Measurement and Analysis of the VoIP Capacity in IEEE 802.11 WLAN - 2009

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Mobilizing Information Technology Lab @ EEDept.@NTUST

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Measurement and Analysis of theVoIP capacity in IEEE 802.11

WLANAdvisor : Dr. Jenq-Shiou Leu

Student : Yun-Sun Yee

Date : 21 Sept. 2009

Sangho Shin; Schulzrinne, H.;

Mobile Computing, IEEE Transactions on

Volume 8, Issue 9, Sept. 2009 Page(s):1265 - 1279

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Outline

l Introduction

l Related Works

l Theoretical Capacity For VoIP Traffic

¡Capacity for CBR VoIP Traffic

¡Capacity for VBR VoIP Traffic

l Capacity For VoIP Traffic VIA Simulations

lCapacity For VoIP Traffic VIA Experiments

l Conclusion

l Appendix








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Introduction

l As many public 802.11 wireless networks have been

deployed not only in buildings but also in parks and

streets

l The importance of usage of VoIP over wireless networkshas been increasing

l The necessity of a large number of wireless clients and

the difficulty of controlling them and collecting data








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Related Works

l A lot of research has been conducted to improve QoS and

increase the capacity for VoIP traffic

¡Hole and Tobagi et al.

lProvides an analytical upper bound on the capacity for VoIP

applications in IEEE 802.11b networks, evaluating a wide

range of scenarios including different delay constraints,

channel conditions and voice encoding schemes using an

analytical method

assuming the long preamble only








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Related Works (cont.)

l Veeraraghavan et al.

¡Analyzed the capacity of a system that uses Point

Coordination Function (PCF) for Constant Bit Rate (CBR)

and Variable Bit Rate (VBR) voice traffic, using Brady'smodel for VBR voice traffic.

¡Used a value of 75 ms and 90 ms as the Contention Free

Period (CFP) interval, which causes a delay that is not

acceptable for VoIP. The capacity for VoIP with a 90 ms

CFP interval was 26 voice calls, but the maximum delay

was 303 ms.








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l Chen et al.

¡Evaluated via simulations the capacity of VoIP with IEEE

802.11e Enhanced DCF (EDCF) and Enhanced PCF

(EPCF), which are called EDCA and HCCA in the finalstandard.

¡Used G.711, G.729 and G.723.1 as voice codecs and

assumed CBR traffic. IEEE 802.11e provides low end-to-

end delay for voice packets even if mixed with best effort

traffic.








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l Garg and Kappes et.al.

¡Experimentally measured the capacity for VoIP traffic with

10 ms packetization interval and the effect of VoIP traffic

on UDP data traffic in 802.11b.¡Found that the capacity of such VoIP traffic is six and the

effective available bandwidth is reduced by ongoing VoIP

connections








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l Kodama and Katsube et. al.

¡Measured the capacity and the performance of their newscheme, Backoff Control and Priority Queuing (BC-PQ)experimentally.

¡To decide the capacity for VoIP traffic, they used thepacket loss rate, which depends on the network buffer sizeof the AP in DCF, unless the wireless link is unreliable

¡Found that the capacity with 20ms packetization interval is10 calls

, which differs from our results and we believe thatthis is because of the effect of the Auto Rate Fallback (ARF)and preamble size

parameters are not mentioned in the paper








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Theoretical Capacity For VoIP Traffic

l Capacity for CBR VoIP Traffic

NCBR = P/(2*Tt)

NCBR = maximum number of CBR calls

P = packetization interval

Tt = total transmission time

Tt = TDIFS + TSIFS + Tv + TACK + Tb

TDIFS = Distributed Interframe Space (DIFS)

TSIFS = Short Interframe Space (SIFS)








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Theoretical Capacity For VoIP Traffic (cont.)

l Capacity for CBR VoIP Traffic

¡The backoff time is the number of backoff slots Ts, where

Ts is a slot time, and the number of backoff slots has a

uniform distribution over (0, CWmin) with an average of CWmin /2.








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Parameters in IEEE 802.11b (11mb/s)








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l Capacity for VBR VoIP Traffic

¡VBR VoIP traffic is characterized by on (talking) and off

(silence) periods

¡defined as the ratio of on-periods and the wholeconversation time

¡Difference with CBR Traffic is the number of packets

generated every second

19.5 packets(0.39 activity ratio)50 packets20ms

VBRCBR









13


ITU-T Recommendation P.59










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Capacity For VoIP Traffic VIA Simulations








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Capacity For VoIP Traffic VIA Simulations (cont.)

l Ethernet portion added 1ms of transmission delay to

assume that the end-to-end delay is essentially the same

as the wireless transmission delay

l Voice codec : G.711, 64kb/s with 20ms packetizationinterval

l Time period : 200s repeated 50 times with different seeds

and VoIP traffic start time








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Capacity For VoIP Traffic VIA Simulations (cont.)

Ninetieth percentile delay and retry rate of CBR & VBR VoIP traffic in simulations.

Number of CBR VoIP Sources








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Capacity For VoIP Traffic VIA Experiments

l Open Access Research Testbed for Next-Generation

Wireless Networks (ORBIT) testbed








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Capacity For VoIP Traffic VIA Experiments (cont.)

l We used the main grid, which consists of 380 nodes withAtheros chipset (AR5212) wireless cards and 20 nodeswith Intel chipset wireless cards

l20 x 20 grid with 1 meter internode distance

l Set up a center node as the AP so that distances betweenthe AP and nodes are within 10 meters, which is closeenough to avoid the effect of signal strength on packetloss








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l Every node has a Pentium IV CPU with 1 GB memory,runs Linux (kernel 2.6.19), and has two wireless and twoEthernet interfaces with the Mad-Wifi driver 0.9.2 used asthe wireless card interface driver.

l Wrote a simple UDP client which sends 172 byte (160 B

VoIP payload +12 B RTP header) UDP packets to a

specified destination.

¡Records the sending time and receiving time in separate

files with the UDP sequence number, which is included as

an UDP packet payload, and the data were used to calculate

the downlink and uplink delay and the packet loss








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Ninetieth percentile delay and retry rate of CBR & VBR VoIP traffic in the experiments.








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Conclusion

l Measured the capacity for VoIP traffic via experiments

with actual wireless clients in the ORBIT testbed and

compared it with the theoretical capacity and simulation

results.l Based on IEEE 802.11b operating at a data rate of 11

Mb/s








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Appendix

l Point Coordinated Function

¡Media Access Control (MAC) technique used in wireless

networks which relies on a central node, often an Access

Point (AP), to communicate with a node listening to see if

the airwaves are free

¡available only in "infrastructure" mode

l HCCA (HCF (Hybrid Coordinator Function) Controlled

Channel Access)

¡QoS-enabled stations have the ability to request specifictransmission parameters (data rate, jitter, etc.) which should

allow advanced applications like VoIP and video streaming

to work more effectively on a Wi-Fi network.










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Appendix (cont.)

l Short Interframe Space

¡Small gap between the data frame and its acknowledgment

l DCF Interframe Space

¡Transmissions being allowed if the medium is

continuously idle for DIFS duration

¡DIFS = SIFS + (2 * Slot time)

9 or 20

9

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Slot time (us)

28 or 5010IEEE 802.11g

3416IEEE 802.11a

5010IEEE 802.11b

DIFS (us)SIFS (us)Standard

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Measurement and Analysis of the VoIP Capacity in IEEE 802.11 WLAN - 2009