Delay Jitter
Ketan Mayer-Patel
Comp 249 - Spring 2003
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
a
b
c
d
e
TS
10
11
12
13
14
ArrivalTime
1000
1025
1050
1095
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
E-policy
Packet
a
b
c
d
e
TS
10
11
12
13
14
Comp 249 - Spring 2003
ArrivalTime
1000
1025
1050
1095
1185
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(offobserved in 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
Talkspurt Example
off + kv
Send
a
offnew + kvnew
b
c
d
e
f
g
h
Receive
Playout
Talkspurt i
Comp 249 - Spring 2003
Talkspurt i+1
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