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
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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
G.8261
G.8271
G.8261.1
G.8271.1
G.8264
G.8275
G.8271.2
G.8265
PTP Profile
G.8265.1
G.8275.1, G.8275.2
Clocks
G.8262
G.8272
G.8263
G.8273,.1,.2,.3
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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 ms
IP Delay monitoring
3
5 ms
LTE TDD (cell >3km)
UTRA-TDD,
LTE-TDD (cell  3Km)
Wimax-TDD (some
configurations)
4
1.5 ms
5
1 ms
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 simulations
Metrics
Network Limits Components (Constant and
Dynamic Time Error)
Failure conditions
Network Rearrangements
Time Sync Holdover
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Noise (Time Error)
Budgeting Analysis
Common Time Reference (e.g. GPS time)
N
Network Time Reference
(e.g. GNSS Engine)
TEA
TEC
Packet
Master
(T-GM)
T-BC: Telecom Boundary Clock
PRTC: Primary Reference Time Clock
T-TSC: Telecom Time Slave Clock
T-GM: Telecom Grandmaster
R4
R3
R2
R1
PRTC
TEB
Packet
Network
R5
Packet
Slave Clock
(T-TSC)
End Application
Time Clock
Typical Target Requirements TED < 1.5 ms
•chain of T-GM, 10 T-BCs, T-TSC (LTE TDD, TD-SCDMA)
Simulation Reference Model:
•with and without SyncE support
Same limit applicable to R3 and R4 (limits in R4
applicable only in case of External Packet Slave Clock)
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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 (t)
1.5 us
|TE|
TEHO or TEREA budget
t
Holdover-Rearr. period
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)
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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)
PTP
C
T-TSC
1 PPS
D
End
Application
TED
Max|TE|
Test Equipment
max|TE’|
1500 ns
Max |TEC (t)| = max|TE’| + TEREA + TEEA < TED
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1100 ns
400 ns
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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
250ns
150 ns
200 ns
BC Internal Errors
(Constant)
550 ns
Link Asymmetries
250 ns
PRTC
100 ns
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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
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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)
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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
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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
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Partial Timing Support
HRM for G.8271.2
Time reference,
e.g. GNSS signal
PRTC
Test output,
T+t1
Packet
Master
Clock
Time Reference, T
TU
NE
TU
NE
Network
Element
with BC
Link containing up to M timingunaware network elements
TU
NE
Test output,
T+t2
TU
NE
Network
Element
with BC
Link containing up to M timingunaware network elements
Up to N network
elements containing BCs
TU
NE
Packet
Slave
Clock
TU
NE
Link containing up to M timingunaware 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)
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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 calibration
Handling of failures in the network
Still some important topics need to be completed
Stability requirements
Partial timing support (G.8271.2)
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Back Up
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Time Synchronization
via PTP
The basic principle is to distribute Time sync
reference by means of two-way time stamps
exchange
M
S
t1
Time Offset= t2 – t1 – Mean path delay
t2
Mean path delay = ((t2 – t1) + (t4 – t3)) /2
t3
t4
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)
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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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