Measuring Time Error. Tommy Cook, CEO.

Size: px

Start display at page:

Download "Measuring Time Error. Tommy Cook, CEO."

Clifford Palmer
6 years ago
Views:

1 Measuring Time Error Tommy Cook, CEO

2 Presentation overview What is Time Error? Network devices. PRTC & Grand Master Clock Evaluation. Transparent Clock Evaluation. Boundary Clock Evaluation. Sources of uncertainty. Evaluation to G performance specification. Slave Clock Evaluation. Summary 2

3 What is Time Error? 3

4 Time Error and Time Interval Error (TIE) Time Error measures the time difference between two clocks Max TE Time Error cte dte 0 Time Time Interval Error measures change of time error Starts at zero by convention, then tracks the dynamic time error Time Interval Error 0 Time Interval 4

5 Time Error Specifications Time error generation of equipment clocks specified using four parameters: Constant Time Error (cte) - specified in nanoseconds. Dynamic Time Error (dte L ) - specified with MTIE & TDEV masks, data pre-filtered by LP 0.1Hz. Dynamic Time Error (dte H ) - specified in nanoseconds, data pre-filtered by HP 0.1Hz. Max absolute time error (max TE ) - specified in nanoseconds, data unfiltered. Network limits on time error at reference points specified using two parameters: Dynamic Time Error (dte L ) - specified with MTIE & TDEV masks, data pre-filtered by LP 0.1Hz. Max absolute time error (max TE ) - specified in nanoseconds, data pre-filtered by LP 0.1Hz. 5

6 Network Devices involved in Time delivery 6

7 G.8275 Networks - Phase and Time Recovery Full Timing Support, (G ): switch/routers containing a T-BC T-GM every switch/router on the path between T-GM and the T-TSC contains a T-BC Partial Timing Support, (G ): ordinary switch/routers T-GM TC not all switch/routers on the path between T-GM and T-TSC contain a T-BC or T-TC 7

8 PRTC and T-GM Testing 8

9 PRTC Specification G.8272 defines the specification for a PRTC: Max TE < 100ns, relative to the applicable primary time standard (e.g. UTC) dte (or wander) limited by the MTIE mask: Limits apply at both the 1pps phase output, and time output if T-GM is integrated with the PRTC, (G.8272 Amendment 1). 9

10 How do you test a PRTC+T-GM? Device under Test: GNSS-Based PRTC+T-GM Timing Monitor Freq. PRC Ref. G.8272 Appendix I suggests three main ways of testing: 1. Comparison to a PRC frequency standard. Only addresses phase wander (i.e. dte). Cannot verify max TE value. Device under Test: GNSS-Based PRTC+T-GM Timing Monitor 1pps + ToD 2. Comparison to a reference receiver Addresses both phase wander and max TE. Uncertainty of the reference receiver may be too high. Device under Test: GNSS-Based PRTC+T-GM Timing Monitor 1pps + ToD National Standard 3. Comparison to a national time standard Addresses both phase wander and max TE. Only available from a certified national time laboratory (e.g. NIST, NPL, PTB, etc). 10

11 GNSS simulators Produces an RF signal similar to what is seen by the receiver Contains full orbital models of satellites Covers multiple constellations (e.g. GPS, Glonass, Beidou) Models atmospheric delays and disturbances Includes full position and navigation data for fixed or mobile receivers Produces a 1pps and ToD signal accurately synchronized to the RF signal Enables timing receiver accuracy testing 11

12 Testing a PRTC with a GNSS Simulator GNSS Simulator RF Device under Test: GNSS-Based PRTC Timing Monitor Stable Frequency Time alignment to < 5ns 1pps Time-of-day Reference Time Input Rb. Oscillator 12

13 Transparent Clock evaluation 13

14 What is a Transparent Clock? Packet Interfaces Siwtch/router Switch/Router Packet Interfaces Messages Transparent Clock Messages Residence Time Bridge Measures time each packet resides in the switch/router SyncE SyncE 14

15 Accuracy of the CorrectionField value: Does it reflect the actual delay experienced by the Sync & Del_Req messages? Theoretical model: CorrectionField precisely reflects the delay through the equipment Ideal case zero net PDV Q1 Q2 Qn Types of CorrectionField inaccuracy; 1. Variable error Caused by packet-to-packet variation in CorrectionField accuracy. Leads to residual PDV when terminated. 2. Fixed Error Caused by CorrectionField value always being greater or less by a fixed value. Results in a fixed delay being measured when terminated. Not a issue if Fixed Error is matched in forward and reverse direction. Differences between forward and reverse Fixed Error will produce asymmetry and hence create a Constant Time Error. 15

Transparent Clock Test Plan Evaluation Test Bed Master Capture Ref. Time & Freq.

Measure the packet-by-packet latency across the TC. 2. Determine the change to the correctionfield value for each message.

Traffic Generator to load DUT G.8273.3 under development in ITU-T. IEEE std C37.238-2011 in Power Systems Applications.

