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## PowerPoint Slideshow about 'Delay Jitter' - piper

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Presentation Transcript

What is jitter?

- Jitter: variation in delay
- Can be measured as the variance of delay.

Sender

Reciever

Comp 249 - Spring 2003

Elements of Delay

- Packetization
- Think of audio.
- Propagation
- Physical transmission.
- Queuing
- Routing, congestion, etc.
- This is the part that is most variable.
- Synchronization
- May need to wait for corresponding samples from other streams.

Comp 249 - Spring 2003

Elements of Delay

Sampling

Compress

Packetization

Transmission

Reception

Decompress

Synchronize

Display

Comp 249 - Spring 2003

Sources of Jitter

- In general, due to queuing effects.
- When queue is building…
- Competing traffic inserted between successive packets of a stream.
- When queue is draining…
- Probe compression.

Comp 249 - Spring 2003

Jitter Buffer

- In general, jitter is removed by buffering in the reciever.
- Sources of delay and jitter considered a black box.

Comp 249 - Spring 2003

Jitter Buffer

Sampling

Compress

Packetization

Transmission

Reception

Decompress

Synchronize

Display

Usually here with

hardware support.

Usually here with

software support.

Comp 249 - Spring 2003

Why?

- Why do we care about jitter anyway?
- Smoothness of playback.
- Inelastic media types.
- What is the cost of a jitter buffer?
- Increased end-to-end latency.
- When will we care about this?
- Interactive streams.
- Live streams.
- Stored playback applications can afford the cost of larger jitter buffers.

Comp 249 - Spring 2003

Media Elasticity

- Extent to which presentation time of one ADU is dependent on presentation times of previous ADU’s.
- Video
- Fairly elastic.
- Audio
- Fairly inelastic

Comp 249 - Spring 2003

Jitter Buffer Design

- On one extreme: static jitter buffer.
- Total delay budget held at some static amount.
- If ADU arrives late, same as if lost (i.e., discard)
- I-policy [Naylor82]
- On the other: constantly increasing buffer.
- Every ADU is played.
- Jitter buffer is adjusted to largest delay seen thus far.
- E-policy

Comp 249 - Spring 2003

I-policy

- Establish jitter buffer on first ADU.

p = t * r + off

- Use first ADU to calculate offset.
- p = playout time
- t = timestamp of ADU
- r = period of ADU clock
- off = jitter buffer offset
- Solve for off for first ADU.
- Add jitter allowance to off.

Comp 249 - Spring 2003

Example

- Variance allowance = 60 ms
- Media timestamp period = 30 ms
- First ADU:
- Media Timestamp = 10
- System Clock = 1000
- Solve for off:
- 1000 = 10 * 30 + off => off = 700
- Add variance allowance
- p = t * 30 + 760

Comp 249 - Spring 2003

I-policy

- Maintain queue of incoming ADU’s and schedule playback of head.
- Example:

Packet TS ArrivalTime

a 10 1000

b 11 1025

c 12 1050

d 13 1095

e 14 1185

Comp 249 - Spring 2003

I-policy

- Playback smoothness is achieved if arriving ADU’s delay average + max jitter is less than first ADU delay + jitter allowance.
- What happens if initial delay is unrepresentative?

Comp 249 - Spring 2003

E-policy

- Solve for off for every ADU.
- If late, adjust off.
- If early, queue.
- Again, maintain a queue and schedule playout of the head.
- Basically keep adjusting playout delay for the largest delay experienced so far.
- Accommodates jitter by definition.

Comp 249 - Spring 2003

Adaptive Buffer Techniques

- Between E and I are adaptive techniques.
- Adaptively estimate delay variance.
- Calculate off on a per ADU basis.
- Maintain a weighted moving average for off.
- Maintain a weighted moving average for variance of off.
- Manage queue using off average + k * off variance where k is typically 2-4

p = t * r + (off + k * v)

Comp 249 - Spring 2003

Moving Averages

offnew estimate=offold estimate+x (offobserved–offold estimate)

vnew estimate=vold estimate+x (|offobserved–offold estimate| –vold estimate)

or

offnew estimate=offold estimate+ (1 – )offobserved

vnew estimate=vold estimate+ (1 – ) x

(|offobserved–offold estimate|)

or

offnew estimate= MIN(offobservedin the recent past)

Comp 249 - Spring 2003

Adjusting Down

- Extending jitter buffer delay is easy.
- How do you make downward adjustments?
- Two basic techniques:
- Play faster
- Drop ADU’s

Comp 249 - Spring 2003

Adaptive Issues

- What should k be?
- Depends on variance distribution.
- What should weighting coefficients be?
- Tradeoff between stability and responsiveness.
- What about inelastic media?
- Truncating audio frames or allowing space between audio frames causes problems.

Comp 249 - Spring 2003

Media Specific Techniques

- For audio, can take advantage of “talkspurt” structure.
- Make playout buffer adjustments in silence periods.

Comp 249 - Spring 2003

offnew+kvnew

Send

a

b

c

d

e

f

g

h

Receive

Playout

Talkspurt i

Talkspurt i+1

Talkspurt ExampleComp 249 - Spring 2003

Agility vs. Stability

- Tradeoff between agile and stable.
- Controlled by gain parameter in moving weighted filters.
- Agile
- Weighs new measurements more than past measurements.
- Reacts quickly to change.
- Can react too quickly to transient changes.
- Stable
- Weighs past measurements more than new measurements
- Change in value bounded to small percentage of current value.
- Takes a while to react to fundamental changes.

Comp 249 - Spring 2003

Kim and Noble 01

- Proposes three different filters which adaptively vary the gain parameter.
- Flip-flop: detects when to use an agile gain and when to use a stable gain.
- Control heuristics come from statistical process control world
- Stability filter: adapts gain relative to current variance measure and recent variance range.
- Error filter: adapts gain to reinforce “good” estimation behavior.
- Concludes that FF filter works best in fairly wide spectrum of applications.
- Work done in the mobile domain but very applicable to all areas of networking.

Comp 249 - Spring 2003

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