Delay Jitter

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Delay Jitter

• Ketan Mayer-Patel

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What is jitter?

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

Sender
Reciever
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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.

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Elements of Delay
Sampling
Packetization
Transmission
Compress
Reception
Decompress
Synchronize
Display
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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.

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Jitter Buffer

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

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Jitter Buffer
Sampling
Packetization
Transmission
Compress
Reception
Decompress
Synchronize
Display
Usually here with hardware support.
Usually here with software support.
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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.

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Media Elasticity

• Extent to which presentation time of one ADU is
  dependent on presentation times of previous
  ADUs.
• Video
• Fairly elastic.
• Audio
• Fairly inelastic

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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

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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.

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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 gt off 700
• Add variance allowance
• p t 30 760

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I-policy

• Maintain queue of incoming ADUs and schedule
  playback of head.
• Example
• Packet TS ArrivalTime
• a 10 1000
• b 11 1025
• c 12 1050
• d 13 1095
• e 14 1185

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I-policy

• Playback smoothness is achieved if arriving ADUs
  delay average max jitter is less than first ADU
  delay jitter allowance.
• What happens if initial delay is
  unrepresentative?

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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.

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E-policy

• Packet TS ArrivalTime
• a 10 1000
• b 11 1025
• c 12 1050
• d 13 1095
• e 14 1185

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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)

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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(offobserved in the recent
  past)

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Adjusting Down

• Extending jitter buffer delay is easy.
• How do you make downward adjustments?
• Two basic techniques
• Play faster
• Drop ADUs

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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.

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Media Specific Techniques

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

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Talkspurt Example
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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.

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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.