Practical Considerations for Smoothing Multimedia Traffic over Packet-Switched Networks

1. Practical Considerations for Smoothing Multimedia Traffic over Packet-Switched Networks Christos Tryfonas tryfonas@sprintlabs.com April 2002 Sprint Advanced Technology Laboratories Burlingame, California

2. Problem Statement Sender Receiver Stream Decoder (Buffer) Video Server Packet-Switched Network Network Interface • Objective: Compute a rate schedule to send a video stream that maximizes some network parameter (e.g. utilization) • Constraints: Avoid decoder buffer under/overflows

3. Work-Ahead Smoothing Algorithms Received Data Decoder Buffer Size Overflow Envelope Jitter Underflow Envelope Size of frame 4 Schedule Time • Most smoothing algorithms compute maximum constant rate segments • Differences in the selection of the starting point of next segment

4. Outline • Practical Observations • Smoothing under variable network delays • Sender-Based Smoothing • Clock-Aware Smoothing • Renegotiation Failure Decisions • Conclusions

5. Practical Observations • Network delay is variable • Current smoothing algorithms compute schedule based on the receiver side • Not all data sent by the sender is utilized by the receiver • Some applications require clock to be recovered at the receiver (e.g. Broadcast TV) • Consideration of Renegotiation Failures

6. Smoothing under Variable Network Delays s r • Jitter usually a function of the reserved rate (e.g. Fair Queueing schedulers) • Currently optimization done with a pre-specified worst-case network jitter, i.e., not optimal • Overflow envelope is a function of the decision, not fixed • The burstiness of the source at the selected rate defines the ambiguity zones

7. Our Approach Overflow Envelope Received Data Decoder Buffer Size Underflow Envelope Schedule – Ambiguity Zones Size of frame 4 Time • Optimization should be done by considering the ambiguity zones resulting from the varying reserved rates • Existing optimization algorithms should consider the variable ambiguity zones for their decisions

8. The Case of Rate Decrease Jitter 2 Schedule Ambiguity Envelope Received Data Jitter 1 < Jitter 2 Jitter 1 Time • New rate corresponds to higher jitter • Lower envelope is changed to accommodate the increase in jitter

9. The Case of Rate Increase Jitter 2 • New rate corresponds to less jitter • A peak-rate segment is inserted before the new ambiguity zone with the new jitter Schedule Ambiguity Envelope Jitter 1 > Jitter 2 Received Data Peak Rate segment Jitter 1 Time

10. Sender-Based Smoothing Sequence Header GOP Header Frame Header Frame Coding Extension Frame Data Frame Coding Extension Frame Header Frame Data Received Data function as computed by optimization Received Data Actual Received Data after Rate Adaptation Time • Not all data transmitted is inserted in the decoder buffer • Two approaches: • Rate Adaptation • Forward Schedule computation

11. Clock-Aware Smoothing Received Data Received Data Clock-Aware Schedule Schedule Time Time Clock Clock Time Time

12. Renegotiation Failure Decisions Convergence Point Received Data Main Schedule Alternate Schedule Intersection Point Renegotiation Failure Time • Dynamic Requantization • Use of scalable-encoded streams • New Approach: Pre-computed alternate schedules with convergence points

13. Conclusions • Current smoothing algorithms produce infeasible schedules in practice • Optimization has to consider: • Variable Network Jitter a function of the reserved rate • Unused data sent by the sender but not utilized by the receiver/decoder • Clock-awareness • Renegotiation Failures • For more www.sprintlabs.com/People/tryfonas