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2012 IEEE Wireless Communications and Networking ConferenceCoverage analysis of Skype VoIP services over 3G cellular networksRenny E. Badra and Hayat AbdullaDepartamento de Electrónica y Circuitos Universidad Simón Bolívar Caracas, Venezuela [email protected] , [email protected] presentation Motivation Skype Traffic Simulation Set-up Results ConclusionsMotivation Skype ® is the most popular commercial platform for peer-to-peer IP telephonyOver 700 million registered users wo
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2012 IEEE Wireless Communications and Networking Conference
Coverage analysis ofSkype VoIP services
over 3G cellular networks
Renny E. Badra and Hayat AbdullaDepartamento de Electrónica y Circuitos
Universidad Simón BolívarCaracas, Venezuela
This presentation
MotivationSkype TrafficSimulation Set-upResultsConclusions
Motivation
Skype ® is the most popular commercial platformfor peer-to-peer IP telephonyOver 700 million registered users worldwideOften used over data-enabled cellular devices
Skype can be significantly bit rate-hungryCellular coverage is compromised for high rate servicesAir interface parameters may affect Skype traffic
This work seeks to perform a side-by-sidecoverage comparison between conventional CSvoice services and Skype over 3G networks
Skype Traffic
CODEC G.729 iSAC SVOPC
Constant / Variable Bit Rate
(CBR/VBR)
CBR VBR VBR
Speech Bandwidth 4 kHz 8 kHz 8 kHz
Codec frame interval [ms] 10 30 or 60 20 - 60
Skype Frame interval Variable
Codec output bit rate [kbps] 8 Variable
10 – 32
Variable
20 – 50
Approx. max. sending rate
(no redundancy) [kbps]
12
34
54
Application Scenario Skype-
out
Skype-
to-Skype
Skype-to-
Skype
(3.1 or
later)
Main Skype Voice Codecs
Skype Traffic
Three factors influence Skype data rate:Choice of voice codecChoice of compression ratio (only for VBR codecs)Skype redundancy ratio
Under random packet loss conditions, Skypedata rate can be modeled by:
rs(t) is Skype’s sending rater(t) is the redundancy ratioLI(t) is the bit rate of the codec’s i-th operating mode
!
rs( t) = (1+ r( t))•L
i( t)
Skype Traffic
Skype’s redundancy ratio reflects doubletransmission of some voice packetsRedundant packets are piggybacked to fresh packets.For random packet loss rate under 2%, r(t) approaches
zero.For random packet loss rate above 5%, r(t) approaches
its maximum value (unity).
This work assumes that VBR codecs operateat their maximum rateThis ensures MOS values comparable to CS voice
(about 3.4 for AMR at 12.2 kbps).
Skype Traffic
CODEC AMR G.729 iSAC SVOPC
B L E R 1% 1% 10% 1% 10% 1% 10%
Max codec rate [kbps]
12.2 8
32 50
Max SKYPE sending rate [kbps]
N/A 12 24 34 68 54 108
Max MAC rate [kbps]
15.6 17 29 39 71 59 113
Required Eb/No [dB]
4.5 4.5 2.7 3.7 2.4 3.4 2.5
Traffic simulation scenarios
Simulation set-up
W-CDMA (release 99) system assumed Skype packets carried by the uplink DPDCH RTP/UDP/IP header compression (down to
about 1.6 kbps) Signaling overhead of 3.4 kbps (average) Small MAC and RLC overhead Two BLER set-point values considered for
Skype service: 1% and 10%.
Simulation set-up
Urban hexagonal macrocells Random system parameters:
Large scale fading (Shadowing)Small-scale fading (Rice/Rayleigh)Line-of-sight condition for outdoors locationsSoft-Handoff gain
520K uniformly spaced Monte Carlo trials percellCoverage probability defined as the fraction of Monte
Carlo trials where minimum required power is less thanmaximum UE transmit power
Simulation set-up
Parameter Value
UE max transmit power 24 dBm (1900 MHz)
UE ant. gain + cable loss 0 dB
Receiver Noise Figure 5.0 dB
Receiver Sensitivity Service-dependent
BS Antenna Type Sectorized, 90° beamwidth
BS Antenna Max Gain 16.1 dBi
BS Cable losses 2 dB
Uplink Load Factor 3.0 dB (50% cell load)
BS ant. diversity gain Included in (Eb/No)req
Link Budget Parameters
Simulation Set-up
Location Outdoors Indoors
Environment Urban
Frequency 1900 MHz (UMTS/PCS uplink bands)
UE location Street-level pedestrian In-building
Linearly decreasing LOS
probability as a function of
distance to cell
LOS/NLOS
LOS NLOS
NLOS at
all
locations
Distance-
dependent
Propag. Loss
Walfisch-Ikegami
LOS model
COST 231
Shadowing
Standard
Deviation
4 dB 8 dB 11 dB
Small-Scale
Fading
Rician with
distance-dependent
KR
Rayleigh
Building
Penetration
Loss
0 dB 0dB 12 dB
Body Loss 3 dB
Propagation Models
Simulation Set-up
Both indoors and outdoors locationsconsidered separately
Two cell-area coverage confidencescenarios: 90% and 95% (outdoors).
Simulation Scenario A B
Cell radius 1480 m 1130 m
Street-level coverage probability
for AMR speech
90 % 95 %
In-building coverage probability
for AMR speech
58 % 69 %
Results: G.729
Scenario: A Scenario: B
Cell-area coverage expressed as a fraction of CS-voice coverage
Results: iSAC
Scenario: A Scenario: B
Cell-area coverage expressed as a fraction of CS-voice coverage
Results: SVOPC
Scenario: A Scenario: B
Cell-area coverage expressed as a fraction of CS-voice coverage
Conclusion
Skype coverage is always a fraction of CS-voicecoverage because of higher peak bit rates:Such fraction is close to 100% for G.729 codecCoverage drops to 80-90% for iSAC codec, and even
less for SVOPC (down to a minimum of 67%)
Smaller cells (and thus, looser link budgets)improve Skype coverage relative to CS-speech
Setting BLER to 1% (as opposed to 10%)improves Skype coverage relative to CS-speechSkype redundant packet transmission is avoided by
lowering the air interface BLER
Conclusion
Coverage is confirmed to be a potential obstacle inmassive deployment of proprietary VoIP over cellularnetworks.
This problem can be significantly mitigated through:Adapting traffic management rules to cellular networks at the
VoIP application (i.e., redundancy)Appropriate setting of air interface parameters
Similar problems are expected in HSPA because ofcoverage sensitivity to bit rates and Eb/NorequirementsHowever, layer-2 retransmissions in HSPA should improve
coverage at the expense of increase packet jitter.
2012 IEEE Wireless Communications and Networking Conference
Coverage analysis ofSkype VoIP services
over 3G cellular networks
Renny E. Badra and Hayat AbdullaDepartamento de Electrónica y Circuitos
Universidad Simón BolívarCaracas, Venezuela