Measurement and Analysis of the VoIP Capacity in IEEE 802.11 WLAN - 2009
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Transcript of Measurement and Analysis of the VoIP Capacity in IEEE 802.11 WLAN - 2009
8/8/2019 Measurement and Analysis of the VoIP Capacity in IEEE 802.11 WLAN - 2009
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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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Related Works (cont.)
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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Related Works (cont.)
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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Related Works (cont.)
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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Theoretical Capacity For VoIP Traffic (cont.)
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
Theoretical Capacity For VoIP Traffic (cont.)
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Theoretical Capacity For VoIP Traffic (cont.)
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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Capacity For VoIP Traffic VIA Experiments (cont.)
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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Capacity For VoIP Traffic VIA Experiments (cont.)
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
20
Slot time (us)
28 or 5010IEEE 802.11g
3416IEEE 802.11a
5010IEEE 802.11b
DIFS (us)SIFS (us)Standard
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