Performance Optimization of
VoIP Using an Overlay Network
Raj Kumar Rajendran
Samrat Ganguly
Rauf Izmailov
Dan Rubenstein
Mentor(s):
Project: Grid Networking (GRIN)/VoIP
Group: IP Networking and Distributed Systems Group
Motivation
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VoIP traffic poised to grow rapidly

FCC projects 27 million residential VoIP lines by 2010
Corporate VoIP usage to go from 4% to 44% by 2010
Skype had 10 billion minutes of calls in first year
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Not clear how it works

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Internet not engineered for delay-sensitive
applications
Skype is proprietary
Hypothesized that VoIP performance can be
improved by
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Path-Diversity
Error-coding
Challenge

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Design VoIP-specific overlay network
Implement
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Path-Diversity
Parity-coding
Deploy
Test performance
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Design test tools and methodology
Test usefulness of overlay
Effect of Path-Diversity
Tradeoffs of Redundancy and Error-Coding
Measuring VoIP Performance

ITU-T E-Model


R-factor
Delay Impairment (Id)


R-Factor
Voice Quality Rating
0<R<100
Best
80<R<90
High
70<R<80
Medium

60<R<70
Low
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50<R<60
Poor
R  94.2  I e  I d
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0.024D unto 177ms
0.11D after 177ms
Loss Impairment (Ie)
Network Loss
Jitter Loss


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Variance in Delay
I e   1   2  ln( 1   3 E )
Non-Linear
Depends on Codec  i
R-factor depends on delay, loss, clustering of loss
Is Non-linear
System
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Nodes play 3 roles
User-node
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Overlay-nodes
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User-nodes
Beacon-node
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Beacon-Nodes
Runs VoIP appl
On startup finds and registers with
beacon-node
Intermediary between overlay and user
Represents registered user-nodes for
routing purposes
Overlay-Node

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Routes VoIP streams
Receives and sends VoIP streams
directly to user-nodes
Architecture
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User-Nodes
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Beacon-node
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Send VoIP stream to overlay
node indicated to it by beacon
With Overlay-nodes periodically
assesses quality of links
between them
Overlay Nodes
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Calculates optimal routes to all
beacon-nodes (directly or
through other overlay-nodes)
Receives VoIP streams directly
from user-nodes and optimally
routes them
Control
Control-Layer
Control-Layer
Route-Compute Layer
Route-Compute Layer
Probe Layer
Data Layer
Overlay-nodes
Probe Layer
Data
Data Layer
Quality
Probe
Probe Layer
Data
User Layer
Beacon-Nodes
Data Layer
Beacon Layer
User-nodes
Data Layer
Beacon Layer
Estimating Link-Quality

Overlay-nodes
periodically asses
quality of link
between each other

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They send a talk-spurt
Estimate quality of
link
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Link Quality-Probe
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Spurt-beginning
132B
8 second talk-spurt (1000
132Byte packets) is sent and
we estimate
Average Delay (d)
Network Loss-rate (n)
Jitter Loss-rate (j)
Delay loss
Loss Clustering (c)
1000 132B packets
Spurt-end
Estimating Path-Quality

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We know quality of link
between individual overlay
nodes
Call may go over multiple-links
What is the end-to-end quality
of the path (R-factor) ?
R-factor is not linear so is
not additive !
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We cannot just add the Rfactor of the links
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We need to estimate
parameters for whole path!
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Overlay
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Delay is additive
Network loss is multiplicative
Cluster-factor can be
computed as a weighted
average
Jitter (delay) loss is hard

User-node
Our experiments indicated
that sum is a good estimate
The Routing Algorithm
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Calculates best
quality (R-factor)
paths.
Uses a modified
Bellman-Ford
(distance-vector)
algorithm
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Do (Every T seconds)
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Measure quality of directpath to beacon, overlay nodes
Repeat
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Exchange delay, loss(
network,cluster) of bestpath with other overlaynodes
If better-path exists
through another overlaynode, update best-path
Until (paths don’t change)
Done
Experimental Setup
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System
Implemented on
PlanetLab
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35 nodes spread
over US, Europe,
Asia
11 nodes were
chosen to be
overlay-nodes
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Tests Used the G.711 codec
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120 Byte frames every 15 ms
(133 f/s)
Each call 170Kbps
(Frame+RTP+UDP+IP)
Created a VoIP tester
application
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Generated a call consisting of
8-second talk-spurts
Statistics are collected for
each talk spurt
Results: Overlay Performance
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Overlay run for 1 day
uninterrupted
Routing algorithm
rerun every 10 minutes

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% Paths that improved
Avg. R-factor
improvement
% Paths where R-factor

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Improved from low to
medium (< 70 to >70)
Improved from
medium to high (< 80
to > 80)
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41% of calls improved
R-factor improved by 26
for these calls
11% improved from “low”
to “medium” quality
11% improved from
“medium” to “high”
The improvement was
stable over time
Usefulness of Path-Diversity


Can we put through
more calls if we use
multiple paths?
We tested 2-calls, 3calls and 4-calls
1
Path-1
1
2
1
2
Path-2
2
2
1
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R-factor improved on
average by 4, 11 and 12
low quality spurts fell by
11%, 21% and 22%
90
80
70
2-Path
3-Path
4-path
Benefit
Single-Path
60
50
edu net com org eur
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Use of Coding
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Can we improve quality by
using more bandwidth

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Path-1
1
Packet-duplication
Parity-codes
2
1
P
1
P
2
2
1
Path-2
2
60
40
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20
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0
4
3
com
2
net
1.66
1.33
1
edu
Duplication deteriorates performance of
regular channels
Parity-coding offers better tradeoff. A
50% increase in BW

(3,2) code channel improves R-factor by 16,
and low-quality spurts fell by 27%
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eur
il
Conclusion
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Quality of VoIP calls can significantly improved by using an
overlay-network
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The Path-diversity offered by overlays can be used to
improve quality
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41% of calls improve, and 10% improve from low to medium
quality and 10% improve from medium to high quality
At large loads 20% fall in low-quality spurts
When bandwidth is available

Parity codes can be used to effectively trade-off bandwidth
for quality
Details: Infocom submission at www.ee.columbia.edu/~kumar/papers/