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Adaptive Delay Aware Error Control for Internet telephony Catherine Boutremans Jean-Yves Le Boudec IP Telephony Workshop’2001. Institute for computer Communication and Applications phone: + 41 21 693.5258; fax: +41 21 693.6610 Catherine.Boutremans@epfl.ch http://icawww.epfl.ch. Framework.
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Adaptive Delay Aware Error Control for Internet telephonyCatherine BoutremansJean-Yves Le BoudecIP Telephony Workshop’2001 Institute for computer Communication and Applications phone: + 41 21 693.5258; fax: +41 21 693.6610 Catherine.Boutremans@epfl.ch http://icawww.epfl.ch
Framework • Real-time audio over Best Effort networks suffers from varying packet loss rates, delays and available bandwidth. • Forward Error Correction (FEC) is an efficient way to recover from packet losses but : • bandwidth requirement • end-to-end delay • Adaptive rate/error control was proposed but it does not try to optimize the end-to-end delay
Motivation • Above a certain threshold (around 150ms): the end-to-end delay is annoying • New differentiated services, such as Alternative Best Effort offer applications the trade-off between receiving lower end-to-end delay (and higher loss rates) or more overall throughput. important to take the end-to-end delay into account in the adaptive control scheme
Adaptive Delay aware error control • Problem definition:develop an error control scheme for audio which is delay aware (namely, which chooses the FEC according to its impact on the end-to-end delay) for Best Effort Internet services such as: 1) Flat 2) ABE • Out of scope:validation of perceptual models
Outline • A.Our Joint rate/error/delay control scheme • 1. Error recovery taken from media specific FEC • 2. An RTCP-based Rate control scheme • 3. A new Delay control scheme • 4. Our Global optimization problem • B.Simulation examples • 1. Flat Network: what do we improve? • 2. ABE: Is it worth to be green?
Start with high rate audio encoding • copy audio, compress, and delay • if packet lost, recover from redundancy ... ... packet stream 5 4 4 3 3 2 3 reconstruction secondary/redundancy encoding 5 4 1.How media specific FEC Works Audio Frame 4 Audio Frame 3 Qmax Qmax Dp Dp Dr Dr Quality Rp Rr Rr Rp Rmax Rmax rate primary/source encoding
p 1-p 0 1 1-q q 1.Efficiency of FEC mainly depends on packet loss process • Loss process of audio packets in the Internet can be modeled as low order Markov chain • In this work: Gilbert Model
2.Our RTCP-based Rate Control scheme • Audio streams have to share bandwidth fairly with TCP connections TCP-Friendly • Equation-Based TCP Friendly rate control (Padhye): • Rate controlled via packet size (constant packet intervals) • Use RTCP for Feedback • EWMA filter of PLR in order to smooth the rate
2. Our Rate control achieves fairness Our scheme shares bandwidth fairly withTCP connections
3.Quality is function of rate and delay • Audio quality is function of end-to-end delay: decreases rapidly above 150ms • Audio quality is function of reconstructed rate (SNR,MOS,...)
3.Quality is function of rate and delay • Consider the user perceived quality (utility) as a function of the reconstructed rate (R) and the overall end-to-end delay (D) • We use several utility curves as we don’t know which one is the best.
4.Our Joint rate/error/delay control Consider: • A source with flexibility to encode audio at rate • Unreliable network characterized by • a loss distribution Gilbert Model : r.v. • a delay distribution • an available bandwidth TCP-Friendly rate constraint • A utility function ofdelay and reconstructed rate
4.Our Joint rate/error/delay control Define: • K = of copies of audio segment sent over the network • ,the delay spent on FEC • the r.v. the set of copies that make it across the network Constrained optimization problem: Under
4.Our Joint rate/error/delay control Solution: • General solution is derived using Lagrange Multipliers for small values of K and via numerical method SQP for K4 • The solution has the following properties: • if (p+q)1 it pays to offset and to put more quality into the end packets • if (p+q)1 better not to offset and
B.Simulation examples • Single bottleneck • Topology with small and long flows • Bottleneck BW = 15Mbps, 5Mbps • variable number of connections d2 d1 d1 d2
1. FLAT: Conclusion • The Delay aware scheme increases utility by avoiding that the source waste delay on the FEC when it is not really needed.
2.ABE: Alternative Best Effort • ABE is a novel service for IP networks which offers the choice between receiving a lower end-to-end delay or more overall throughput. • Packets are marked either green or blue. • Green packets receive a low, bounded queuing delay but they receive more losses during bouts of congestion. • Blue packets receive more throughput (and less losses) but also more delay jitter.
Is it worth being green? • Green packets receive a lower delay but they experience more losses (and hence, receive less throughput). • Losses are repaired using FEC but FEC increases the e2e delayand the BW requirement.
Higher RTT (2 bottlenecks): green is better if load not too high.
2. ABE: Conclusions • It is worth accepting to receive less bandwidth (and more losses) except in trivial cases where • the RTT is small anyway • the network is badly congested • Need for adaptive Color choosing algorithms
Conclusions • We proposed an adaptive Delay aware error control scheme. • We showed it could prevent a source from wasting delay on FEC when not necessary. • It helped us to figure out that it was worth trading throughput for delay.