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CONFIDENTIAL INFORMATION
Training Session
Packet BasedSynchronization
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Overview
Drivers for Packet based synchronisation Traditional Vs Packet based synchronisation
PTP and NTP
Related Standards
Oscillator requirements for Packet based synchronisation
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CONFIDENTIAL INFORMATION
Time Division Multiplexing
Time Slot (Channel) :for an established conversation, 8 bits of information (voice sample) are transmitted during a specific time slot.To maintain voice quality, voice samples HAVE to be transmitted every 125s, regardless of the number of channels in the stream
In order to assign more than one subscriber to one or two pairs of cable wire, each subscriber is allocated aparticular time slot (channel) for sending and receiving information.
Central Office
Tx0
Tx1
Tx31
Time Slot 0
Time Slot1
Time Slot 31
E1 Stream of information
CH31CH1CH0
125s
CH0
125s
125s
125s
SLIC CODEC
SLIC CODEC
CODECSLIC
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CONFIDENTIAL INFORMATION
TDM World to Packet World
Time Division Vs Statistical Efficient and low cost
No perfect synchronization required
Bursty traffic like email or internet traffic
Higher level flow control like pause frames
XO
DATA DATA
PHY Layer
Layer 2 - 7
PHY Layer
Layer 2 - 7
XO+/-100ppm +/-100ppm
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Mobile Backhaul is most affected
Requirements for Air interface & Backhaul 16ppb for network and 50ppb for air interface E1/T1
connections provided synchronization
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Base Transceiver Station
(BTS)
Base Transceiver Station
(BTS)
Base Station Controller
(BSC)
Mobile Switching Centre
(MSC)
To CO
Base Transceiver Station(BTS)
Base Transceiver Station
(BTS)
Base Station Controller
(BTS)
Mobile Switching
Centre (MSC)
To CO
(Node B )
(RNC ) (SGSN )(GGSN )
To ISP
2G
3G
4G
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SyncE and Timing over Packets
SyncE Extending traditional Sync to Ethernet Point to point physical layer technology
Transfers frequency
Timing over packet network
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PLL
Timing Distribution Using ToP
BITS/SSU
PLL
ZL30106
ZL30106
1GbEPHYZL30106
ZL30106
1GbEPHY
PRS
TimingPackets
+/-100ppm
DataPackets
ZL30106
ZL30106
1GbEPHY
+/-100ppm
ServerToP
Engine
PLL
TimingPackets
TimingPackets
ZL30106
ZL30106
1GbEPHY
SlaveToP
Engine
Synchronous Ethernet
ZL30106
ZL30106
1GbEPHY
BITS/SSU
PRS
Data Data
ZL30106
ZL30106
1GbEPHY
ZL30106
ZL30106
1GbEPHYZL30106
ZL30106
1GbEPHYZL30106
ZL30106
1GbEPHY
SyncEDPLL
SyncEDPLL
SyncEDPLL
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Packet timing
Precision Time Protocol (IEEE 1588) and Network Time
Protocol (NTP)
Both uses Time Stamps Time encoded in packets when they leave interface
Protocol ensures the flow of time stamps and is standard Algorithm handles and generates clocks and is proprietary
Phase and Time are known Since there is two way transfer, the round trip delay is known
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Master
Slave
t1t2
t3
t4
Round Trip Delay =2
)34()12( tttt
Offset =2
)34()12( tttt
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Types of PTP Clocks
Grand Master, Boundary Clock, Transparent Clock &Ordinary Clock
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GRAND
Ordinary
Clock
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Clock Recovery methods
Filtering of significant instances inputs to filter
For Physical clocks
The transitions on the physical line
For Packet clocks
The packet arrival times
The time stamps at the source and destination
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1
10
20
6
15
25
t1=4
Sync message containing an
approximation of t1
t2=11
Follow_Up message containing the
precise sending time (t1)
t3=20Delay_Req message
Delay_Resp message containing t4
t4=17
MasterSlave
t2-t1=Delay+Offsett4-t
3=Delay-Offset
Offset=[(t2-t
1)-( t
4-t
3)]/2
Delay=[(t2-t
1)+( t
4-t
3)]/2
In this example
Delay = 2
Offset = 5
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What is fundamentally different?
