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Network Measurements of a Wireless Classroom Network

Carey Williamson Nuha Kamaluddeen

Department of Computer ScienceUniversity of Calgary

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Introduction Wireless technologies are prevalent

today; continued growth in popularity Example: IEEE 802.11b WLAN (“WiFi”) Economical, convenient, flexible

solution for tetherless network access (11 Mbps)

Enabler for mobile computing Two possible modes of usage:

Infrastructure mode Ad hoc mode

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Infrastructure Mode

Carey

Internet

Access Point (AP)

cnn.com

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Motivation Observation: The same wireless

technology that allows clients to be mobile also allows servers to be mobile

Hybrid networking paradigm, combining client-server and ad hoc networking, without general Internet infrastructure

Portable, short-lived, ad hoc networks “Portable networks” Is this useful? How well does it work?

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Portable Network (1 of 2)

CareyAccess Point (AP)

mystuff.com

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Portable Network (2 of 2)

Carey

mystuff.com

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Portable Networks Concept Set up when needed, tear down after Typically needed for minutes or

hours When and where not known a priori No existing network infrastructure General Internet access not

available, but not required either Pre-defined content; target audience Modest number of users; mobile too

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Example Usage Scenarios

Classroom area network (e.g. “legacy classroom”)

Press conferences, media events Conventions and trade shows Disaster recovery sites Recruiting events Schools Voting...

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Classroom Experiment

Winter 2003 CPSC 641 graduate course at U of C

(Performance Issues in High Speed Networks) Course content available online Mirrored copy of Web site provided in

classroom using wireless Web server Students download desired content Review lecture notes Begin work on assignment (large trace file)

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Experimental Setup

IEEE 802.11b wireless LAN Ad hoc mode Web server (Apache) 13 students, sharing

6 laptops and 2 PDAs Wireless network analyzer

Web Server Sniffer

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Results Analysis Overview

Aggregate traffic profile Per-client traffic profile

TCP-level analysis TCP connection-level statistics Throughput analysis

HTTP-level analysis Web user behaviour Persistent HTTP connections

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Aggregate Traffic Profile (Feb’03)

Aggregate Packets Transmitted on WLAN

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Time in Seconds

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Packets Transmitted by Client IP 200

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Packets Transmitted by Client IP 204

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Packets Transmitted by Client IP 207

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Packets Transmitted by Client IP 212

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Packets Transmitted by Client IP 220

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Packets Transmitted by Client IP 208

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Per-Client Traffic Profiles

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TCP-level Analysis

Trace file Info about each TCP/IP packet

exchanged 1400 seconds (~ 23 minutes) 262 TCP connections observed

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Tutorial: HTTP and TCP

TCP is a connection-oriented protocol

SYN

SYN/ACK

ACKGET URL

YOUR DATA HERE

FINFIN/ACK

ACK

Web Client Web Server

Example Web Page

Harry Potter Movies

As you all know,the new HP moviecame out in Juneand then there willbe a new bookshortly after that…

“Harry Potter andthe Bathtub Ring”

page.html

hpface.jpg

castle.gif

Client Server

The “classic” approachin HTTP/1.0 is to use oneHTTP request per TCPconnection, serially.

TCP SYN

TCP FIN

page.htmlGet

TCP SYN

TCP FIN

hpface.jpgGet

TCP SYN

TCP FIN

castle.gifGet

Client Server

The “persistent HTTP”approach can re-use thesame TCP connection forMultiple HTTP transfers,one after another, serially.Amortizes TCP overhead,but maintains TCP statelonger at server.

TCP FIN

Timeout

TCP SYN

page.html

Get

hpface.jpg

Get

castle.gif

Get

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TCP – Connection Duration

56% lasted < 10 seconds

Some lasted > 3 minutes

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TCP – Number of Packets

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TCP – Number of Bytes

92% transferred less than 2 MB

One connection transferred 50 MB Contributed 20%

of total traffic! Heavy-tailed

behaviour

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TCP – Connection Throughput

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HTTP Requests per TCP Conn.

Persistent connection Download multiple

HTTP objects in one TCP connection

Most TCP connections are non-persistent, but most HTTP transfers are on persistent connections

Heavy-tailed or power-law behaviours

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HTTP – Number of Packets

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HTTP – Number of Bytes

Up to 24 MB

Majority transferred < 2 KB

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HTTP Transfer Throughput

Higher average than TCP connection throughput

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Summary and Conclusions

Portable networks: a novel paradigm for the use of wireless ad hoc networks

Adequate performance for small WLAN classroom area network environment

Works well even for wireless media streaming of audio and video with up to 8 clients (French 217, March 2004)

Future work: exploring other scenarios using this networking paradigm