v1.0 - 20050426

Telecommunications Industry Association TR41.N.n-YY-MM-XXX

Document Cover Sheet

Project Number PN-3-0062-RV2 (TIA-921-B)

Document Title Packet Delay Modeling—From a network synchronization perspective

Source Anue Systems

Contact Name: Chip Webb Complete Address: 9111 Jollyville Rd Austin, TX 78759

Phone: 512-527-0453x102 Fax: Email: cwebb@anuesystems.com

Distribution TR-30.3

Intended Purpose of Document (Select one)

For Incorporation Into TIA Publication x For Information Other (describe) -

The document to which this cover statement is attached is submitted to a Formulating Group or sub-element thereof of the Telecommunications Industry Association (TIA) in accordance with the provisions of Sections 6.4.1–6.4.6 inclusive of the TIA Engineering Manual dated March 2005, all of which provisions are hereby incorporated by reference.

Abstract

An update on delay models for packet networks with a focus on network synchronization ..

Telecommunications Industry Association TR-30.3/09-03-004Arlington, VA, March 30-31, 2009

Packet Delay ModelingFrom a network synchronization perspective

TIA TR 30.3Arlington, VAMarch 2009

Outline

• Introduction• Status

• Need common test suites

• Issues• Burst Definition

• Simple way to understand PDV• Matlab approximation to help demystify

• Synthetic test cases• Some example results• Interesting 3D surface plots

• Recap

State of Sync

• Deployment of CES/TDMoIP/PTP has begun• Strong financial incentive to do so• Trials are ongoing with good success

• But is the technology really ready?• Not yet.• Examples:

• SLA metrics are insufficient to ensure success with Sync• Need common conformance and interoperability test suites• Allocate end-to-end PDV budget to individual NEs

• Drives future equipment development

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Need common test suites

• Three different approaches• Collect real world traces from actual networks

• Good because it is realistic• But no way to know if such traces are really worst case• Service providers are sensitive about publishing such measurements

• G.8261 Appendix VI test bed• Measure delays in a test bed under controlled conditions• Realism and repeatability depends on many factors

• Synthetic test cases• Create test cases that introduce controlled amounts of impairment• Can be unrealistic but still very useful

• E.g: sinusoidal tolerance tests for SONET/SDH• Can be realistic

• E.g: modem testsd

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Real world measurements

• Actual measurements are samples of network behavior. • Good for research and initial development• Not so good for ensuring interoperability or performance

margin in a real network.

• What if conditions change?• Network loads & characteristics increase• Technologies change (e.g. DOCSIS3.0, WiMAX, xDSL, PON)

• And: Harder to control• e.g. no control over results (e.g. minTDEV)

Real World Measurements

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GEM

RNC

Capture CES or PTP and save to PDVBidi & Multi stream

PDV #1

PDV #2

1

2

Real world measurements

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G.8261 App VI Test case: Seems straightforward

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Build & Measure a G.8261 App VI Test bed

• Test case descriptions in Appendix VI are incomplete• It probably seemed clear enough when written.• And it wasn’t important enough to be normative anyway.• Now, everyone is doing it.• Differently.

• Top three issues1. Load Definition

2. Burst Definition

3. Burst Definition

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A test bed• Measure delay under G.8261 Appendix VI test conditions…..

Definition of Load Percentage

• Load percentage is• A ratio: number of bits / max # bits possible• Measured over a specified time interval

• Longest possible time is to measure over a whole test case (one value/average)• Shortest possible time is to measure from one frame to the next (many values)

• Why does this matter?• Because we need to know what 80% load means

• If 80% is the peak load• Then 80% is max and applies only during the burst.• During gap, assume that 64 and 576 byte generators stay same

• If 80% is an average over the whole test case (nominal), • Then load measured during a single burst must be greater than 80%.• The amount by which it exceeds 80% determines the burst duty cycle.

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Bursts not fully specified

“Maximum size packets will occur in bursts lasting between 0.1s and 3s.”

• Two related questions:• How long between bursts?• What is the burst density?

• If “Load” means peak load• Then there’s no guidance on the gaps between bursts• Could pick fixed or random value, or just assume 50% duty cycle

• If “Load” means average load• Then gap time will depend on burst density

• proportional to the amount by which the load during a burst exceeds the nominal load

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Burst Parameters

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time

Load

Per

cent

age

Burst Length

Burst Load

Gap Length

NominalLoad

Burst Examples (30 sec. each: 20%, 50%, 80%)

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Burst Examples (30 sec. each: 20%, 50%, 80%)

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Burst Examples (30 sec. each: 20%, 50%, 80%)

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Are there limits on Burst Density? • Yes• For two reasons• TM1 has 64-byte disturbance

packets at a constant rate (CBR).

