Analysis and Evaluation of Skype and Google Talk
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Analysis and Evaluation
of Skype and Google Talk
Batu Sat and Benjamin W. Wah
Department of Electrical and Computer Engineering
and the Coordinated Science Laboratory
University of Illinois at Urbana-Champaign
Ref: B. Sat and B. W. Wah, “Analysis and Evaluation of the Skype and Google
Talk VoIP Systems,” Proc. IEEE Int’l Conf. on Multimedia and Expo, July 2006.
Problems Studied
z Motivations
z Skype (1.3.0.65) & Google Talk (Beta) are widely used but
proprietary
z Understanding their limitations help design better VoIP systems
z Problem statement
z Study the various components of Skype and Google Talk
z Identify their limitations
z Propose new VoIP system
z Adapt better to a large range of network conditions
July 9-12, 2006 ICME 2006 2
Outline
z Experimental Setup
z Network evaluations
z Architecture of VoIP Systems
z Speech Codecs
z Packetization
z Play-out Scheduling
z Loss Concealments
z Experimental Results
z Future Work
July 9-12, 2006 ICME 2006 3
Network Traces from PlanetLab
Collecting Internet traces from various geographical locations via PlanetLab nodes
Intra-continental paths:
• Intra-Asia, intra-Europe, intra-America
Inter-continental paths:
• North America – Europe/Asia/South America
• Asia – Europe/South America
July 9-12, 2006 ICME 2006 4
Observations
z Highly varying packet delays and jitters ⇒ dynamic jitter control
z < 50ms for most intra-continental paths
z > 250 ms for some inter-continental and intra-Asian paths
z Highly varying packet loss rates ⇒ dynamic loss concealment
z < 1% for most intra-continental paths
z > 33% for some inter-continental and intra-European paths
z Fast changing loss rates ⇒ real-time estimation & frequent feedbacks
July 9-12, 2006 ICME 2006 5
Connection with High Loss Rate
Bursty Losses: Single: 12.8%, Double: 7.8%, Triple: 0.7%, Quadruple: 12.2%
July 9-12, 2006 ICME 2006 6
Connection with Very Low Loss Rate
Bursty Losses: Single: 1.5%, Double: 0.1%, Triple: 0.1%
July 9-12, 2006 ICME 2006 7
Connection with High Delay
Out-of-order packets: 13%
July 9-12, 2006 ICME 2006 8
Connection with Very Low Delay
Out-of-order packets: 0%
July 9-12, 2006 ICME 2006 9
Experimental Setup
4
Troll: Emulating
Network Conditions
3 5
VoIP Client (A) VoIP Client (B)
2 Router (C) 6
7
1
Perceptual
Speech Quality
PESQ
8
July 9-12, 2006 ICME 2006 10Outline
z Experimental Setup
z Network evaluations
z Architecture of VoIP Systems
z Experimental Results
z Future Work
July 9-12, 2006 ICME 2006 11Speech Codecs
ITU G.729 low bit-rate codec
z High efficiency by removing redundancies across frames
z Long propagation of errors once a frame is lost
Internet Low Bit-Rate Codec (iLBC—RFC3951)
z Encode self-decodable frames and remove internal
states in receiver
z 66 - 90% higher bit-rate than G.729
z Achieve shorter recovery time and robustness to losses
z Inadequate without redundancies
July 9-12, 2006 ICME 2006 12Performance Under IID Losses
PESQ G729 3-way
24 Kbps
iLBC 2-way
26.6 Kbps
G729 3-way Sound (OLE2)
is better than Wave Sound
Wave Sound
iLBC 2-way
with similar Sound (OLE2)
bit rate
For loose end-to-end delay conditions, G729 performs better than iLBC for LR>10%
July 9-12, 2006 ICME 2006 13Packet Delays and Redundancy
• Skype and Google Talk use 60-ms packet period
z Skype packs two 30-ms iLBC frames in a packet (14.4 Kbps)
z Google Talk packs variable bit-rate PCM (24 Kbps)
Frame Per Packet No 2-way 3-way 4-way
Packet Period Redundancy
1 30 ms 0 ms 30 ms 60 ms 90 ms
2 60 ms 30 ms 90 ms 150 ms 210 ms
• 3- and 4-way redundancies
• Cannot be used with 60-ms periods
• Can be used with 30-ms periods without extensive delay
Loss rate not affected by
reducing packet period
from 60 to 30 ms
July 9-12, 2006 ICME 2006 14Play-out Scheduling
Skype uses simple FIFO play-out buffer
z 1st packet not received 60-ms later than the expected arrival time
z Apply iLBC-level loss concealment (RFC3951)
z Subsequent packets not received beyond the 60-ms expected time
z Played-out when received, causing the whole speech to shift
Time
1 3 2 4
1 3 2 4
Original Waveform
Degraded Waveform
Google Talk employs a jitter buffer similar to that of Skype
July 9-12, 2006 ICME 2006 15Skype: Loss Adaptation & Concealment
Feedback Control
z Packets in Skype not arriving 600 ms beyond
expected time are considered lost
z Receiver sends decision to increase or decrease
redundancy degree to sender every 2 seconds
5 Bytes 2 sec 2 sec
July 9-12, 2006 ICME 2006 16Skype: Adaptation to Loss-Rate Changes
z Loss rate: Step function [0% - 15% - 0%] 5 min steps
Faster Increase Slower Decrease
Redundancy Degree
Very slow loss adaptation due to
• Small step sizes to increase/decrease redundancy degrees
• Infrequent feedbacks
z Loss rate: Step function [0% - 25% - 0%] 5 min steps
• Can only track loss-rate changes of 1% (in 2 seconds)
60 sec
200 sec
Time
July 9-12, 2006 ICME 2006 17Google Talk: No adaptation to Loss-Rate
Changes
z No change in packet period and payload size with loss-rate changes
z No network feedback
0% 5% 10% 15% 20% 25% 30% 40% 50%
July 9-12, 2006 ICME 2006 18Proposed iLBC Prototype
z 30-ms packet period
z Packets are sequenced
z 1-way to 4-way redundancy decision made at receiver
z Relayed with each reverse-path UDP speech packet
z Extra 90ms delay for 4-way concealment in the most conservative case
z Packets late over 60ms are discarded
z Near-term improvement opportunities
z Decide on redundancy rate based on loss rate, UCFLR (unconcealable
frame loss rate), jitter-buffer size
z Relay decision when condition changes (repeated to assure reception)
z Adaptively change play-out schedule to minimize delay with a
prescribed level of UCFLR
July 9-12, 2006 ICME 2006 19Outline
z Experimental Setup
z Network evaluations
z Architecture of VoIP Systems
z Speech Codecs
z Packetization
z Play-out Scheduling
z Loss Concealments
z Experimental Results
z Future Work
July 9-12, 2006 ICME 2006 20Internet Traces (#2): Low Loss Rate July 9-12, 2006 ICME 2006 21
Internet Traces (#5): High Loss Rate July 9-12, 2006 ICME 2006 22
Conclusions
z Internet delays and losses are diverse
z Skype and Google Talk
z Too conservative loss adaptation and feedback
control
z Ineffective play-out scheduling
z Proposed prototype
z More aggressive frame rate and feedbacks
z 1-4 ways of redundancy
July 9-12, 2006 ICME 2006 23You can also read
