A Closer Look at PDV and Oscillator for Packet Equipment Clocks

zarlink.com

A Closer Look at PDV and Oscillator for Packet Equipment Clocks

VOICE & TIMING SOLUTIONS

For a New Global Network

A Closer Look at PDV and Oscillator for

Packet Equipment Clocks

WSTS 2010

Peter Meyer

peter.meyer@zarlink.com


Frequency, Phase & Time Synchronization over

Packet Networks

� 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/PRC

PRTC

PEC-M

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PEC-B

PEC-B

PEC-B

PEC-B

PEC-B

PEC-B

PEC-B

PEC-S


Network Limits

� Network limits specify the maximum noise present on the timing signal at

different points in the network

PRS/PRC

PRTC

PEC-M

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PEC-B

Network Limits

PEC-B

PEC-B

PEC-B

PEC-B

PEC-B

PEC-B

PEC-S


Equipment Clock Characteristics

Equipment clock specifications define requirements for clock engines

� Jitter & Wander (Noise) Generation

– Output from PEC when synchronized to noise-free

input

� Jitter & Wander Transfer

– Output from PEC given a known input

� Jitter & Wander Tolerance

– Error free operation of the equipment with a known

input of maximum noise output

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ideal signal measured generation

PEC

known noise measured noise transfer

PEC

network noise limit error free operation

PEC


PEC-S 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

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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

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Eliminate Noise from

Packet Timing Signal


Network Limits & Characteristics – Metrics

� Performance of PEC-S depends heavily on PDV, or the

noise on the packet timing signal

� How to define the network limits & characteristics for PDV

– New metrics

– Under study at ITU-T Study Group 15 Question 13

– Existing metrics

– Definitions exist for delay variation, loss, availability, etc.

– Driven by non-synchronization applications (voice, video, data,

wireless)

– 3GPP: ‘all’ selection (e.g. 99% of packets less than 10 ms delay)

– MEF 10.2: “Ethernet Services Attributes Phase 2” FD, IFDV, FLR

– ITU Y.1540: “IP packet transfer and availability performance

parameters”

– IETF RFC3393: “IP Packet Delay Variation Metric for IP

Performance Metrics (IPPM)” IPDV

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Classic PDV Histogram


PEC-S Functional Model:

Packet Selection vs. ‘All’ Selection

� Several proposals to select packets experiencing the least delay,

known as the floor delay

� These packets deemed to be least affected by PDV and therefore would

best filter the noise from the PSN

� Seems reasonable …..

– Example where even an odd band of packets (10%-30%), a subset of the total

range, can improve the result

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PEC-S Functional Model:

Packet Selection vs. ‘All’ Selection

� … Except when then ‘floor delay’ is unstable or such ‘lucky’ packets are

few and far between

� The larger the time between ‘lucky’ packets the lower the sample rate

and the less ability to see local oscillator movement

� The use of more packets would allow to better filter the noise caused by

the PSN and better track local oscillator movements

� Example of network comparing ‘all’ packets with ‘floor delay’ packets

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Some Networks Resist Packet Selection

� xDSL networks, even if unloaded, have characteristics that resist the

concept of floor packet selection

� xDSL native asymmetry in upstream and downstream direction

� Access technologies such as GPON and EPON have their own

mechanisms to transfer frequency & phase that may be used

– PEC-S in the OLT and a PEC-M in the ONT

� Early interest in xDSL to also natively transfer frequency & phase

between DSLAM and modem (e.g. NTR)

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PDV Topics to Investigate

� Handling of transient detection & suppression, to address dynamic

packet network bursts, re-routes and outage phenomena

� Dealing with a variety of PSN PDVs that have different characteristics

from slow movements to fast movements

� Tackling a variety of physical layer transport technologies such as

xDSL

Ramp Ramp

Square On/Off

Fast Ramp Square On/Off

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Congestion


The Local Oscillator

� Oscillator datasheets generally specify

performance in a few categories

– Frequency: clock output (e.g. 20 MHz or 25 MHz)

– Initial offset: absolute frequency offset at

manufacture

– Ageing: relative frequency change over different

periods of time, such as 24 hours and 10 years

– Temperature: relative frequency change over a

specified temperature range

– Supply voltage: relative frequency change over a

specified power rail voltage range

– Load: relative frequency change over a specified load

change

– Warm-up: the time required after power-up to meet

the specification values – can range from minutes to

days to a month!

– Overall: worse case absolute frequency over ‘lifetime’

due to all causes

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The Local Oscillator – Specified in the

Frequency Domain for use in the Time Domain

� Generally oscillators do not specify performance parameters in the time

domain … whereas most ITU-T requirements are specified in the time

domain!

