ACM Multimedia October 4, 2001 Real-time Voice Communication over the Internet Using Packet Path...

ACM MultimediaOctober 4, 2001

Real-time Voice Communication over the Internet Using Packet Path Diversity

Yi Liang, Eckehard Steinbach, and Bernd Girod

Image, Video, and Multimedia Systems GroupInformation Systems LaboratoryDepartment of Electrical EngineeringStanford University

Liang, Steinbach, Girod: Real-time Voice Communication over the Internet Using Packet Path Diversity 2

Overview

Requirements of VoIP

Packet path diversity for low delay VoIP

Adaptive multi-stream playout scheduling

Internet experiments

Demo


Requirements of VoIP

Small end-to-end delay for conversational services ( <150ms ) Delay variations (jitter) have to be smoothed using receiver buffer Late packets are lost, no time for retransmissions Small residual packet loss rate is ok Trade-off between end-to-end delay and late loss rate

Sender

Receiver

Playout

Time

1 2 3 4 5 6 7 8

1 2 3 4 5 6 7 8

Missed deadlineReceiver buffer

Time

TimePacketization

time


Motivation for Packet Path Diversity for VoIP In up to 80% better alternative path

[Savage ‘99] Multi-path routing [Sidhu ‘91, Bahk ‘92] Uncorrelated packet loss on independent

paths [Apostolopoulos ‘01] Efficiency of FEC limited by packet loss

and delay correlation [Bolot ‘93, Bolot ‘99] Delay jitter is the major killer of delay

sensitive applications Our contributions:

Exploitation of statistically independent jitter behavior for VoIP

Adaptive multi-stream playout scheduling technique

R

S

Secondpath

1 2

Speech

Packet Path Diversity

Default path


Two-path speech coding

Redundant description of voice stream (MDC) Two bitstreams for two paths

Stream 1: [Jiang 2000] Even samples: (8-bit, PCM) Odd samples: (2-bit, ADPCM)

Stream 2: Vice versa 25% Overhead Loss of one packet small reduction in speech quality Loss of both packets error concealment

Es1

s2

O E O E O

O E O E O E

E O

Packet length

Time


Packet Path Diversity for Low Delay VoIP

Time1 2 3 4 5 6

Sending on path 1

Receiving on path 1

Packet Path Diversity reduces effective delay jitter and therefore late loss rate

Packet Path Diversity reduces effective delay jitter and therefore late loss rate

Playout1 2 3 4 65

1 2 3 4 5 6Sending on path 2

Receiving on path 2


Adaptive Playout Scheduling

Adaptive playout scheduling and speech scaling allow us touse more packets for playout at given mean target delay

Adaptive playout scheduling and speech scaling allow us touse more packets for playout at given mean target delay

Time1 2 3 4 5 6Sending on path 1

Receiving on path 1

1Constant Playout

stretching compressing

Adaptive Playout1 2 3 4 65

If past delay values indicate congestion delay playout of next packet(s) by stretching speech signal

If past delay values are small advance playout of next packet(s) by compressing speech signal

2 3 4 65


Modification of Playout Speed Based on time-domain interpolation algorithm WSOLA

[Verhelst et al., 1993, Liang 2001]

Output packet

1/20/1 2/3 3 4

Original packet

Pitch-period

0 21 3 4

Template


Adaptive Two-stream Playout Scheduling

1

2

1 1 2 2 1 2 2 1

Pr |

Pr |

ˆ ˆ ˆ ˆ ˆ ˆ( (1 ) (1 ))

i i iplay play

iplay

iplay

C d both descriptions lost d

only one description lost d

d p p p p p p

playout deadlineplayout deadline

estimate of loss probabilityestimate of loss probability

1p̂

Histogram of past delay values

Combination of Packet Path Diversity and Adaptive Playout Minimization of Lagrangian cost function

Delayiplayd

Variation ofiplayd


QwestQwest

Internet Experiment

Explicit path selection using relay server [Apostolopoulos ‘01] UDP packets with payload of 240 bytes

Exodus

Comm.

