Adaptive Playout Scheduling Using Time- scale Modification in Packet Voice Communications Yi J. Liang, Nikolaus Farber, Bernd Girod Information Systems Laboratory, Department of Electrical Engineering, Stanford University ICASSP 2001

Objective To improve tradeoff between delay and loss. Adaptively adjusting the play out schedule of individual packet according to the varying network condition, even during voiced period.

Different Playout Scheduling

Notations t i s : sent time t i r : received time t i p : played out time L 0 = t i+1 s - t i s : packetization time (constant) d i b = t i p – t i r : buffering delay d i n = t i r – t i s : network delay L i = t i+1 p - t i p : achieved length of audio packet I d i max : maximum allowable total delay

Timing Relationship for Adaptive Scheduling

Two Issues of Evaluating Different Scheduling Schemes Average buffering delay where Late loss rate R : total packets received N : total packets sent

Basic Operation of the Playout Process To estimate d i+1 n and t i+1 p with d i n, d i-1 n, … To calculating L i by definition and scaling current packet i if necessary If d i max is chosen close to d i n, the buffering delay can be reduced.

Basic Operation of the Playout Process (continued) According to the user-specified loss rate, d i+1 max is set to the small value satisfying h i (d n ) is the histogram of past delays accumulated at time i

Basic Operation of the Playout Process (continued) The expected buffering delay is

Scaling of Voice Packets

Performance Comparison

Performance Comparison (continued)

Concluion The proposed algorithm effectively reduces buffering delay and loss rate by adapting the playout schedule to the varying network delay n high dynamic way. Simulation results based on Internet measurement have shown significant reductions in average buffering delay and loss rate produced by the proposed playout scheme.