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Oscillator Impact on PDV and Design of Packet Equipment Clocks

ITSF 2010Peter Meyer

[email protected]

[] C O N F I D E N T I A LOctober 15, 2010

Protocol Layer Synchronization• When deployed and inter-connected within the packet network the packet

equipment clocks will allow frequency, phase and time to be transferred over the packet network

• Different types of packet equipment clocks (PEC)– PEC-M the input is physical timing and the output is packet timing signal– PEC-B the input is a packet timing signal and the output is a packet timing signal– PEC-S the input is a packet timing signal and the output is a physical timing signal

PRS/PRCPRTC

PEC-M

PEC-B

PEC-B

PEC-B

PEC-B

PEC-B

PEC-B

PEC-B

PEC-BPEC-B

PEC-BPEC-B

PEC-BPEC-B

PEC-BPEC-B

PEC-BPEC-B

PEC-BPEC-B

PEC-BPEC-B

PEC-S


Frequency & Time Transfer over PSN

• Two approaches– A PSN may be inserted between the server and client, that is not aware of protocol

layer synchronization packets (e.g. IEEE 1588-2008)

– The PSN has ‘on-path support’ where each switch / router is aware of protocol layer synchronization packets (e.g. IEEE 1588-2008)

PEC Model & Generic Requirements


Protocol Layer EC Functional Model

• ITU-T G.8263 (draft) Annex includes a functional model of a PEC-S packet-based clock

• PEC differs from traditional EC with introduction of a packet selection block has been included

• The PLL filters the network wander with a low pass filter

• This means the PLL acts as a high pass filter for the local XO

Low Pass filter Oscillator

Local Timescale

Output ClockPacket TimingSignal

PacketSelection

Local reference

Time ScaleComparator

PEC-S


PEC-S Functional Model: Packet Selection & Low Pass Filter• Goal of the packet selection block is to select from all the input packets to

the packet equipment clock a certain subset that are the least affected by the packet switched network

• These packets would thus best reflect the timing signal at the transmitter• Both the packet selection block and the low pass filter function to remove

noise from the packet timing signal to faithfully re-create the timing source• The ‘cleaned’ timing signal can then be used to discipline the local oscillator

Eliminate Noise fromPacket Timing Signal


Equipment Clock Specifications

• Definition of EC– Jitter & Wander Generation– Jitter & Wander Transfer– Jitter & Wander Tolerance– Holdover– Transients– Freerun

• Oscillator dominant factor in meeting parts of the specification– Wander Generation (both MTIE & TDEV)– Holdover Stability (both constant & variable temperature)– Freerun Accuracy

Packet EC Model


Oscillator-Dependent EC Characteristics

• Wander Generation– The amount of wander generated by the EC when locked to an

ideal reference– Oscillator noise measured in the time domain using MTIE & TDEV

metrics• Holdover Stability

– The stability of an EC when after losing lock to its input reference– Oscillator drift due to ageing, temperature, voltage and other

effects measured in the frequency domain• Freerun Accuracy

– The accuracy of an EC without using an input reference– Oscillator error due to all error sources in the frequency domain


Example: Oscillator Requirements for Stratum 3E• Looking at Stratum 3E EC, with a focus on the oscillator, yields the

following requirements to be met by the oscillator specification

• Other ECs (Stratum 3, SMC, etc.) would have similar requirements

• Requirements– Free-run Frequency Accuracy

• ±4.6 ppm

– Wander Generation• MTIE & TDEV masks specified in ITU-T G.812 Type III & Telcordia GR-1244-

CORE Stratum 3E, using 1 mHz clock bandwidth

– Holdover Stability• ± 1 ppb/day at constant temperature (1.16x10^-5 ns/s^2)• 10 ppb over temperature range

Design Considerations of Packet Equipment Clock


PEC Design Considerations

• Trade-off between PDV noise (LPF) and XO noise (HPF)

• Effects of XO on packet selection• Possible PEC characteristics & XO

requirementst

Before Filtering

tAfter Filtering


Trade-off Between PDV and XO• PDV and XO noise can be shown on a frequency spectrum plot• Network PDV has wide frequency spectrum

– Ramp test case has 12 uHz fundamental frequency– On/Off test case has 139 uHz fundamental frequency

• XO has increasing magnitude at low frequency

f0

Network PDVOCXO

TCXO

XO

f0

Network PDVOCXO

TCXO

XO

Low PassFilter Value

• Where to place the loop filter?– May be governed by wander generation of XO– No specification defined for PEC– Common specification may not apply for mobile backhaul

TCXO> HPF

PDV< LPF


1.00E-09

1.00E-08

1.00E-07

1.00E-06

1.00E-05

1.00E-02 1.00E-01 1.00E+00 1.00E+01 1.00E+02 1.00E+03 1.00E+04 1.00E+05 1.00E+06

Observation time (s)

TDEV

(s)

E1, TDEV, G.823, SECE1, TDEV, G.8261, EEC Option 1

E1, TDEV, G.823, Sync PDH T1, TDEV, G.824, Option 2 SECT1, TDEV, G.8261, EEC Option 2T1, TDEV, T1.105.09, SONET Ref

T1, TDEV, T1.101, OC-N Sync Ref T1, TDEV, G.824, Sync ReferenceT1, TDEV, T1.101, DS1 Sync Ref

