๐๐๐๐๐๐ ๐ ๐๐ท๐๐๐๐ฆ๐๐๐๐๐๐ ๐
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p. 116McCall, Stanton with Woods, Weingartner, and SchlechterIEEE 802.1 TSN Interim, Virtual, September 2021 Available at http://www.ieee802.org/1/files/public/docs2021
Formulae โ meanLinkDelayerror
๐๐ท๐๐๐๐ฆ๐๐๐๐๐๐๐
๐ก4๐๐ท๐๐๐๐๐ ๐ก1๐๐ท๐๐๐๐๐ ๐ก3๐๐ท๐๐๐๐๐ ๐ก2๐๐ท๐๐๐๐๐
2
๐ก1๐๐ท๐๐๐๐๐ ๐ป๐บ๐ฎ๐ฌ๐ป๐ฟ ๐ซ๐ป๐บ๐ฌ๐ป๐ฟ ๐ก2๐๐ท๐๐๐๐๐ ๐ป๐บ๐ฎ๐ฌ๐น๐ฟ ๐ซ๐ป๐บ๐ฌ๐น๐ฟ
๐ก3๐๐ท๐๐๐๐๐ ๐ป๐บ๐ฎ๐ฌ๐ป๐ฟ ๐ซ๐ป๐บ๐ฌ๐ป๐ฟ ๐ก4๐๐ท๐๐๐๐๐ ๐ป๐บ๐ฎ๐ฌ๐น๐ฟ ๐ซ๐ป๐บ๐ฌ๐น๐ฟ
๐๐ท๐๐๐๐ฆ๐๐๐๐๐๐๐ ๐
๐๐๐ ๐ ๐๐๐๐๐
10๐๐ซ๐๐๐๐๐น๐๐๐๐๐๐๐ ๐๐๐
2๐๐ท๐๐๐๐ฆ๐๐๐๐๐๐๐
ns
ns
ns
ns
ns
๐๐ท๐๐๐๐ฆ๐๐๐๐๐๐ ๐
๐ ๐ ๐๐๐๐๐
10๐๐ท๐๐๐๐ฆ 1 ๐๐ซ๐๐๐๐๐๐๐๐๐๐ช๐๐๐๐๐๐๐๐๐ ๐๐ท๐๐๐๐ฆ๐๐๐๐๐๐๐ ๐๐ท๐๐๐๐ฆ๐๐๐๐๐๐๐ ๐ ns
๐๐๐ ๐ ๐๐๐๐๐
๐๐ซ๐๐๐๐๐น๐๐๐๐๐๐๐2
๐๐ท๐๐๐๐ฆ๐๐๐๐๐๐๐
10 ns
๐๐๐๐๐ฟ๐๐๐๐ท๐๐๐๐ฆ๐๐๐๐๐ 1 ๐๐๐๐๐ณ๐๐๐๐ซ๐๐๐๐๐๐๐๐๐๐ช๐๐๐๐๐๐๐๐๐ ๐๐ท๐๐๐๐ฆ๐๐๐๐๐๐๐ ๐๐ท๐๐๐๐ฆ๐๐๐๐๐๐๐ ๐ ๐๐ท๐๐๐๐ฆ๐๐๐๐๐๐ ๐
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p. 117McCall, Stanton with Woods, Weingartner, and SchlechterIEEE 802.1 TSN Interim, Virtual, September 2021 Available at http://www.ieee802.org/1/files/public/docs2021
Formulae โ residenceTimeerror
ns๐๐๐ ๐๐๐๐๐๐๐๐๐๐๐๐๐๐๐ ๐๐๐ ๐๐๐๐๐๐๐๐๐๐๐๐๐๐๐๐๐ ๐๐๐ ๐๐๐๐๐๐๐๐๐๐๐๐๐๐๐๐ ๐
๐๐๐ ๐๐๐๐๐๐๐๐๐๐๐๐๐๐๐๐๐ ๐ก1๐๐๐๐๐๐ ๐ก2๐๐๐๐๐๐
๐ก2๐๐๐๐๐๐ ๐ป๐บ๐ฎ๐ฌ๐น๐ฟ ๐ซ๐ป๐บ๐ฌ๐น๐ฟ ๐ก1๐๐๐๐๐๐ ๐ป๐บ๐ฎ๐ฌ๐ป๐ฟ ๐ซ๐ป๐บ๐ฌ๐ป๐ฟ
๐๐๐ ๐๐๐๐๐๐๐๐๐๐๐๐๐๐๐๐ ๐
๐ ๐ ๐๐๐๐๐
10๐๐๐๐๐ ๐๐๐๐๐ป๐๐๐ 10 ๐๐๐ ๐๐๐๐๐๐๐๐๐๐๐๐๐๐๐๐๐
ns
ns
๐ ๐ ๐๐๐๐๐ ๐๐๐๐๐ ๐๐๐๐๐ป๐๐๐๐๐๐ ๐๐๐๐๐๐๐๐๐๐๐๐๐๐๐๐๐
10
ns
ns
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p. 118McCall, Stanton with Woods, Weingartner, and SchlechterIEEE 802.1 TSN Interim, Virtual, September 2021 Available at http://www.ieee802.org/1/files/public/docs2021
Formulae โNRRerror
๐๐๐ ๐ ๐๐๐๐๐ ๐ ๐ ๐๐๐๐๐น๐๐๐๐ฌ๐๐๐๐๐๐๐๐๐๐๐๐๐๐
๐๐ซ๐๐๐๐๐ฐ๐๐๐๐๐๐๐๐ ๐๐๐
๐ต๐น๐น๐ฐ๐๐๐๐๐๐๐๐๐๐๐๐
๐๐๐ ๐๐๐๐๐๐ซ๐๐๐๐ ๐ 1 ๐๐๐๐๐๐ซ๐๐๐๐ ๐ ppm
๐ต๐น๐น๐ฐ๐๐๐๐๐๐๐๐๐๐๐๐ ๐ก4๐๐ท๐๐๐๐๐ ๐ก4๐๐๐๐ฃ๐๐๐ข๐ ๐๐ท๐๐๐๐๐ ๐ก3๐๐ท๐๐๐๐๐ ๐ก3๐๐๐๐ฃ๐๐๐ข๐ ๐๐ท๐๐๐๐๐
๐ก3๐๐๐๐ฃ๐๐๐ข๐ ๐๐ท๐๐๐๐๐ ๐ป๐บ๐ฎ๐ฌ๐ป๐ฟ ๐ซ๐ป๐บ๐ฌ๐ป๐ฟ ๐ก4๐๐๐๐ฃ๐๐๐ข๐ ๐๐ท๐๐๐๐๐ ๐ป๐บ๐ฎ๐ฌ๐น๐ฟ ๐ซ๐ป๐บ๐ฌ๐น๐ฟ
The factors ๐ก3๐๐ท๐๐๐๐๐ and ๐ก4๐๐ท๐๐๐๐๐ are the same as in the ๐๐ท๐๐๐๐ฆ๐๐๐๐๐ calculation.(Same value. No need for new random numbers, as the same pDelayResp message is used to measure NRR.)
