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Playout Buffer Management

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Network

Encoder

SenderMiddlebox

Receiver

Decoder

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How to recv and play?open socketwhile not done

if socket is readable read packet from socketremove RTP headerdecodeplay back

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What’s Wrong?packet orderingpacket lossnext packet arrive in-time?

Especially bad for audio applications

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Detour: A Brief Intro to Audio Conferencing

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Audio CompressionNormally uncompressed

Telephone quality: 8-bit audio 8Khz

20-30ms per packet

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Common TechniqueSilence Detection

No need to send if there is no sound at the input

Talkspurt Sequence of consecutive audio

packets (in between silence)

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Detecting Talkspurt in RTPMarker bit

Not reliable as packet with marker bit could be lost

Deduce from timestamp and sequence number

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Delay Jitter

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What causes Jitter?Network delay =

Propagation Delay (fixed) +Queueing Delay (variable)

Delay jitter is caused by variable queueing delay

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Delay Jitter

Time

TransitTime

small jitter

large jitter

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Spike

Time

TransitTime

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Buffer: The Jitter Absorber

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If Zero Jitter

Time

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With Jitter

TimeLOSS

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With Jitter + Buffer

Time

Buffer Size

Playout Delay

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Q: How to set playout delay?

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Adapting Playout DelayWhen jitter is low, reduce delayWhen jitter is high, increase delay

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Fundamental Trade-off

Latency vs Packet Loss

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Playout DelayOnce you set the playout delay,

cannot change!

NOT true: can change at beginning of talkspurt

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Adapting Playout Delay

SEND

RECV

PLAY

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Adaptation Algorithm

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Variables and Notations

Tsend(i)

Tplay(i)

Tbuffer(i)

Tarrive(i)

Tdelay(i)

Tnet(i)

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First Packet in the TalkspurtTdelay(i) = Enet(i) + 4 Vnet(i)Tplay(i) = Tsend(i) + Tdelay(i)

Tsend(i)

Tplay(i)

Tbuffer(i)

Tarrive(i)

Tdelay(i)

Tnet(i)

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Subsequent PacketsTplay(j) = Tplay(i) + Tsend(j) – Tsend(i)

Tsend(i)

Tplay(i)

Tbuffer(i)

Tarrive(i)

Tdelay(i)

Tnet(i)

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How to estimate Enet(i)

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Algorithm 1 (Jacobson’s)Enet(i) = aEnet(i-1) + (1-a)Tnet(i)

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Algorithm 2 if Tnet(i) > Enet(i)

Enet(i) = bEnet(i-1) + (1-b)Tnet(i)

else

Enet(i) = aEnet(i-1) + (1-a)Tnet(i)

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Algorithm 3Enet(i) = min { Tnet(j) }

(over all packets in previous talkspurt)

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Algorithm 4 (Ramjee’s)Ramjee’s Proposal

Observation: Algorithm 1-3 take too long to react to spike.

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Spike

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Ramjee’s IdeaWorks in 2 modes

SPIKE NORMAL

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Three QuestionsWhen to switch from normal to

spike mode?

When to switch from spike back to normal?

How to estimate during spike mode?

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Normal to Spike if difference in delays is large

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Normal to Spike if difference in delays is large

difference in delays:Tnet(i) – Tnet(i-1)

large:800 + 2Vnet(i)

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Another View of Spike (ZOOM)

Tnet(i)

Tarrive(i)

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Spike to Normal slope = slope/2 +

|2Tnet(i) – Tnet(i-1) – Tnet(i-2)|/8

if slope < 64 switch to normal

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Estimation in Spike ModeEnet(i) = Enet(i-1) + Tnet(i) – Tnet(i-1)

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First Packet in the TalkspurtTdelay(i) = Enet(i) + 4 Vnet(i)Tplay(i) = Tsend(i) + Tdelay(i)

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First Packet in the TalkspurtTdelay(i) = Enet(i) + 4 Vnet(i)Tplay(i) = Tsend(i) + Tdelay(i)

Tsend(i)

Tprop(i)

Tplay(i)

Tbuffer(i)

Tarrive(i)

Tq(i)

Tdelay(i)

Tnet(i)

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How to estimate Vnet(i)

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Variation of DelayVnet(i) =

aVnet(i-1) + (1-a)|Enet(i) – Tnet(i)|

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Comparisons of 4 Algorithms

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Playout Delay vs Loss Rate

Tdelay(i)

Loss Rate

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Algorithm 5 (Moon’s)

