1. 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
Geneva, Switzerland, 13 July 2013

2. 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

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
Frequency
Phase/Time
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 G
ITU-T Q13/15 focusing on this topic after December 2011 (when most of frequency sync aspects were completed)
The following main aspects are addressed
Network Requirements
Architecture
PTP Profiles
Clocks
The work is tentatively planned to be completed in the 2013/2014 time frame
Several aspects also involving other SG15 Questions, e.g.:
Time sync Interfaces
Time sync over access technologies
Time sync over OTN
G.8271 scope
Time and phase synchronization aspects in packet networks
Target applications
Methods to distribute the reference timing signals
It also specifies the relevant time and phase synchronization interfaces and related performance.
Physical characteristics have been into G.703 (Q11/15)
G.8271 is the first document of the G.827x series to be released (Published in 02/2012)
Amendment planned for July 2013 (additional details and alignments with G )
G Scope
maximum network limits of phase and time error that shall not be exceeded.
minimum equipment tolerance to phase and time error that shall be provided at the boundary of these packet networks at phase and time synchronization interfaces.
Related Information (HRM, simulation assumptions, etc.)
Consented in July 2013 ? (TD xxx)
Details on Simulations in G.Supp
Planned for 2013
Geneva, Switzerland, 13 July 2013
© Ericsson AB 2013

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

6. Noise (Time Error) Budgeting Analysis
Noise (Time Error) Budgeting Analysis
Common Time Reference (e.g. GPS time) N
TEA
TEB
Same limit applicable to R3 and R4 (limits in R4 applicable only in case of External Packet Slave Clock)
TEC
Network Time Reference
(e.g. GNSS Engine) R4 R5 R1 R2 R3
Simulation Reference Model:
chain of T-GM, 10 T-BCs, T-TSC
with and without SyncE support
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
Typical Target Requirements TED < 1.5 ms (LTE TDD, TD-SCDMA)
Geneva, Switzerland, 13 July 2013
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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
TE (t) t
|TE|
Holdover-Rearr. period
TEHO or TEREA budget
1.5 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)
Geneva, Switzerland, 13 July 2013
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8. Sync Standards Coordination
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.
End Application
T-TSC
TED D
TE(t)
PTP
1 PPS C
Max|TE|
max|TE’|
Test Equipment
1500 ns
Max |TEC (t)| = max|TE’| + TEREA + TEEA < TED
Geneva, Switzerland, 13 July 2013
1100 ns
400 ns

9. Time Error Budgeting 1.1 us Network Limit (max |TE|)
Time Error Budgeting
1500 ns
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)
Holdover PTP Rearrangements
250ns
End Application
150 ns
1.1 us
Network Limit (max |TE|)
Dynamic Noise accumulation
200 ns
BC Internal Errors (Constant)
550 ns
Link Asymmetries
250 ns
PRTC
100 ns
Geneva, Switzerland,13 July 2013
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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 2013

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)
Geneva, Switzerland, 2 13 July 2013

12. Sources of Asymmetries
Frequency and time/phase synchronization in Packet Networks
10/23/
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
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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
Geneva, Switzerland, 13 July 2013

14. Partial Timing Support
HRM for G
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 calibration
Handling of failures in the network
Still some important topics need to be completed
Stability requirements
Partial timing support (G )
Geneva, Switzerland, 13 July 2013

16. Back Up
Geneva, Switzerland, 13 July 2013

17. Time Synchronization via PTP
Frequency and time/phase synchronization in Packet Networks
10/23/
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
Geneva, Switzerland,13 July 2013
© Ericsson AB 2013

18. Metrics TE (t) Max |TE| t
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
Geneva, Switzerland, 13 July 2013
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19. Measurement of Newtork Limits at ref. Point C
Frequency and time/phase synchronization in Packet Networks
Measurement of Newtork Limits at ref. Point C
10/23/
Option a C
End Application
...
T-TSC
PRTC
T-GM
T-BC
T-BC
1pps
Direct comparison of PRTC Time
with 1 PPS
PRTC
Test Equipment
...
PRTC
T-GM
T-BC C
Monitoring method (e.g. tapped)
PTP Monitor
Test Equipment
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
In alternative the 4 timestamps
could be used:
TE = (TM2 – T1 – T4 + TM3)/2
Geneva, Switzerland,13 July 2013
© Ericsson AB 2013

20. Frequency and time/phase synchronization in Packet Networks
10/23/
Measurement of Newtork Limits at ref. Point C
Option c
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). C
End Application
...
T-TSC
PRTC
T-GM
T-BC
T-BC
Monitoring method (active probe)
PRTC
Test Equipment
PTP Probe
Geneva, Switzerland,13 July 2013
© Ericsson AB 2013

21. Example of Time Error Accumulation
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.
40 ns Time Error due to asymmetry per hop (Constant Time Error)
Source: WD25 (Anue), York, September 2011
no asymmetry (only Dynamic component)
Geneva, Switzerland,13 July 2013
v = max asymmetry per hop
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