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Assessing The Real Impact of 802.11 WLANs:A Large-Scale Comparison of
Wired and Wireless Traffic
Maria Papadopouli *Assistant Professor
Department of Computer ScienceUniversity of North Carolina at Chapel Hill (UNC)
* This work was partially supported by the IBM Corporation under an IBM Faculty Award 2004It was done while visiting the Institute of Computer Science, Foundation for Research
and Technology-Hellas, Greece
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Collaborators & Coauthors
Felix-Hernandez Campos
Department of Computer Science
University of North Carolina at Chapel Hill (UNC)
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Roadmap
Motivation & Research Objectives Testbed & Data Acquisition Data Analysis Contributions Future Work
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Motivation
Increasingly deployment of 802.11 wireless networks Plethora of novel research issues such as mobility, power
management, capacity planning, QoS support Need for benchmarks More accurate and realistic characterizations of production
wireless networks & their performance More representative assumptions in theoretical &
simulations studies
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Research Objectives
Characterize packet-level performance• volume• packet loss• unnecessary retransmissions• delay
Contrast • wireless vs. wired • WAN vs. LAN
Perform large-scale passive measurements
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Infrastructure
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Testbed & Data Set
729-acre campus: 26,000 students, 3,000 faculty, 9,000 staff Diverse environment 14,712 unique MAC addresses 488 APs (Cisco 1200, 350, 340 Series) 175 GB packet headers in a 7-day trace 9,766,507 TCP connections from wired clients 21,396,174 TCP connections from wireless clients ~ 33% of the connections: pathological cases with no useful
payload (~0.1% of bytes) Wireless/wired TCP connections carried ~500GB (each)
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TCP Connection Payload (Bytes)Wired Clients vs. Wireless Clients
Wireless Clients
Wired Clients
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TCP Connection Payload (Bytes)
Download
Upload
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Connection Size (# Packets)
Connections with 100 packetsrepresent < 5% of all connectionsbut carry > 85% of the total bytes
≥
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One-Side Transit Time Measurement
WAN OSTT
LAN OSTT
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Minimum One-Side Transit Time
WAN
Wired LAN
Wireless LAN
[6ms,250ms]
[0.7ms, 1ms]
[1ms,7ms][7ms, 250ms]
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One-Side Transit Time Statistics
Wireless LAN
Wired LANHeavy max & avg
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Maximum One-Side Transit Time
Wired Clients
Wireless Clients
TCP delay ACK mechanismintroduces extra delays of ~ [100ms, 200ms]
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Large Delay Variability on Wireless LAN
Wired LANmad
Wireless LANmad
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Fraction of Round-Trip Time from LANUsing Medians & Means
Wireless Clients
Wired Clients
Using medians
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Packet Losses
2% losses were observed for • 17% of the connections for wired clients• 23% of the connections for wireless clients
802.11 link layer retransmission is very effectiveThe high delay variability suggests that several losses were recovered
They may be higher under special conditions Client mobility High traffic load at AP
Computed based on retransmissions and triple duplicate ACKs (3DUP)
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Unnecessary Retransmissions (% Tot. Pck)
Enough samples for amore conservative timeout that reduces the # of unnecessary retransmissions
Wireless Clients
Wired Clients
100+ connections
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Contributions Large-scale passive measurement study on TCP connection
characteristics on• volume• delays• losses• unnecessary retransmissions• lack of termination
Wireless vs. Wired LANs Wireless LANs have substantially higher delay variability significant more unnecessary retransmissions only marginally greater packet losses
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Future Work
Characterization of wireless flows under certain conditions (mobility, application, AP, time)
Flow modeling Forecasting traffic load using flow-related information Contrast connection models from different wireless
environments (campus, institute, metropolitan area, conference)
UNC/FORTH Data repository with traces & models
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More Info
http://www.cs.unc.edu/~maria
http://www.ics.forth.gr/mobile/
Thank You!