Dynamic versus Static Traffic Policing: A New Approach for Videoconference Traffic over Wireless Cellular Networks

Author: Polychronis Koutsakis

Anukrati Gupta / Manoj Maskara

CS: 6204 - Mobile Computing, Fall 2009

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Outline

• Introduction• Policing Mechanism• System Model• Model Definition• Channel Error Model• The Token Bucket – Basic, Dual & Triple• Jumping Window• Sliding Window• Conclusions

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Introduction…1/2• Video traffic is becoming a major portion of the traffic carried

over wired and wireless networks

• Video traffic calls for:

– New sets of control procedures

– Strict Qos requirements

– Strict packet delay requirements

– Strict packet dropping requirements

• Video traffic is modeled using various statistical models like – lognormal, Gamma, and a hybrid Gamma/lognormal distribution

• Videoconference traffic needs to be modeled differently – reason?

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Introduction…2/2

• Inherent Characteristics of Videoconference – very high autocorrelation

• Video coding standards – H.261, H.262, H.263, H.264• Latest version H.264• Widely used H.263 – Compresses moving picture

component of audio/video services at low bit rates.

“This is the first work in the literature which addresses the problem of modeling single H.263 videoconference traces” – Koutsakis

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Policing Mechanism…1/2

Static and Dynamic – based on traffic flow

• Token Bucket and its variations

• Jumping Window

• Sliding Window (Moving Window)

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Policing Mechanism…2/2

• Focus on traffic control mechanism at the entrance of the system to prevent congestion

• Implemented all three traffic policing mechanism and proposed modification to improve performance

• Used GBAR process (gamma distribution)

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System Model….1/3MSC

Parking Lot

BS

MS MSMS

BS

MS MSMS

•The study focuses on one cell of the network, also called picocell

•Source terminals are spatially dispersed and share a radio channel

•Radio channel connects the source terminals to the fixed Base Station (BS)

•The BS allocates channel resources, delivers feedback information, and provides an interface to the Mobile Switching Center (MSC)

•The MSC provides access to the fixed network infrastructure

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System Model….2/3

•Downlink wireless channel (20Mbps) – BS to MS

•Divided into time frames of equal length

•Each frame has a duration of 12ms and 566 information slots

•Information slot contains 1 fixed length packet with a header & information

•3% or 16 slots (control Interval) of the bandwidth is used for uplink

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System Model….3/3

• The information Interval of the frame = 19.43 Mbps needs efficient traffic policing mechanism

• Used computer simulations to study the performance of the traffic policing mechanisms

• Used C programming language to conduct simulations

• Each simulation point is the result of an average of 10 independent runs

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Model Definition…1/2

• Video conference traffic is modeled using GBAR (1) model as it takes advantage of the properties of Pearson V distribution

GBAR (1) model is based on following results

shape parameter

scale parameter

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Model Definition…2/2

• If Xn-1 = Ga(β, λ), and

An = Be(α, β- α) and

Bn = Ga(β- α, λ) , and all are mutually independent, then GBAR(1) process is defined by {Xn}

where Xn = An Xn-1 + Bn

Since the current value is determined by only one

previous value, this is an autoregressive process of

order 1

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Pearson V Distribution

• The parameters (α, β) of the pearson V is computed using GBAR(1) mean and variance

Mean = β/(α – 1)Variance = β2/[(α – 1)2 (α – 2)]

• Pearson V distribution (Zn) is also known as the inverted gamma distribution which is defined as

Zn = 1/ Xn

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Traces Used

• A video stream from Office Cam

• A video stream from Lecture Cam

• A video stream from Parking Cam

• A video stream from N3 Talk

• A video stream from ARD Talk

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ComparisonBurstiness = peak/mean

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Channel Error Model…1/2

15 States Markov Model Diagram

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Channel Error Model…2/2

• State S0 represents “good state” and all other states represents “bad states”

• When channel is in S0 it can either remain in S0 or can transition to state S1

• When channel is in bad state it can transition to either the next higher state or back to state S0 based on the received packet

• pgood is 0.99995 due to very strict QoS requirements

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Concepts to help understand Token approach

• Mean rate—Also called the committed information rate (CIR), it specifies how much data can be sent or forwarded per unit time on average.

