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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/

maria@cs.unc.edu

Thank You!