• Bandwidth limit (1G) of the disturbance load generators.• Though you could have separate

generators for each packet size ($)

MaxBurstLoad = 100% - NomLoad*(F64+F576)

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G.8261 App VI Test cases: What matters most?

• Primary factor: Configuration of disturbance load generator• Bursts (and gaps)• Disturbance packet size(s)

• Secondary factors• Type of switch (L2 or L2/L3)• CBR (almost or exactly … or not)

• Other important factors• Non-ethernet links (especially asymmetric access technologies)

• DSL, Cable Modem, PON (beating effects w.r.t. packet schedulers)• Uplink versus downlink

• Store & Forward (traditional ethernet) vs. cut-through (MPLS)

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What to expect PDV to look like? Why?function V = PDV(NumPkt, PktSz,

DistLd, TM,

NumSw, MinDelSw, UncDelSw)

MinDly = NumSw * (PktSz + MinDelSw); % Store-fwd delay

MaxLen = max(TM(:,1));

X = zeros(1,MaxLen); % X is PDV for one switch

for d=1:size(TM,1)

len = TM(d,1);

frac = TM(d,2);

X = X + (frac/len)*[ones(1,len), zeros(1,MaxLen-len)];

end

V = zeros(1,MaxLen*(NumSw-1)); % V is end-to end PDV

V(1) = binopdf(0,NumSw-1,max(1e-4,DistLd));

for j=1:NumSw-1

tmp = X;

for k=2:j

tmp = conv(tmp,X);

end

weight = binopdf(j,NumSw-1,max(1e-4,DistLd));

V = V +

weight*[tmp,zeros(1,MaxLen*(NumSw-1)-size(tmp,2))];

end

V = [zeros(1,MinDly) NumPkt*V];

V = conv(V,ClkUncer(NumSw,UncDelSw));

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What to expect PDV to look like?

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Model-based impairment profiles

• Build a bottom-up model of each network element• Construct test scenarios by connecting together various model

elements and run a simulation• Validate end-to-end results

• Challenges• As network changes, model params must also change• New technologies may require new model elements• Harder to make a general model than to measure one sample

Discrete event simulation of test bed..

• One way to model is to use a discrete event simulator• Develop models for the switches and dummy traffic generators. • Anue has developed one such model• This is just the beginning of modeling

• Further refinements are possible

• Plots shown are for G.8261 test beds configured per Appendix VI• TM2 bursts assume Bursts that have constant load (70%-60%)• 10 store/fwd GigE switches, 20 disturbance load generators (fwd/rev)• Clock uncertainty (each switch is asynchronous)• No priority or congestion avoidance (QoS, RED/WFQ, VOQ)

Example model results (20% load TM2)

Reference (test bed)S/W Model Results

G.8261 Test Case 2 (TM2) Load Steps

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G.8261 Test Case 3 (TM2) Slow Load Ramp 24hr

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G.8261 TC3: variations showing 3D PDV

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Delay50us

150us

20%

80%Load

G.8261 TC3: variations showing 3D PDV

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Delay

50us

150us

20%

80%Load

G.8261 TC7 (TM2 E1 16ppb) - Beating

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G.8261 TC8 partA (Blocking)

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Bursts of max len pkts:(this dimension is governedby the burst density andmax packet size)

Gap between bursts:(this dimension is governedby the min and mediumsize packets)

3D Surface plot for TC8 (blocking)

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Delay50us

150us

0%

50%

Load

3D Surface plot for TC8 (blocking)

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Delay

50us

150us

0%50%

Load

G.8261 TC13 TM2 (fwd/rev steps)

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Recap

• Deployment has started• Need metrics for measuring PDV• Need common, repeatable test cases

• Focus on conformance and interoperability

• Burst parameters need better specification• Simple matlab code can predict first order PDV behavior

• Helps understand measurements from lab

• Synthetic model• Results are quite realistic

Thank You!

Questions?

• Contact me!• Chip Webb

Anue Systems9111 Jollyville RdSuite 100cwebb@anuesystems.com (preferred)+1 (512) 527-0453 x102

Backup Slides

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Effect of reverse traffic:For L2/L3 enterprise switch

Capture

Packets

Effect of reverse traffic:For L2/L3 enterprise switch

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Effect of reverse traffic: minTDEV

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