16 Transparent Clock Test Plan Evaluation Test Bed Master Capture Ref. Time & Freq. T-TC Slave Capture Development Test Procedure to characterise actual performance 1. Measure the packet-by-packet latency across the TC. 2. Determine the change to the correctionfield value for each message. 3. Accuracy is the difference in the actual latency compared to the change in Correctionfield value. Traffic Generator to load DUT G under development in ITU-T. IEEE std C in Power Systems Applications. Annex A: TC TimeInaccuracy 50nsec. Measure impact of CorrectionField on Sync PDV. 1. Vary traffic packet size. 2. Vary traffic priority. 3. Vary traffic utilisation. Repeat for Sync & Del_req PDV. Test in 1-Step and 2-Step modes. 16

17 Boundary and Slave Clocks 17

Slave Master Boundary Clock, T-BC SyncE Master Slave EEC G.8273.

18 Slave Master Boundary Clock, T-BC SyncE Master Slave EEC G Telecom Boundary FrequencyClock Time/Phase (T-BC) (T1/E1/SyncE) (1pps) 1pps SyncE Boundary Clocks reduce PDV accumulation by: Terminating the flow and recovering the reference time Generating a new flow using the recovered time No direct transfer of PDV Slave/Master combination Telecom BCs (T-BCs) use SyncE to: Improve stability Improve holdover T-BCs specified in ITU-T Recommendation G

19 Slave Master T-BC sources of uncertainty Sources of uncertainty: Timestamp noise in slave e? e? e? e? 1pps Phase noise and distortion in internal oscillator SyncE phase wander Path delay uncertainty in 1pps signal path SyncE e? SyncE Timestamp noise in master EEC G Telecom Boundary Clock (T-BC) Time Error is the combination of all these uncertainties 19

T1/E1 /SyncE T-BC: Performance measured at the, 1pps & Freq. outputs.

20 Slave Master Slave Master Boundary Clock & Slave Clock comparison Phase 1pps Phase 1pps SyncE EEC G Telecom Boundary Clock (T-BC) Freq. T1/E1 /SyncE SyncE EEC G Telecom Boundary Time Slave Clock (T-BC) (T-TSC) Freq. T1/E1 /SyncE T-BC: Performance measured at the, 1pps & Freq. outputs. T-TSC: Performance measured at the 1pps & Freq. outputs. G Telecom Time Slave Clock (T-TSC) 20

21 T4 cte = +162nsec Testing Time Error, 1pps or? Important to verify performance on output and 1pps output. When evaluating T-GM & T-BC: used downstream therefore important as used to transfer time. 1pps Monitor point, but used once installed to verify behaviour and track down problems. Therefore, need to verify both! 2W cte = 21nsec T1 cte = -119nsec 1pps cte = 207nsec 21

22 Boundary Clock evaluation 22

23 G T-BC Specification Five key elements to every ITU-T clock specification: Noise generation The intrinsic noise generated by the clock itself with an ideal reference at the input Noise tolerance The maximum amount of noise the clock can tolerate at its input Noise transfer The transfer function of the clock; usually defined as the bandwidth Transient response The response of the clock to a transient at its input Holdover How long a clock should maintain its output within specification after loss of the input signal For time clocks, noise means time error. 23

Time Interval Error Time Error Time Error Slave Master G.8273.2 T-BC Noise Generation Ideal Time Ref Time 1pps Time Ideal Freq.

(unfiltered) cte: 50ns (class A), or 20ns (class B) (mean over 1000s) dte L : 40ns MTIE, 4ns TDEV* (after filtering by 0.

24 Time Interval Error Time Error Time Error Slave Master G T-BC Noise Generation Ideal Time Ref Time 1pps Time Ideal Freq. Ref Interval SyncE EEC SyncE Maximum noise generation at both and 1pps outputs: Max TE : 100ns (class A), or 70ns (class B) (unfiltered) cte: 50ns (class A), or 20ns (class B) (mean over 1000s) dte L : 40ns MTIE, 4ns TDEV* (after filtering by 0.1Hz LP) dte H : 20ns (after filtering by 0.1Hz HP) (proposed in draft revision) * - TDEV specified in Appendix hence not mandatory. 24

25 Time Interval Error Time Error Time Error Slave Master G T-BC Noise Tolerance Noisy Time Ref Time 1pps Time Noisy Freq. Ref Interval SyncE EEC SyncE With maximum noise on both and SyncE inputs, check that: No alarms are generated Clock does not switch references or go into holdover There are no performance limits to check 25

26 G Noise Tolerance Limits Noise tolerance: T-BCs must tolerate the maximum dte accumulation over the chain of T-BCs Specified using an MTIE mask in G Section 7.3: MTIE 1µs 660ns 360ns 250ns Figure 2 from G : dte network limit SyncE Noise tolerance as defined in G.8262: MTIE [ ms] 100ns τ (s) Figure 5 from G.8262: Input wander tolerance for EEC Option Observation interval t[s] G.8262-Y.1362(10)_F05 26

