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!