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Time Sync Network Limits: Status, Challenges

Joint IEEE-SA and ITU Workshop on Ethernet. Time Sync Network Limits: Status, Challenges. Stefano Ruffini, Ericsson Q13/15 AR. Contents. Introduction on G.8271 and G.8271.1 Definition of Time sync Network Limits Challenges for an operator Next Steps. Time Sync: Q13/15 Recommendations.

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Time Sync Network Limits: Status, Challenges

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

  2. Contents • Introduction on G.8271 and G.8271.1 • 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 Phase/Time Frequency General/Network Requirements Architecture and Methods PTP Profile Clocks G.8261 G.8261.1 G.8264 G.8265 G.8265.1 G.8262 G.8263 G.8271 G.8271.1 G.8275 G.8275.1, G.8275.2 G.8272 G.8273,.1,.2,.3 G.8271.2

  4. Target Applications Geneva, Switzerland, 13 July 2013 4

  5. 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 Typical Target Requirements TED< 1.5 ms (LTE TDD, TD-SCDMA) Common Time Reference (e.g. GPS time) N TEA TEB TEC Network Time Reference (e.g. GNSS Engine) R5 R4 R3 R2 R1 Packet Slave Clock(T-TSC) Packet Network PRTC PacketMaster(T-GM) End Application Time Clock T-BC: Telecom Boundary Clock PRTC: Primary Reference Time Clock T-TSC: Telecom Time Slave Clock T-GM: Telecom Grandmaster Same limit applicable to R3 and R4 (limits in R4 applicable only in case of External Packet Slave Clock)

  7. Rearrangements and Holdover TE (t) TEHO or TEREA budget 1.5 us |TE| t Holdover-Rearr. period • 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 Failure in the sync network TEHO applicable to the network (End Application continues to be locked to the external reference) TEREA applicable to the End Application (End Application enters holdover)

  8. MAX |TE| based Limits C TE(t) D T-TSC End Application PTP 1 PPS TED 1500 ns Max|TE| max|TE’| Max |TEC (t)| = max|TE’| + TEREA + TEEA < TED Test Equipment • 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. 400 ns 1100 ns

  9. Time Error Budgeting Budgeting Example (10 hops) 1500 ns HoldoverPTP Rearrangements 250ns End Application 150 ns Dynamic Noise accumulation 200 ns BC Internal Errors (Constant) 550 ns Link Asymmetries PRTC 100 ns 250 ns • 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) 1.1 us Network Limit (max |TE|)

  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

  11. 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

  13. 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 Need to define new metrics (e.g. 2-ways FPP) HRM for G.8271.2

  15. Summary • G.8271.1 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.8271.2)

  16. 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: t2 – t1 = t4 – t3 • Any asymmetry will contribute with half of that to the error in the time offset calculation (e.g. 3 ms asymmetry would exceed the target requirement of 1.5 ms) M S t1 Time Offset= t2 – t1 – Mean path delay Mean path delay = ((t2 – t1) + (t4 – t3)) /2 t2 t3 t4

  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) Max |TE| t

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