Empirical Study - Multimedia Networking - Lecture Slides, Slides of Computer Science

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An Empirical Study of Delay Jitter

Management Policies

Introduction

• Want to support interactive audio
• “Last mile” is LAN (including bridges, hubs) to

desktop

• Study that
• (Me: 1995 LANs looked a lot like today’s WANs)
• Transition times vary, causing gaps in playout
• Can ameliorate with display queue (buffer)

Gaps versus Delay

• Can prevent gaps by having constant delay
  • Network reserves buffers
    • Ala telephone networks
    • But not today’s Internet
• Plus
  • will still have (unreserved) LAN as “last mile”
  • OS and (de)compression can still cause jitter
• Thus, tradeoff between gaps and delay must be explicitly managed by conferencing system - Change size of display queue - The larger the queue, the larger the delay and the fewer the gaps and vice versa

This Paper

• Evaluates 3 policies for managing display queue - I-policy , E-policy from [NK92] - ( I is for late data ignored, E is for expand time) - Queue Monitoring from this paper
• Empirical study
  • Audioconference on a LAN
  • Capture traces
• Simulator to compute delay and gaps

The Effect of Delay Jitter

• If display latency worse than largest end-to- end latency, then no gaps - (When is this not what we want?)
• Playout with low latency and some gaps preferable to high-latency and no gaps
• What if a frame arrives after its playout time?
• Two choices:
  • I-Policy – single fixed latency, so discard
  • E-Policy – late frames always displayed, so expand playout time

(Queue parameter is 2)

(3 gaps, display latency of

I-Policy

I-Policy (2)

One event, but latency still low

(e, f, g, …)

E-Policy (2)

One event, latency higher

Outline

• Introduction (done)
• The I- and E-policies (done)
• The Queue Monitoring policy (next)
• Evaluation
• The Study
• Summary

Adjusting Display Latency

• Audioconference with silence detection can be modeled as series of talkspurts - Sound and then silence
• Adjust display latency between talkspurts
• [NK92] said observe last m fragments, discard k largest delays and choose display latency as greatest delay - Recommend m > 40 and k = 0.07 x m
• (Other approaches proposed, since)

Outline

• Introduction (done)
• The I- and E-policies (done)
• The Queue Monitoring policy (done)
• Evaluation (next)
• The Study
• Summary

Comparing Policies

• If A has lower latency and gaps than B, then A is better
• If A lower latency, but A higher gaps than which is better? - Depends upon - relative amounts - resolution - application requirements - Few standards

Outline

• Introduction (done)
• The I- and E-policies (done)
• The Queue Monitoring policy (done)
• Evaluation (done)
• The Study (next)
• Summary

The Study

• Run videoconference
  • Use audio only
• Record end-to-end delay
• Input into simulator to evaluate policy