Comp Spring 2003 Delay Jitter Ketan Mayer-Patel

Comp Spring 2003 What is jitter? Jitter: variation in delay –Can be measured as the variance of delay. Sender Reciever

Comp 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 Spring 2003 Elements of Delay SamplingPacketizationTransmission ReceptionDecompressSynchronizeDisplay Compress

Comp 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 Spring 2003 Jitter Buffer In general, jitter is removed by buffering in the reciever. Sources of delay and jitter considered a black box.

Comp Spring 2003 Jitter Buffer SamplingPacketizationTransmission ReceptionDecompressSynchronizeDisplay Compress Usually here with hardware support. Usually here with software support.

Comp 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 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 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 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 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 *

Comp Spring 2003 I-policy Maintain queue of incoming ADU’s and schedule playback of head. Example: PacketTSArrivalTime a b c d e141185

Comp 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 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 Spring 2003 E-policy PacketTSArrivalTime a b c d e141185

Comp 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 Spring 2003 Moving Averages off new estimate = off old estimate +  x (off observed – off old estimate ) v new estimate = v old estimate +  x (|off observed – off old estimate | – v old estimate ) or off new estimate =  off old estimate + (1 –  ) off observed v new estimate =   v old estimate + (1 –  ) x (|off observed – off old estimate |) or off new estimate = MIN(off observed in the recent past)

Comp 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 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 Spring 2003 Media Specific Techniques For audio, can take advantage of “talkspurt” structure. Make playout buffer adjustments in silence periods.

Comp Spring 2003 Talkspurt Example Talkspurt iTalkspurt i+1 abcdefgh Send Receive Playout off + kv off new + kv new

Comp 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 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.