The pdf (Probability Distribution function) Is Stationary in nature (Defined mean and variance) for
physical clocks
Packet based significant events Packet delay variation Notstationary
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Selected packets as significant events
Packets with minimal delay Selected for filtering
Networks are required to meet performance conditions 1% of the timing packets sent by the packet master remain in the 150 s fixed cluster range, starting at the
floor delay in every observation window of 200 s.
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-4
-3
-2
-1
0
1
2
3
4
0 50 100 150 200 250 300 350 400 450 500Delay
Time
Floor Delay
150 S
200s 200s 200s 200s 200s
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Challenges
Packet Clock Recovery Challenges Master Accuracy
Timing Packet Rates
Number of nodes from Master to Slave
Packet Size mix in the network
Queuing techniques in Switches & Routers
Underlying transport mechanism (DSL, Microwave)
Asymmetry of the network (Fibre delays)
Incomplete Standards bodies directions
Many current implementations are on field trials
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Standards Activities
PEC Packet Equipment Clock
On Path Support / Aware All intermediate nodesBC/SyncE
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Category PEC with Frequency,Phase/Time
(Unaware Networks)
PEC Frequency ONLY
(No On-Path Support /
Unaware Networks)
PEC with Phase/Time
(With On-Path Support / Aware)
Network
Limit
Various network configurations
Implementations with
1. 5 switch, no SyncE
2. 10 switch + 10 SyncE
G.8261 and G.8261.1
(Timing and synchronization
aspects in packet networks
(frequency))
G.8271
(Time and phase synchronization
aspects in packet networks)
Equipment
Limit
No Standards available
Proprietary implementations
G.8263.1 [Master]
G.8263.2 [Slave]
Packet Master and Slave
Performance guidelines
G.8272 [PRTC]
G.8273.1 [Master]
G.8273.2 [BC w/SyncE]
G.8723.2 [BC wo/SyncE]
G.8273.3 [TC]
G.8273.4 [Slave]
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Traditional filtering Vs packet clocks
Traditional Stratum 3 filters are 0.1 Hz to 10z
Time constants are 0.01s to 1.6s
Packet Clocks has narrower filters due to nature ofsignificant events 1mHz to .05mHz or lower!
Time constants are 160 seconds to 54 minutes
This means the PLL control will change the output at alower rate
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f2
1
t
F(t)
t
F(t)
54 minutes
63%PLL
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Impact of the Oscillator
Stratum 3 time constant is ~2 seconds The oscillator has not much effect
Packet clocks have time constant of, say, 54 minutes
The oscillator has big effect especially temperature changes
A change of 0.5C/min is about 30C!
F v T performance and Aging directly impacts the packetbased clocks
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Phase detector
&
Low Pass Filter
DCO
XO
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Temperature effects of Oscillators
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TemperatureSensor
CompensationNetwork or
Computer
XO
Temperature Compensated (TCXO)
-450Cf
f
+1 ppm
-1 ppm
+1000CT
Oven
control
XO
Temperature
Sensor
Oven
Oven Controlled (OCXO)
-450Cf
f+1 x 10-8
-1 x 10-8
+1000CT
Voltage
Tune
Output
Crystal Oscillator (XO)
-450C
-10 ppm
+10 ppm
250C
T
+1000C
f
f
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Oven Control methods
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TemperatureLower
Turnover
Point (LTP)
UpperTurnover
Point (UTP)
f (UTP)
f (LTP)
Freq
uency Oven Set Point
-40C 85C
-40C 85C
Oven temperature variation
due to external temperature
change
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PDV filtering with Various Oscillator types
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+
Filter
=
Filter
+
Filter
=
Filter
+
Filter
=
Filter
PDV of Network
PDV of Network
PDV of Network
Oscillator Noise - XO
Oscillator Noise - TCXO
Oscillator Noise -OCXO
PLL Output Noise
PLL Output Noise
PLL Output Noise
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Oscillator dependence
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Free-run Accuracy The accuracy of an PEC without using an input
reference Oscillator error due to all error sources in the frequency
domain
Wander Generation The amount of wander generated by the PEC when
locked to an ideal reference Oscillator noise measured in the time domain using
MTIE & TDEV metrics
Holdover Stability The stability of an PEC when after losing lock to its
input reference Oscillator drift due to ageing, temperature, voltage and
other effects measured in the frequency domain
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Oscillator noise effect on frequency
The frequency accuracy of the system is directlyimpacted by (Oscillator related)
Temperature variations
Ageing of the oscillator
Loop bandwidth
Eg. An oscillator with temperature stability 100ppb,ageing 10ppb/24 hours.
