experimental measurement of voip capacity in ieee 802.11 wlans sangho shin henning schulzrinne...
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Experimental Experimental Measurement of VoIP Measurement of VoIP Capacity in IEEE 802.11 Capacity in IEEE 802.11 WLANsWLANs
Sangho ShinHenning Schulzrinne
Department of Computer ScienceColumbia University
VoIP over Wireless LANsVoIP over Wireless LANs
InternetInternet
AP (Access Point)
PBX
WIFI
Motivation and goalMotivation and goal
Check the VoIP capacity using wireless cards and compare it with theoretical and simulation results
Identify all factors that affect the VoIP capacity in experiments and simulations
OutlineOutline
Theoretical capacity for VoIP traffic VoIP capacity via simulations VoIP capacity via experiments ‘Hidden factors’ that affect
experiments and simulations Conclusion
Packetization interval
1 2 3 N 1 2 3 N……. …….MAC
Theoretical capacityTheoretical capacity
parameters value
Voice codec 64 kb/s
Packet size 160B
Packetization interval
20ms
Transport layer UDP
PHY data rate 11 Mb/s
RTS/CTS No
bt TT
PN
2max
Capacity (calls)
Packetization Interval (ms)
= 15 calls
PLCP = Physical Layer Convergence Procedure
PLCP MAC IP UDP Voice ACKPLCPbackoff
DIFS SIFS
Tt
Tb
RTP
Simulation setupSimulation setup
WIFIWIFI
WIFI
WIFI
Ethernet-Wireless
parameters value
Voice codecG.711 (64 kb/s)
Packet size 160B
Packetization interval
20ms
Transport layer UDP
PHY data rate 11Mb/s
RTS/CTS No
WIFI
IEEE 802.11b
QualNet simulator v3.9
Simulation resultsSimulation results
CapacityNumber of VoIP sources
90th
per
cent
ile d
ela
y (m
s)
Downlink delay
Uplink delay
ExperimentsExperimentsNJ Rutgers University
ExperimentsExperiments
80 ft
70 ft
Atheros
Intel
Experimental setupExperimental setup
parameters value
Voice codecG.711 (64 kb/s)
Packet size 160B
Packetization interval
20ms
Transport layer UDP
PHY data rate 11Mb/s
RTS/CTS No
client
client clientclient client
clientclientclient
clients clientAPclient
client clientclientclient
IEEE 802.11bAtheros chipsetMadWifi-0.9.3
Experimental resultsExperimental results
Capacity
90th
per
cent
ile d
ela
y (m
s)
Downlink delay
Uplink delay
FactorsFactors ARF (Auto Rate Fallback) Preamble size PHY data rate of ACK frames Offset of VoIP traffic start time Signal strength Scanning APs Retry limit Network buffer size
90th
per
cent
ile d
ela
y (m
s)
Fixed rate
ARF (AMRR)
Threshold for capacity
ARFARF ARF (Auto Rate Fallback)
PHY data rate are automatically changes When frame loss is caused by bad link quality, it helps When frame loss is caused by congestion, it makes worse
Problems The effect varies according to algorithms
Turned off in simulations Turned on in wireless cards
Experimental results 8% of frames were transmitted with lower rates
AMRR=Adaptive Multi-Rate Retry
Preamble sizePreamble size IEEE 802.11b : long and short
preamble QualNet, NS-2 Long preamble Atheros + MadWifi driver Short
preamble Theoretical capacity with the long
preamble = 12 calls Experimental results
Long Short
Preamble size 144 us
72 us
Header size (us) 48 us 24 us
Total size (us) 192 us
96 us
Fraction in a VoIP (size)
9% 6%
Fraction in a VoIP (time)
53% 36%
PLCP = Physical Layer Convergence Procedure
90th
per
cent
ile d
ela
y (m
s)
Short
Long
PHY data rate for ACK PHY data rate for ACK framesframes ACK frames
Required for ARQ Theoretical VoIP capacity
using 11 Mb/s for ACK frames 16 calls
Experimental results
PLCP MAC
14B
2Mb/s 152 us = 57% of a VoIP packet11Mb/s106 us = 39% of a VoIP packet
Type : 01 Subtype 1101
90th
per
cent
ile d
ela
y (m
s)
11 Mb/s
2 Mb/s
MadWifi2Mb/sQualNet11Mb/sNS-21Mb/s
Offset of VoIP traffic start Offset of VoIP traffic start timetime
1 2 3 4
Packetization interval
1 2 3 4Application layerOffset
MAC layer data backoff
SIFS
ACK
DIFS
data
VoIP source 1
VoIP source 2
VoIP source 3
VoIP source 4
1
2
3
4
1
2
3
4
MAC layer 1 2 3 4 1 2 3 4collisions
Offset of VoIP traffic start Offset of VoIP traffic start timetime
Uplink retry rate
650 μs = the optimal offset (20ms/(15 sources*2))
Offset of traffic start time (μs)
Simulation results with 15 VoIP sources
90th
per
cent
ile d
ela
y (m
s)
FactorsFactors ARF (Auto Rate Fallback) Preamble size PHY data rate of ACK frames Offset of VoIP traffic start time Signal strength Scanning APs Retry limit Network buffer size
Fixed
Short
2Mb/s
Randomized
Signal strengthSignal strength
Scanning APsScanning APs Scan APs based on
signal strength transmission failure Regularly (e.g. every min) Hard to determine the
algorithms Problems
Management frames have a higher priority than data frames causes delay
Increases the traffic make channels congested
1 probe request and 1 ~ 2 probe responses per channel
APclientProbe request (broadcast)
Probe response (unicast)
(fo
r 10
0 s)
Retry limitRetry limit Wireless nodes retransmit frames until the number
of retransmission reaches the retry limit Long retry limit - frame size > RTS threshold Short retry limit - frame size ≤ RTS threshold
Effect More retransmissions reduces packet loss, but
increases congestion Less retransmissions Increases the packet loss
Experimental results
(4)(7)
Network buffer sizeNetwork buffer size Packet loss happens mostly because of the buffer
overflow at the AP Small buffer increase the packet loss Bigger buffer reduces packet loss, but increase the
delay Buffer size needs to be big enough to allow 60ms of
delay Simple static queuing analysis
avgS
BD
1
max
Maximum queuing delay
Buffer size
Packet size
Average service rate
µ = 1/500D = 60msS = 200BBmin = 5.8KB < 10KB MadWifi
ConclusionConclusion Need to consider the following factors when
measuring the VoIP capacity experimentally ARF Preamble size PHY data rate of ACK frames Offset of VoIP traffic start time Scanning APs Retry limit Network buffer size
By adjusting all the factors, we can achieve the same experimental, simulation, theoretical capacity
Our study can be used in any 802.11 experiments and the analysis and comparison
Thank you!Thank you!