Joint IEEE-SA and ITU Workshop on Ethernet

Time Sync Network Limits: Status, Challenges Stefano Ruffini, Ericsson Q13/15 AR

Geneva, Switzerland, 13 July 2013

Contents

Introduction on G.8271 and G.8271.1

Definition of Time sync Network Limits Challenges for an operator Next Steps Geneva, Switzerland, 13 July 2013 2

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

Frequency Phase/Time 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.8271.2

G.8275.1, G.8275.2

G.8272

G.8273,.1,.2,.3

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Target Applications

Level of Accuracy 1 2 3 4 5 6 Time Error Requirement (with respect to an ideal reference) 500 ms 100 m s 5 m s Geneva, Switzerland, 13 July 2013 1.5 1 m m s s < x ns (x ffs) Typical Applications Billing, Alarms IP Delay monitoring LTE TDD (cell >3km) UTRA-TDD, LTE-TDD (cell  3Km) Wimax-TDD (some configurations) Wimax-TDD (some configurations) Location Based services and s ome LTE-A features (Under Study) 4

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 Geneva, Switzerland, 2 13 July 2013 5

Noise (Time Error) Budgeting Analysis

N Common Time Reference (e.g. GPS time) Network Time Reference (e.g. GNSS Engine)

R1

TE A

R2

TE B

R3

TE C

R4 R5

PRTC Packet Master (T-GM)

Packet Network

Packet Slave Clock (T-TSC )

End Application Time Clock

T-BC: Telecom Boundary Clock PRTC: Primary Reference Time Clock T-TSC: Telecom Time Slave Clock T-GM: Telecom Grandmaster

Simulation Reference Model: •chain of T-GM, 10 T-BCs, T-TSC Typical Target Requirements TE D (LTE TDD, TD-SCDMA) < 1.5 m s •with and without SyncE support

Same limit applicable to R3 and R4 (limits in R4

Geneva, Switzerland, 13 July 2013

applicable only in case of External Packet Slave Clock)

6

TE (t)

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 Failure in the sync network TE HO or TE REA budget 1.5 us |TE| t Holdover-Rearr. period TE HO TE REA applicable to the network (End Application continues to be locked to the external reference) applicable to the End Application (End Application enters holdover) Geneva, Switzerland, 13 July 2013 7

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.

TE(t)

C

T-TSC End Application

D

PTP 1 PPS TE D Max|TE| Test Equipment max|TE’| 1500 ns Max |TE C (t)| = max|TE’| + TE REA + TE EA < TE D Geneva, Switzerland, 13 July 2013 8 1100 ns 400 ns

Time Error Budgeting

Budgeting Example (10 hops)

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|) Geneva, Switzerland,13 July 2013 1500 ns Holdover PTP Rearrangements End Application Dynamic Noise accumulation BC Internal Errors (Constant) Link Asymmetries PRTC 250ns 150 ns 200 ns 550 ns 250 ns 100 ns 9

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 10 Geneva, Switzerland,13 July 2013

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) Geneva, Switzerland, 2 13 July 2013 11

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 2013 12

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 Geneva, Switzerland, 13 July 2013 13

Partial Timing Support

HRM for G.8271.2

Time reference, e.g. GNSS signal Test output, T+  t1 Test output, T+  t2 PRTC Packet Master Clock TU NE TU NE Network Element with BC Time Reference, T Link containing up to

M

timing unaware network elements TU NE TU NE Link containing up to

M

timing unaware network elements Up to

N

network elements containing BCs Network Element with BC TU NE TU NE Packet Slave Clock Link containing up to

M

timing unaware network elements Time Output, T+  ts Path containing up to

P

timing unaware network elements in total Need to define new metrics (e.g. 2-ways FPP) 14 Geneva, Switzerland,13 July 2013

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) Geneva, Switzerland, 13 July 2013 15

Geneva, Switzerland, 13 July 2013

Back Up

Time Synchronization via PTP

The basic principle is to distribute Time sync reference by means of two-way time stamps exchange

M S

t 1 Time Offset= t 2 – t 1 – Mean path delay t 2 Mean path delay = ((t 2 – t 1 ) + (t 4 – t 3 )) /2 t 3 t 4 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 m s asymmetry would exceed the target requirement of 1.5 m s) 17 Geneva, Switzerland,13 July 2013

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 TE (t) Max |TE| t 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 ?

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