• Burst size—Also called the committed burst (Bc) size, it specifies for each burst how much data can be sent within a given time without creating scheduling concerns.

• Time interval—Also called the measurement interval, it specifies the amount of time in seconds per burst.

mean rate = burst size / time interval

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Token Bucket Basic ApproachStatic Approach

Conforming –> Size of Token = Packet Size

Non Conforming –> Size of Token < Packet Size

Packets either wait or discarded

Max no. of Tokens in bucket = Size of Bucket

Overflow – New incoming tokens are discarded

Token

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The Dual Token Bucket ApproachProposed method of generating tokens with the use of Pearson V-based variant of the GBAR model

Token

Token

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Packet Loss Comparison Static versus Dual Token Bucket

bucket size equal to the declared peak frame size of the source

Increase in bucket size results in decrease in video packet dropping and the difference between the static approach and dynamic method decreases.

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High packet dropping - Solutions

Three solutions to deal with high packet droppingrate in dual token bucket as seen in previous slide. Sol 1 - Infinite buffer sizeThird bucket is introduced for other two solutionsSol 2 - Use Triple Token Bucket where third bucketis of fixed sizeSol 3 - Use Triple Token Bucket where third bucketis of dynamic nature whose size varies dependingon source transmission rate – DYNAMIC

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The Triple Token Bucket Approach

Behavior of Bucket 1 & Bucket 2 is similar to Dual Bucket Scheme.

When Bucket 2 is full, it start marking packets with “non-conform”

All 3 buckets are connected in parallel

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Properties of 3rd Bucket

• Responsible for source packet drop• Token rate = average size of “token frame”• Token frame is equal to the average no. of token

needed in each video frame for the source to lose only 0.01% of packets

• In case of dynamic bucket size varies:– If source transmit at higher rate than its mean,

size of bucket is reduced in order to prevent long overuse of network resources by malicious users

– If source transmit at lower rate, size of bucket is increased to help accommodate anticipated bursts

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Function of Buckets in Different Approaches

Dual Token Bucket Approach

Triple Token Bucket Approach

Bucket 1 Same as Token Bucket basic approach

Same as Token Bucket basic approach

Bucket 2 Once full, packets are “discarded”

Once full, packets are marked as “non-conform”

Bucket 3 ---- It handles packets that are marked “non-conform”

Static Bucket Size

Dynamic Bucket Size

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Jumping Window

• Size of window is of fixed length T, placed side by side through time

• During a window only K bytes (or packets) are submitted by the source to the network

• Source transmits > K bytes = Packets are dropped/marked non conformed

• Mechanism is implemented using Token Counter

• In each new window, packet counter starts with initial value of zero

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Modification in Jumping Window

Token Bucket Jumping Window

Reason: Significantly larger window size (40 frames, 3.2 seconds)

• If less than K bytes are transmitted within a window then token counter is not set to 0

• It starts with an initial value equal to the remaining tokens

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Sliding Window• Similar to Jumping Window• Difference – Each video frame size is remembered for the width of

exactly one window, starting with specific video frame and ending T frames later

• This mechanism can be interpreted as a window which steadily moves along the time axis, with the requirement that the frame size of T frames are stored for the duration of one window

• Complexity is directly related to the window size. • Strictest bandwidth enforcement.

> Jumping Window

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Modification of Sliding Window

To implement more dynamic policy:

in case of less than K bytes transmitted by

the source within one window W, the tokens

left in the bucket are not discarded, but they

are added to the token bucket of the next

window (W+1)

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Conclusion• Dynamic approach provides significantly better results in

policing the burstiness of video traffic sources than static traffic policing mechanisms

• Proposed triple token bucket scheme, which showed the best performance

• Showed results using various example and studied the Token Bucket, Jumping Window and Sliding Window schemes

• Low paying users – Triple Token Bucket or Triple Sliding Window

• High paying users – Triple Jumping Window• Token Window less complex to implement rather than

Sliding Window but is more sticker.• Videoconference applications are very greedy in terms of

bandwidth requirements

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Thanks and wish you all Good Luck for exam next week !!!