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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Contents

Introduction on G.8271 and G.8271.1Definition of Time sync Network Limits Challenges for an operatorNext Steps

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.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

Frequency Phase/Time

G.8271.2

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Target Applications

Level of Accuracy

Time Error Requirement(with respect to an ideal reference)

Typical Applications

1 500 ms Billing, Alarms

2 100 s IP Delay monitoring

3 5 s LTE TDD (cell >3km)

4 1.5 sUTRA-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 some LTE-A features

(Under Study)

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Time sync Network Limits

Aspects to be addressed when defining the Network Limits

Reference network (HRM) for the simulationsMetricsNetwork Limits Components (Constant and Dynamic Time Error)Failure conditions

Network RearrangementsTime Sync Holdover

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 ClockPRTC: Primary Reference Time ClockT-TSC: Telecom Time Slave ClockT-GM: Telecom Grandmaster

End ApplicationTime Clock

Packet NetworkPRTC

Common Time Reference (e.g. GPS time)

Network Time Reference(e.g. GNSS Engine)

N

R1 R3 R4 R5

PacketMaster(T-GM)

R2

Typical Target Requirements TED < 1.5 s

(LTE TDD, TD-SCDMA)

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Same limit applicable to R3 and R4 (limits in R4 applicable only in case of External Packet Slave Clock)

TECTEA TEB

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 ScenariosPTP traceability to the primary master is lost; the T-BC or the End Application switches to a backup PTP reference

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TE (t)

t

|TE|

Holdover-Rearr. period

TEHO or TEREA budget1.5 us

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)

MAX |TE| based LimitsThe 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 |TEC (t)| = max|TE’| + TEREA + TEEA < TED

Max|TE|max|TE’|

Test Equipment

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1500 ns

End Application

T-TSC

TED

DTE(t)

PTP

1 PPS

C

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 supportIt looks feasible to control the max |TE| in the 200 ns range

Constant Time Error (cTE)Constant Time Error per node: 50 nsPRTC (see G.8272): 100 ns End Application: 150 nsRearrangements: 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

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1.1 usNetwork Limit (max |TE|)

HoldoverPTP Rearrangements 250ns

Stability Requirements

Additional requirement on stability of the timing signal is needed and is under studyApplicable to the dynamic component (d(t))In terms of MTIE and TDEVPossible Jitter requirementsImportant for End Application Tolerance

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Challenges for an operator

Distribution of accurate time synchronization creates new challenges for an operator

Operation of the networkHandling 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 proceduresNetwork performance and Node performance requires new methods and test equipment Some aspect still under definition (e.g. G.8273.x)

Sources of AsymmetriesDifferent Fiber Lengths in the forward and reverse direction

Main problem: DCF (Dispersion Compensated Fiber)

Different Wavelengths used on the forward and reverse directionAsymmetries added by specific access and transport technologies

GPONVDSL2MicrowaveOTN

Additional sources of asymmetries in case of partial support :Different load in the forward and reverse directionUse 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

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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

Partial Timing Support

HRM for G.8271.2

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TU NE

Packet Master Clock

PRTC

Packet Slave Clock

Time Reference, T

Network Element with BC

Network Element with BC

Time Output, T+ts

Test output, T+t1

Test output, T+t2

TU NE

TU NE

TU NE

Link containing up to M timing-unaware network elements

Up to N network elements containing BCs

Path containing up to P timing unaware network elements in total

TU NE

TU NE

Time reference, e.g. GNSS signal

Link containing up to M timing-unaware network elements

Link containing up to M timing-unaware network elements

Need to define new metrics (e.g. 2-ways FPP)

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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 calibrationHandling of failures in the network

Still some important topics need to be completedStability requirementsPartial timing support (G.8271.2)

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

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 s asymmetry would exceed the target requirement of 1.5 s)

t1

t2

t4

t3

M S

Time Offset= t2 – t1 – Mean path delay

Mean path delay = ((t2 – t1) + (t4 – t3)) /2

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MetricsMain 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 importantMTIE and TDEVRelated 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|

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