geneva, switzerland, 13 july 2013 time sync network limits: status, challenges stefano ruffini,...
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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
Geneva, Switzerland, 13 July 2013 2
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
Geneva, Switzerland, 13 July 2013 3
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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Geneva, Switzerland, 2 13 July 2013 5
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
Geneva, Switzerland, 13 July 2013 81100 ns 400 ns
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
Geneva, Switzerland,13 July 2013 9
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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Geneva, Switzerland, 2 13 July 2013 11
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
Geneva, Switzerland,13 July 2013 12
Geneva, Switzerland, 13 July 2013 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
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)
Geneva, Switzerland, 13 July 2013 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 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
Geneva, Switzerland,13 July 2013 17
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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