Experimental Measurement of VoIP Capacity in IEEE 802.11 WLANs
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Transcript of Experimental Measurement of VoIP Capacity in IEEE 802.11 WLANs
Experimental Experimental Measurement of VoIP Measurement of VoIP Capacity in IEEE 802.11 Capacity in IEEE 802.11 WLANsWLANs
Sangho ShinHenning Schulzrinne
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)
PLCP MAC IP UDP Voice ACKPLCPbackoff
DIFS SIFS
Tt
Tb
= 15 calls
PLCP = Physical Layer Convergence Procedure
Simulation setupSimulation setup
WIFIWIFI
WIFI
WIFI
Ethernet-Wireless
parameters value
Voice codecG7.11 (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
Delay 90th percentileRetry rate average
ExperimentsExperimentsNJ Rutgers University
ExperimentsExperiments
80 ft
70 ft
Atheros
Intel
Experimental setupExperimental setup
param value
Voice codecG7.11 (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 802.11b
Experimental resultsExperimental results
Capacity
ComparisonsComparisonsSimulation Experiments
Delay
Downlink delay is larger than uplink delay
Very low Increases sharply
Gradually increases
Retry rate
Uplink retry rate is higher than downlink retry rate
Uplink:2~5%Downlink:1~3%
Uplink:7~11%Downlink:4~7%
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
ARFARF ARF (Auto Rate Fallback)
PHY data rate are automatically changes Transmission failure decrease rate Successful transmission restore the rate
When frame loss is caused by bad link quality, it helps
When frame loss is caused by congestion, it makes worse
No way to tell the reason for frame losses Problems
The effect varies according to algorithms Turned off in simulations Turned on in wireless cards
The algorithms are mostly implemented in drivers
ARFARFARF=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
Long Short
Preamble size 144 us 72 us
Header size (b) 48 bits 48 bits
Header coding rate 1 Mb/s 2 Mb/s
Header size (us) 48 us 24 us
Total size (us) 192 us 96 us
Portion in a VoIP (size)
9% 6%
Portion in a VoIP (time)
53% 36%
PLCP Preamble PLCP Header
PLCP Preamble PLCP Header
Long preamble
Short preamble
144us
72us
48bits
48bits
= 48us
= 24us
192us
96us
PLCP MAC IP UDP Voice
PLCP = Physical Layer Convergence Procedure
Preamble sizePreamble size
PHY data rate for ACK PHY data rate for ACK framesframes
ACK frames Required for ARQ (Automatic Repeat-reQuest)
Theoretical VoIP Capacity using 11 Mb/s for ACK frames 16 calls
PLCP MAC
14B
2Mb/s 152 us = 57% of a VoIP packet11Mb/s106 us = 39% of a VoIP packet
Type : 01 Subtype 1101
PHY data rate of ACK PHY data rate of ACK framesframes
Offset of VoIP traffic Offset of VoIP traffic startimestartime
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
650us > 31 x 20us
Offset of traffic start time (us)
Simulation results with 15 VoIP sources
≈ 20 ms / (15 * 2)
Key FactorsKey Factors 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 Scanning APs
When signal strength decreases below a threshold When the retransmission rate increases above a
threshold Regularly (e.g. once per 30 seconds) 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 (unicaset)
Scanning APsScanning APs
Retry limitRetry limit Wireless nodes retransmit frames until the
number of retransmission reaches the retry limit
Long retry limit For the packets whose size is bigger than the RTS
threshold Short retry limit
For the packets whose size is smaller than or equal to the RTS threshold
Effect More retransmissions
Might reduces packet loss, but increases congestion Less retransmissions
Increases the packet loss
(4)
(7)
Retry limitRetry limit
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 avoid
the effect Simple static queuing analysis
avgS
BD max
Maximum queuing delay (ms)
Buffer size (B)
Packet size
Average transmission time of a packet
u = 2msD = 60msS = 200BB = 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
Thank you!Thank you!