Ramp (TC13) and Square (TC14) Fundamental Frequency 25000s tau:

12 uHz Ramp TC142158 tau:139 uHz Square TC13

0.1 mHz1 mHz10 mHz100 mHz


1.00E-11

1.00E-10

1.00E-09

1.00E-08

1.00E-07

1.00E-06

1.00E-05

1.00E-02 1.00E-01 1.00E+00 1.00E+01 1.00E+02 1.00E+03 1.00E+04 1.00E+05 1.00E+06


TDEV

(s)

E1, TDEV, G.823, SECE1, TDEV, G.8261, EEC Option 1

E1, TDEV, G.823, Sync PDH 1 mHz: TC13, Square, TM2 0.1 mHz: TC13, Square, TM2

Ramp (TC13) TDEV25000s tau:12 uHz Ramp TC14

2158 tau:139 uHz Square TC13

0.1 mHz1 mHz10 mHz100 mHz


Ramp (TC13) MTIE

1.00E-10

1.00E-09

1.00E-08

1.00E-07

1.00E-06

1.00E-05

1.00E-04

1.00E-02 1.00E-01 1.00E+00 1.00E+01 1.00E+02 1.00E+03 1.00E+04 1.00E+05


MTI

E (s

)

E1, MTIE, G.823, SECE1, MTIE, G.8261, EEC Option 1

E1, MTIE, G.823, Sync PDH 0.1 mHz: TC13, Square, TM2 1 mHz: TC13, Square, TM2

Relationship between PDV, XO and Clock Bandwidth


Wander Generation vs. Clock Bandwidth• With a 10 mHz loop filter this oscillator has a low TIE and TDEV noise

• With a 1 mHz loop filter there is significantly MORE noise contributed by the oscillator A lower the loop filter will filter LESS oscillator noise

• Cannot keep lowering the loop filter to be more robust against PDV without increasing the cost of the equipment!

0 0.5 1 1.5 2 2.5 3 3.5 4

x 105

-60

-40

-20

0

20

40

60

Seconds

TIE

(ns)

10 mHz Loop Filter

10-2 100 102 104 10610-3

10-2

10-1

100

101

102

103

104

Observation interval (seconds)

TDE

V (n

s)

TDEV for 10 mHz Loop Filter

Computed TDEVG.824 Envelop (T7/F5)G.824 Envelop (T6/F4)G.823 Envelop (T11/F9)G.823 Envelop (T13/F11)

0 1 2 3 4 5 6 7 8

x 104

-600

-400

-200

0

200

400

600

Seconds

TIE

(ns)

1 mHz Loop Filter

10-2 100 102 104 10610-1

100

101

102

103

104

Observation interval (seconds)

TDE

V (n

s)

TDEV for 1 mHz Loop Filter

Computed TDEVG.824 Envelop (T7/F5)G.824 Envelop (T6/F4)G.823 Envelop (T11/F9)G.823 Envelop (T13/F11)


Wander Generation vs. Clock Bandwidth

• Wander Generation MTIE– 3 mHz, 1 mHz, 0.3 mHz & 0.1 mHz clock bandwidths– 1 mHz 0.1 mHz results in 10x more wander @ 8000 s


• Wander Generation TDEV– 3 mHz, 1 mHz, 0.3 mHz & 0.1 mHz clock bandwidths– 1 mHz 0.1 mHz results in >4x more wander @ 1000 s

Wander Generation vs. Clock Bandwidth

Oscillator Selection Impact on Packet Selection


Packet Selection vs. Oscillator

• ‘Cleaned’ packet timing signal used to discipline local oscillator

• Will the oscillator movement impact on the packet selection to reduce estimated performance– If there was originally a stable floor delay, how does it appear to

move based on a non-ideal local oscillator?– What is inter-packet gap between selected packets and how

should this be adjusted to match the non-ideal local oscillator?

+ =

Packet Delay(Zoom)

Oscillator Observed Packet Delay(Zoom)


Packet Selection vs. Oscillator: Histogram• Two Oscillators• Same Clock Bandwidth, Packet Selection, PDV

• Observation: FWPR is reduced


Packet Selection vs. Oscillator: MAFE• Two Oscillators• Same Clock Bandwidth, Packet Selection, PDV

• Observation: Frequency accuracy not greatly impacted for typical mobile backhaul application

4 ppb1000 seconds

3 ppb1000 seconds


Packet Selection vs. Oscillator: Packet Timing Signal MTIE• Two Oscillators• Same Clock Bandwidth, Packet Selection, PDV

• Observation: MTIE substantially impacted relative to synchronization performance requirements


Packet Selection vs. Oscillator & Clock Bandwidth• Summary

– XO directly impacts wander generation conformance, a parameter defined in the time domain

• Absence of time domain characterization in XO makes component selection difficult

– Time domain is significantly impacted by oscillator selection vs. packet selection & clock bandwidth

• Lack of standard for PEC results in freedom to optimize clock bandwidth based on custom design choices

– Frequency domain performance is less impacted by oscillator selection vs. packet selection & clock bandwidth

• Specifically the mobile backhaul application (< 50 ppb accuracy)• Target application is very forgiving of XO selection

– Lowest hanging fruit• PEC for applications requiring only frequency accuracy, such as

mobile basestation, are easier to design based on traditional XO characterization information

Thank-you for Your Time & Attention

ITSF 2010

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