Exodus

Comm.

BBN PlanetBBN Planet

Netergy Networks

192.84.16.176MIT

18.184.0.50

Harvard140.247.62.110

(5ms)(45ms)

(40ms) (5ms)

(5ms)Sender

Relay Server

Receiver


Measured Packet Delay Trace

Delay in ms

Packet number


Adaptive Two-stream Playout Scheduling

Delay in ms

Packet number


Comparison: Single-path Transmission with FEC

2 3 41 1 2 3

2 41 1 3

2 3 41

Stream received with packet loss

Stream reconstructed

3

Stream sent

Packets protected with FEC

FEC: adds redundancy by sending one or more copies of the source signal in the following packet(s) [Bolot ‘96]

FEC protected single-stream

For fair comparison

Primary copy: quantized at fine resolution (8-bit)

Secondary copy quantized at coarser resolution (2-bit)

Same data rate as transmission with Packet Path Diversity

Same adaptive playout scheduling technique


Results

delay (ms)

Packet loss ratein %


Demo

Original

Average total end-to-end delay: 84 ms Error concealment: speech segment repetition

Average total end-to-end delay: 84 ms Error concealment: speech segment repetition

Path Diversity Single-stream with FECat same data rate


Conclusions Packet Path Diversity for real-time voice communication over IP

Multiple Description Coding of speech signal Quality improves with each description received

Exploitation of statistically independent jitter behavior Improvement of delay versus speech quality trade-off

Adaptive Playout Scheduling Flexible playout deadline using time-scale modification Reduction of late loss rate for given target delay Lagrangian cost function for two-path transmission

Internet experiments Implementation of path diversity using relay server Observation of largely independent delay jitter behavior Significant speech quality improvement in comparison to single-path

transmission with FEC at the same data rate and end-to-end delay


PESQ Results

Perceptual Evaluation of Speech Quality (ITU-T Rec. P.862, Feb. 2001)

PESQ can be used for end-to-end quality assessment

Ranges from –0.5 to 4.5 but usually produces MOS-like scores between 1.0 and 4.5


Internet Experiment II

VBNS IP Backbone

Service

VBNS IP Backbone

ServiceDANTE

Operations

DANTE Operations

UUNET

Tech.

UUNET

Tech.

Erlangen131.188.130.136

Harvard140.247.62.110(7ms)

(40ms)

AT&TAT&T

(5ms)(5ms)

(10ms)

New Jersey165.230.227.81

Path 1 (direct): N. J. – Erlangen Path 2 (alternative): N. J. – Harvard – Erlangen


Result

Path 1 (direct): N. J. – GermanyPath 2 (alternative): N. J. – Harvard –

Germany

Mean delay

61.3/65.0 ms link loss

0.6% / 1.1% Significant reduction of

late loss and end-to-end delay by packet path diversity


Constant Playout

Trade-off between packet loss and delay

Constant playout deadline

late loss


Adaptive playout deadline

Adaptive Playout

Adaptation to delay variation (jitter)


Late Loss Mean Delay


Speech Scaling


Demo 2

Original

Average total end-to-end delay: 108 msAverage total end-to-end delay: 108 ms

Path Diversity(PESQ: 4.1)

Single-stream with FEC(PESQ: 3.5)


Voice over IP (VoIP)

VoIP is rapidly growing 900% 1998-1999 5000% 1999-2004

2004

1999

1998

135 billion minutes

2.7 billion minutes

310 million minutes

[Source: IEEE Spectrum, Mai 2000]


Limits of Speech Stretching

More than 25% is annoyingMore than 25% is annoying

Original Stretching: s=1.3


Speech and Audio Scaling

Speech scaling

Audio scaling

original stretched: s=1.3 compressed: f=0.7

original stretched: s=1.3 compressed: f=0.7

ACM Multimedia October 4, 2001 Real-time Voice Communication over the Internet Using Packet Path...

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