27 Time Interval Error Time Error Time Error Slave Master G T-BC Noise Transfer Sinusoidal Time Error Time 1pps Time Sinusoidal Wander Interval SyncE EEC SyncE Measures the transfer function of the clock, i.e. the bandwidth Measured by applying sinusoidal signals at different frequencies and measuring the relative attenuation 27

28 Slave Master G : Noise Transfer T-BC to flow 1pps output SyncE EEC SyncE SyncE to SyncE flow input is: Noisy at high frequencies (e.g. packet jitter, timestamp quantization) Quiet at low frequencies (because traceable to a time reference) Time Clock is a phase (time) locked loop: Smooth out packet noise at high frequencies by following SyncE or local osc. (i.e. input is low-pass filtered) follows input at low frequencies, correcting SyncE or oscillator wander (i.e. SyncE/oscillator noise is high-pass filtered) SyncE input low-pass filtered by EEC, then high-pass filtered by Time Clock Net result is a band-pass filter 28

29 G : T-BC Clock Bandwidth Noise Transfer to : Low pass filter, bandwidth between 0.05 to 0.1Hz Gain peaking 0.1dB Amplitude, db Frequency, Hz SyncE to SyncE (G.8262): Low pass filter, bandwidth between 1 to 10Hz Amplitude, db 1 10 Frequency, Hz SyncE to : Bandpass filter, lower cutoff 0.05 to 0.1Hz, upper 1 to 10Hz Gain peaking 0.1dB Amplitude, db Frequency, Hz 29

30 G Transient Response -to- Transient Response for further study. Specifies that a T-BC should reject a SyncE transient on its input. This means it should not follow the SyncE input until the signal is restored. Determined by monitoring the QL of the SyncE signal Defines an acceptable transient mask that the T-BC should remain within when a SyncE transient is applied at the input. Maximum phase error (ns) T-BC output phase mask for first output phase error transient after start of SyncE rearrangement Time after start of first output phase error transient (s) 30

31 G Holdover Specification G does not yet define holdover period for a T-BC Holdover provided at the end node rather than at each T-BC Assisted holdover may be provided using SyncE Long term stability of the SyncE signal helps output to remain within specification Even with assisted holdover, the clock will not remain in specification for more than a few seconds 31

32 Slave clock evaluation 32

33 G T-TSC Specification Five key elements to every ITU-T clock specification, (the same as T-BC but no output to check, only 1PPS): Noise generation The intrinsic noise generated by the clock itself with an ideal reference at the input Noise tolerance The maximum amount of noise the clock can tolerate at its input Noise transfer The transfer function of the clock; usually defined as the bandwidth Transient response The response of the clock to a transient at its input Holdover How long a clock should maintain its output within specification after loss of the input signal For time clocks, noise means time error. 33

Slave T-TSC Time Error Evaluation Evaluation Test Bed Master Impair Ref. Time & Freq. TE: Difference between time output to ingress time; Max TE (unfiltered) Constant TE Dynamic TE.

34 Slave T-TSC Time Error Evaluation Evaluation Test Bed Master Impair Ref. Time & Freq. TE: Difference between time output to ingress time; Max TE (unfiltered) Constant TE Dynamic TE. Capture Capture Verify Performance specification in G ; Noise Generation Noise Tolerance Noise Transfer Transient Response Holdover Phase 1pps, ToD DUT T-TSC EEC Freq. T1/E1/SyncE 34

Summary Time Error Metrics Max TE (0.1Hz filtered for networks, unfiltered for equipment) Constant TE, cte Dynamic TE; dte L (MTIE & TDEV), dte H (peak value) G.

35 Summary Time Error Metrics Max TE (0.1Hz filtered for networks, unfiltered for equipment) Constant TE, cte Dynamic TE; dte L (MTIE & TDEV), dte H (peak value) G.8272: PRTC and PRTC+T-GM performance parameters specified. Time Error (max TE ) & Wander Generation (dte). G (draft): T-TC Prove accuracy of inserted CorrectionField value. {c specifies accuracy today.} G ; Both T-BC and T-TSC performance parameters specified. Noise Generation (max TE, cte, dte L, dte H ) Noise Tolerance (dte) Noise Transfer (-to-, SyncE-to-, SyncE-to-SyncE) Transient Response (-to-, SyncE-to-, SyncE-to-SyncE) Holdover (tbd) 35

36 INTEGRITY TIME MEASUREMENTS REQUIRE TRUE PRECISION Tommy Cook +44 (0)

ITU-T G /Y

ITU-T G /Y I n t e r n a t i o n a l T e l e c o m m u n i c a t i o n U n i o n ITU-T TELECOMMUNICATION STANDARDIZATION SECTOR OF ITU G.8273.2/Y.1368.2 (01/2017) SERIES G: TRANSMISSION SYSTEMS AND MEDIA, DIGITAL