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Geneva, Switzerland, 13 July 2013 Time Sync Network Limits: Status, Challenges Stefano Ruffini, Ericsson Q13/15 AR Joint IEEE-SA and ITU Workshop on Ethernet.

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Presentation on theme: "Geneva, Switzerland, 13 July 2013 Time Sync Network Limits: Status, Challenges Stefano Ruffini, Ericsson Q13/15 AR Joint IEEE-SA and ITU Workshop on Ethernet."— Presentation transcript:

1 Geneva, Switzerland, 13 July 2013 Time Sync Network Limits: Status, Challenges Stefano Ruffini, Ericsson Q13/15 AR Joint IEEE-SA and ITU Workshop on Ethernet

2 Geneva, Switzerland, 13 July Contents Introduction on G.8271 and G Definition of Time sync Network Limits Challenges for an operator Next Steps

3 Time Sync: Q13/15 Recommendations Analysis of Time/phase synchronization in Q13/15: G.8260 (definitions related to timing over packet networks) G.827x series General/Network Requirements Architecture and Methods PTP Profile Clocks G.8261 G G.8264 G.8265 G G.8262 G.8263 G.8271 G G.8275 G , G G.8272 G.8273,.1,.2,.3 Frequency Phase/Time G Geneva, Switzerland, 13 July

4 Target Applications Level of Accuracy Time Error Requirement (with respect to an ideal reference) Typical Applications 1500 msBilling, Alarms s IP Delay monitoring 3 5 s LTE TDD (cell >3km) s UTRA-TDD, LTE-TDD (cell 3Km) Wimax-TDD (some configurations) 5 1 s Wimax-TDD (some configurations) 6< x ns (x ffs) Location Based services and s ome LTE-A features (Under Study) Geneva, Switzerland, 13 July

5 Geneva, Switzerland, 2 13 July Time sync Network Limits Aspects to be addressed when defining the Network Limits Reference network (HRM) for the simulations Metrics Network Limits Components (Constant and Dynamic Time Error) Failure conditions Network Rearrangements Time Sync Holdover

6 Noise (Time Error) Budgeting Analysis Simulation Reference Model: chain of T-GM, 10 T-BCs, T-TSC with and without SyncE support Packet Slave Clock (T-TSC ) T-BC: Telecom Boundary Clock PRTC: Primary Reference Time Clock T-TSC: Telecom Time Slave Clock T-GM: Telecom Grandmaster End Application Time Clock Packet Network PRTC Common Time Reference (e.g. GPS time) Network Time Reference (e.g. GNSS Engine) N R1 R3 R4 R5 Packet Master (T-GM) R2 Typical Target Requirements TE D < 1.5 s (LTE TDD, TD-SCDMA) Geneva, Switzerland, 13 July Same limit applicable to R3 and R4 (limits in R4 applicable only in case of External Packet Slave Clock) TE C TE A TE B

7 Rearrangements and Holdover The full analysis of time error budgeting includes also allocating a suitable budget to a term modelling Holdover and Rearrangements Time Sync Holdover Scenarios PTP traceability is lost and and the End Application or the PRTC enters holdover using SyncE or a local oscillator PTP Master Rearrangement Scenarios PTP traceability to the primary master is lost; the T-BC or the End Application switches to a backup PTP reference Geneva, Switzerland, 13 July TE (t) t |TE| Holdover-Rearr. period TE HO or TE REA budget 1.5 us Failure in the sync network TE HO applicable to the network (End Application continues to be locked to the external reference) TE REA applicable to the End Application (End Application enters holdover)

8 MAX |TE| based Limits The Constant Time Error measurement was initially proposed as could be easily correlate to the error sources (e.g. Asymmetries), however Complex estimator (see G.8260) Different values at different times (e.g. due to temperature variation) Max |TE| has then been selected : The measurement might need to be done on pre-filtered signal (e.g. emulating the End Application filter, i.e. 0.1 Hz). This is still under study. Max |TE C (t)| = max|TE| + TE REA + TE EA < TE D Max|TE| max|TE| Test Equipment Geneva, Switzerland, 13 July ns 400 ns 1500 ns End Application T-TSC TE D D TE(t) PTP 1 PPS C

9 PRTC 100 ns BC Internal Errors (Constant) 550 ns Dynamic Noise accumulation 200 ns Time Error Budgeting Dynamic Error (dTE (t)) simulations performed using HRM with SyncE support It looks feasible to control the max |TE| in the 200 ns range Constant Time Error (cTE) Constant Time Error per node: 50 ns PRTC (see G.8272): 100 ns End Application: 150 ns Rearrangements: 250 ns (one of the main examples) Remaining budget to Link Asymmetries (250 ns) 1500 ns Budgeting Example (10 hops) End Application 150 ns Link Asymmetries 250 ns Geneva, Switzerland,13 July us Network Limit (max |TE|) Holdover PTP Rearrangements 250ns