ns
ns
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Formulae โ RRerror
๐ ๐ ๐๐๐๐๐ ๐ ๐ ๐๐๐๐๐๐๐ ๐ ๐ ๐ ๐๐๐๐๐๐ท๐๐๐๐ก ppm
๐๐๐๐๐ฆ๐๐๐ ๐ _๐๐ฆ๐๐~๐ 0, 1 ๐๐ซ๐๐๐๐๐น๐๐๐๐บ๐๐๐๐๐๐๐๐๐๐๐๐๐ ๐๐ซ๐๐๐๐๐ฐ๐๐๐๐๐๐๐
๐ ๐ ๐๐๐๐๐๐๐ ๐ ๐ ๐๐๐๐๐น๐๐๐๐ฌ๐๐๐๐๐๐๐๐๐๐๐๐๐๐
๐๐ซ๐๐๐๐๐ฐ๐๐๐๐๐๐๐๐ ๐๐๐
๐ต๐น๐น๐๐๐๐๐๐๐๐๐๐๐๐๐
๐๐๐ ๐๐๐๐๐๐ซ๐๐๐๐๐ฎ๐ด ๐๐๐๐๐๐ซ๐๐๐๐
๐ ๐ ๐๐๐๐๐๐ท๐๐๐๐ก ๐ ๐ ๐ ๐๐๐๐๐๐ท๐๐๐๐ก ๐ 1 ๐ ๐๐๐๐๐น๐๐๐๐ฌ๐๐๐๐๐๐๐๐๐๐๐๐๐๐
๐๐๐๐๐ฆ๐๐๐ ๐ _๐๐ฆ๐๐
10๐๐๐๐๐๐ซ๐๐๐๐๐ฎ๐ด ๐๐๐๐๐๐ซ๐๐๐๐ ๐
ppm
ppm
ms
The factor ๐ต๐น๐น๐๐๐๐๐๐๐๐๐๐๐๐๐ is the same as in the ๐ต๐น๐น๐๐๐๐๐ calculation as it is the same error.
Does not include any effect of changing clock drift (ppm/s2) duringResidence Time. See speaker notes in ๐ ๐ ๐๐๐๐๐ section for details.
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Special Cases
๐๐๐๐๐๐ซ๐๐๐๐ 0 ๐๐๐๐๐๐ซ๐๐๐๐๐ฎ๐ด
๐๐๐๐๐ ๐๐๐๐๐ป๐๐๐๐๐๐๐๐ ๐ฅ 0
For the first hop (๐ ๐), ๐ ๐ ๐, i.e. the first device in chain, which is the GM.
For the last hop (๐ ๐), the Sync message is not passed on so there is no Residence Time Error.
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McCall, Stanton with Woods, Weingartner, and SchlechterIEEE 802.1 TSN / 60802 Interim, September 2021 Available at http://www.ieee802.org/1/files/public/docs2021
p. 121McCall, Stanton with Woods, Weingartner, and SchlechterIEEE 802.1 TSN Interim, Virtual, September 2021 Available at http://www.ieee802.org/1/files/public/docs2021
Of Interestโฆ
โข What happens when there is GM Clock Drift?โข The ability to control clockDriftGM separately allows this to be investigated.
โข How much do the various factors influence DTE?โข Also: how do they accumulate along the chain?
โข The separation of various factors until late in the process allows detailed analysis
โข How effective are the various correction fields?
Careful construction of the model will help when it comes to extracting the values of interest (minimums, maximums, means, etcโฆ) as the
model is run, without taking excessive time or using excessive memory.
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