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Problems of Existing Algorithms Jacobson’s react too slowlyRamjee’s follow the delay too

closely

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Moon’s IdeaCollect statistics on packets that

have arrived

Find t such that q% of last w packets has Tnet(i) < t

Tdelay(i) = t

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Example (w =50, q = 90%)

num of packets

delay 1 2 3 4 5 6 7 8 9 10 11 12

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Spike ModeTdelay(i) = Enet(1)

SPIKE NORMAL

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Spike Mode

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Switching ModeNormal to Spike

if (Tnet(i) > k*Tdelay(i))

Spike to Normal

if (Tnet(i) < k’ * OLDTdelay(i))

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Loss Rate vs. DelayTdelay(i)

Loss Rate

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Theoretical Lower Bound

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Dynamic Programming

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Fibonacci Numbers

0 1 …

0 1 2

…

i-2 i-1 i

otherwiseii

FFF

ii

i 10

10

21

+

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Fibonacci Numbersfib(n)

x[0] = 0x[1] = 1for (i = 2; i < n; i++)

x[i] = x[i-1] + x[i-2]return x[i]

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Binomial Coefficient

nor 0 if 1

111

kkn

kn

kn 1

1 1

1 1

1 1

1

1

k

n

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Change-Making Problem [Weiss] 7.6

For a currency with coin C1, C2, .. Cn (cents), what is the min number of coins needed to make K cents of change?

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Example C = {1, 5, 10, 20, 50} K = 76 cents Give 4 coins 50 + 20 + 5 + 1 = 76

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Formulation To make a change of K cents, either

make a change of (K-50) cents, or make a change of (K-20) cents, or make a change of (K-10) cents, or make a change of (K-5) cents, or make a change of (K-1) cents

Number of coins for K = 1 + minimum of all the above choices

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Dynamic Programming)}({min1)( ii

CKcoinUsedKcoinUsed

… …

min +1

KK-5K-10K-20

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Theoretical Lower Bound

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Goal Input:

A packet trace A number of packet losses

What is the minimum average playout delay Tdelay(.) ?

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Trace

kM talkspurts

1,k 2,k 3,k j,k nk,k

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New NotationsM:

Number of TalkspurtNpacket(k) or nk

Number of packets in talkspurt kNtotal

Total number of packets

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Define..d(k, i)

minimum average playout delay for choosing i packets to be played out from k-th talkspurt

k.. M..

1,k 2,k 3,k j,k nk,k

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Define..D(k, i)

minimum average playout delay for choosing i packets to be played out from k-th to M-th talkspurt

k.. M..

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Dynamic Programming

1

2

3

M

0 1 2 N

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Formulation for Base Caseif i = 0 then

D(k,i) = 0if k = M

if i ≤ Npacket(M) thenD(k,i) = d(k,i)

elseD(k,i) = ∞

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Formulation for Recursive Case

k.. M..

i

jkjdjikDjiikDij

),(),1()(min),(0

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Loss Rate vs. DelayTdelay(i)

Loss Rate1%

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Video Playout

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Two Methods I-Policy

Display at fixed playout delay Drop late frames

E-Policy Display late frame at next period Playout delay increase

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I-Policya b c d e f g h

1 2 3 4 5 6 7 8 9 10 a c e f h

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E-Policya b c d e f g h

1 2 3 4 5 6 7 8 9 10 11 a b d f gc e h

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I-Policya b c d e f g h

1 2 3 4 5 6 7 8 9 10 a b e f hg

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E-Policya b c d e f g h

1 2 3 4 5 6 7 8 9 10 a b c d fe

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Pro and Cons I-Policy: Fixed latencyE-Policy: No Loss Frame or Gap

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Ideaif queue length > L for T seconds

drop incoming frames

long queue -> small Tshort queue -> large T

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Queue Monitoring

3

2

3

3

3

4

2

5

2

6

2

7

2

8

0 0 0 0 0 0 0

queue length

threshold

waiting time

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Queue Monitoring

3

2

3

3

3

4

2

5

2

6

2

7

2

8

0 0 0 0 0 0 0

queue length

threshold

waiting time

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Queue Monitoring

3

2

3

3

3

4

2

5

2

6

2

7

2

8

1 0 0 0 0 0 0

queue length

threshold

waiting time

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Queue Monitoring

3

2

3

3

3

4

2

5

2

6

2

7

2

8

2 1 0 0 0 0 0

queue length

threshold

waiting time

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Queue Monitoring

3

2

3

3

3

4

2

5

2

6

2

7

2

8

3 2 1 1 0 0 0

queue length

threshold

waiting time

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SummaryPreliminary work by Stone and

Jeffay on Video Playout

Ramjee’s and Moon’s adaptive audio playout algorithm

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Possible Survey and Project Topic

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Adapting PlaybackBesides adapting buffering time,

we can adapt playback time

Estimating and eliminating clock drift