10 Stability Requirements Additional requirement on stability of the timing signal is needed and is under study Applicable to the dynamic component (d(t)) In terms of MTIE and TDEV Possible Jitter requirements Important for End Application Tolerance Geneva, Switzerland,13 July

11 Geneva, Switzerland, 2 13 July Challenges for an operator Distribution of accurate time synchronization creates new challenges for an operator Operation of the network Handling of asymmetries (at set up and during operation) Planning of proper Redundancy (e.g. Time sync Holdover is only available for limited periods (minutes instead of days). Exceeding the limits can cause service degradation New testing procedures Network performance and Node performance requires new methods and test equipment Some aspect still under definition (e.g. G.8273.x)

12 Sources of Asymmetries Different Fiber Lengths in the forward and reverse direction Main problem: DCF (Dispersion Compensated Fiber) Different Wavelengths used on the forward and reverse direction Asymmetries added by specific access and transport technologies GPON VDSL2 Microwave OTN Additional sources of asymmetries in case of partial support : Different load in the forward and reverse direction Use of interfaces with different speed Different paths in Packet networks (mainly relevant in case of partial support) Traffic Engineering rules in order to define always the same path for the forward and reverse directions Geneva, Switzerland,13 July

13 Geneva, Switzerland, 13 July Next Steps Work is not completed Dynamic components in terms of MTIE and TDEV; Jitter? Testing methods (G.8273 provides initial information) Partial Timing support

14 Partial Timing Support HRM for G Geneva, Switzerland,13 July Need to define new metrics (e.g. 2-ways FPP)

15 Geneva, Switzerland, 13 July Summary G consented this week: Max |TE| Time sync limits are available The delivery of accurate time sync presents some challenges for an operator Asymmetry calibration Handling of failures in the network Still some important topics need to be completed Stability requirements Partial timing support (G )

16 Geneva, Switzerland, 13 July 2013 Back Up

17 Time Synchronization via PTP The basic principle is to distribute Time sync reference by means of two-way time stamps exchange Symmetric paths are required: Basic assumption: t 2 – t 1 = t 4 – t 3 Any asymmetry will contribute with half of that to the error in the time offset calculation (e.g. 3 s asymmetry would exceed the target requirement of 1.5 s) t1t1 t2t2 t4t4 t3t3 MS Time Offset= t 2 – t 1 – Mean path delay Mean path delay = ((t 2 – t 1 ) + (t 4 – t 3 )) /2 Geneva, Switzerland,13 July

18 Metrics Main Focus is Max Absolute Time Error (Max |TE|) (based on requirements on the radio interface for mobile applications) Measurement details need further discussion Stability aspects also important MTIE and TDEV Related to End Application tolerance Same Limits in Reference point C or D ! Same limits irrespectively if time sync is distributed with SyncE support or not ? TE (t) t Max |TE| Geneva, Switzerland, 13 July

19 Measurement of Newtork Limits at ref. Point C... PRTC T-GM T-BC C 1pps Test Equipment PRTC T-TSC End Application Direct comparison of PRTC Time with 1 PPS In alternative the 4 timestamps could be used: TE = (TM2 – T1 – T4 + TM3)/2 Option a... PRTC T-GM T-BC C Monitoring method (e.g. tapped) PTP Monitor Test Equipment PRTC T-TSC End Application X TE = TM1 - T1 – X = TM4 – T4 -X Sync timestamp Monitor timestamp of Sync Delay Resp (t4 timestamp) Monitor timestamp of Delay Request Option b Geneva, Switzerland,13 July

20 ... PRTC T-GM T-BC C PTP Probe Test Equipment PRTC T-TSC End Application Monitoring method (active probe) from the two-way PTP flow via an active measurement probe (e.g. prior to the start of the service, or connecting the active monitor to a dedicated port of the T-BC). Option c Measurement of Newtork Limits at ref. Point C Geneva, Switzerland,13 July

21 Example of Time Error Accumulation Source: WD25 (Anue), York, September 2011 Accumulation of maximum absolute time error over a chain of boundary clocks for different values of asymmetry bias. The physical layer assist involves SEC/EEC chain with bandwidth 10Hz. v = max asymmetry per hop 40 ns Time Error due to asymmetry per hop (Constant Time Error) no asymmetry (only Dynamic component) Geneva, Switzerland,13 July

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