� Oscillator performance specifications cover frequency variation in the

frequency domain

– May see ADEV specified over 1 to 100 second tau

� ITU-T specification requirements generally provided in time domain

– TIE, MTIE, TDEV MTIE w/ppb

TDEV (s)

1.00E-05

1.00E-06

1.00E-07

1.00E-08

1.00E-09

1.00E-10

1.00E-11

TDEV

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

E1, TDEV, G.823, SEC

E1, TDEV, G.8261, EEC Option 1

Observation time (s)

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

T1, TDEV, T1.101, DS1 Sync Ref

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E1, TDEV, G.823, Sync PDH T1, TDEV, G.824, Option 2 SEC

T1, TDEV, G.8261, EEC Option 2

T1, TDEV, T1.105.09, SONET Ref

MTIE (s)

1.00E-04

1.00E-05

1.00E-06

1.00E-07

1.00E-08

1.00E-09

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

E1, MTIE, G.823, SEC

E1, MTIE, G.8261, EEC Option 1

E1, MTIE, G.8261, Case 2A

E1, MTIE, G.8261, Case 3

T1, MTIE, G.824, Sync Reference

T1, MTIE, T1.101, DS1 Sync Ref

16 ppb 50 ppb

Observation time (s)

E1, MTIE, G.823, Sync PDH E1, MTIE, G.8261, Case 1

E1, MTIE, G.823, Traffic PDH T1, MTIE, T1.101, OC-N Sync Ref

T1, MTIE, G.8261, Case 1

T1, MTIE, G.8261, Case 2a

T1, MTIE, G.8261, Case 3

T1, MTIE, G.824, Traffic PDH

T1, MTIE, T1.403, Traffic PDH


FFO (pbb)

A Frequency Domain Look at a Filtered

Oscillator

� What does the oscillator look like after high

frequency components have been filtered?

� Underlying movement is more clear once the

short term variation is filtered

15

10

-5

� Existing oscillator specifications may useable

for a definition of a mask for frequency

accuracy, such as mobile basestation

5

0

1 Hz Loop Filter

0 1 2 3 4 5 6 7 8

x 10 4

-10

Seconds

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FFO (pbb)

8

6

4

2

0

-2

-4

10 mHz Loop Filter

σ y (τ) (pbb)

0 1 2 3 4 5 6 7 8

x 10 4

-6

Seconds

10 -8

10 -9

10 0 10 -10

FFO (pbb)

8

6

4

2

0

-2

-4

Allan Deviation after 1 Hz Filtering

10 2

τ (sec)

1 mHz Loop Filter

10 4

0 1 2 3 4 5 6 7 8

x 10 4

-6

Seconds


A Time Domain Look at an Oscillator

� What does the oscillator look like in the time domain?

– How to specify the oscillator for the packet selection and loop filter?

� Method used to view the oscillator in the time domain information presented in

the next slides

– Measure Oscillator TIE in lab over 24hrs

– Extract FFO using a Simulink model

– Filter FFO based on different loop filter settings

– Subtract frequency offset for calculated FFO and estimate TIE

– Estimate TIE for each loop filter Setting

– Calculate TDEV based on estimated TIE

– Calculate ADEV based on calculated FFO

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TIE data from oscillator

+ +

-

to measure TDEV

Loop Filter

+

to measure FFO

D


A Time Domain Look at an Oscillator

� With a 10 mHz loop filter this oscillator has a low TIE and TDEV

noise

TIE (ns)

60

40

20

0

-20

-40

10 mHz Loop Filter

0 0.5 1 1.5 2 2.5 3 3.5 4

x 10 5

-60

Seconds

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TDEV (ns)

10 4

10 3

10 2

10 1

10 0

10 -1

10 -2

10 -3

10 -2

Computed TDEV

G.824 Envelop (T7/F5)

G.824 Envelop (T6/F4)

G.823 Envelop (T11/F9)

G.823 Envelop (T13/F11)

10 0

TDEV for 10 mHz Loop Filter

10 2

Observation interval τ (seconds)

10 4

10 6


TIE (ns)

A Time Domain Look at an Oscillator

� With a 1 mHz loop filter, equivalent of Stratum 3E, 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!

600

400

200

0

-200

-400

1 mHz Loop Filter

0 1 2 3 4 5 6 7 8

x 10 4

-600

Seconds

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TDEV (ns)

10 4

10 3

10 2

10 1

10 0

10 -2

10 -1

Computed TDEV

G.824 Envelop (T7/F5)

G.824 Envelop (T6/F4)

G.823 Envelop (T11/F9)

G.823 Envelop (T13/F11)

10 0

TDEV for 1 mHz Loop Filter

10 2

Observation interval τ (seconds)

10 4

10 6


Effect of the Local Oscillator on the Packet

Selection

� ‘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)

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+ =

Oscillator Observed Packet Delay

(Zoom)


Oscillator Topics to Investigate

� Beneficial to have oscillator performance specified in the time

domain

� Optimizations for packet-based clocks requirements, which may

be different than traditional electrical-based clocks (e.g. Stratum

clocks)

� Matching well the metrics and loop filter selection to an

acceptable price point

– Contributions for PEC-S without on path support at ITU-T generally

have selection windows and/or loop filters around 1 mHz (GR-1244

Stratum 3E / G.812 Type III) and 3 mHz (G.812 Type I)

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Thank-you for Your

Time & Attention

WSTS 2010

VOICE & TIMING SOLUTIONS

For a New Global Network

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