Assuming the loop bandwidth is small enough not to remove theageing effect, we can approximate a 5 day frequency stability
as: 100ppb + 5 days * 10ppb = 150ppb is the worst case stability
after 5 days
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Stratum Hierarchy
Stratum 3E 12ppb
Stratum 3 370ppb
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Pluto
Mercury
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Oscillator impact on Phase
Phase error is accumulation of frequency error
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F1
F2
PhaseE
r
ror
t
F1
F2
Constant frequency difference
t
F1
F2
Linear frequency difference
Phase is polynomial
of degree 2
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Impact of Slope Spec on Phase
Slope spec has parabolic effect on Phase
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0
5
10
15
20
25
0
10
20
30
40
50
60
70
80
90
100 200 300 400 500 600 700 800 900 10001100120013001400150016001700180019002000210022002300240025002600270028002900
F1
F2
F1 Phase
F2 Phase
Phase and Frequency (F1)
Phase and Frequency (F2) Phase
Error Limit
Time to
reach
limit = 550s
Time to reachlimit = 1550s
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Effect Slope Spec of Oscillator
The OCXO1 is better than OCXO2, because of slope spec
Saturday, 31 August 2013 24
-40 85
OCXO1
OCXO2
Temp window of operation
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Rakon Standards Efforts ITU T
Contribute to ITU with a temperature profile
Oscillator output variations within G.8263 limits To be added as an appendix in the next release
Limited temperature excursions It is proposed therefore that a limit of +/-20C movement at
(0.5C/min or 1C/min as required) be taken to cover both theexternal and internal environmental effects.
Where ts is the test stabilisation time, tL is the time required forthe loop to recover, T is the maximum temperature excursion,T/ t is the ramp rate
Saturday, 31 August 2013 25
tL
tL
tL
tLT
tL
ts T/ t
T
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Mercury G.8263 Compliant*
* When the temperature profile is applied
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Rakon Standards Effortsd
The d parameter
Used to be historically mentioned in standards without mentionof how to measure.
Rakon supported to get rid of this in new G.8263
Enables Mercury to comply with G.8263 holdover requirements
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Rakon Standards Efforts Others
The following contributions are under consideration
Rakon supports the Wander generation specification
to be adjusted for 11ppb including the temperature efforts, fromthe current 10ppb
Gives a bit more room for the loop
Oscillator start up conditions for PDV tolerance testing
Appendix I of G.8263 about PDV tolerance testing methodologymay contain the following text
The recommended oscillator startup considerations may beconsulted before the testing of the PDV tolerance starts.
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Suggested Selection Table
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Oscillator Type Rakon OscillatorFamily
Oscillator Parameters
Time to reach phase error
limit with +/-20 temperature
variation @ 10C/hour
Total Phase
movement
Cost
Indicator
Freq v Temp Daily
Ageing
1S 3 S 7 S 24 Hours
(Constant
Temp)
OCXO ROX Stratum 20.1ppb
0.05ppb/day
12
Hours
48 hours 144
Hours
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Summary
Technology change from Circuit Switched to packetswitched networks
Challenges for Synchronization
Methods for Packet Synchronization
Importance of Oscillators on Packet Synchronization
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The End
Thank you