Portable Networks and the Wireless Web

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Portable Networksand the Wireless Web

• Carey Williamson
• Department of Computer Science
• University 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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Example Infrastructure Mode
cnn.com
Internet
Access Point (AP)
Carey
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Example Ad Hoc Mode

• Multi-hop ad hoc networking

Mike
Carey
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Example Ad Hoc Mode

• Multi-hop ad hoc networking

Mike
Carey
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Example Ad Hoc Mode

• Multi-hop ad hoc networking

Mike
Carey
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Example Ad Hoc Mode

• Multi-hop ad hoc networking

Mike
Carey
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Main Idea

• 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)
mystuff.com
Access Point (AP)
Carey
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Portable Network (2 of 2)
mystuff.com
Carey
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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 1

• 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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2 Wireless Media Servers
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3 Wireless Gaming
Multi-player gaming over an ad hoc wireless
network
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4 Home Networking

• Even for homes without Internet access, wireless
  media servers and ad hoc networks could be quite
  useful
• Possible usage scenarios
• use PDA to read recipe while in the kitchen
• page your kids for supper time
• work while on your back porch (backups)
• music in any room of the house
• portable media player for parties
• family gaming fun

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Research Objectives

• Assess feasibility of portable networks
• Benchmark the capabilities and limitations of
  wireless content servers in an ad hoc
  IEEE 802.11b WLAN
• Identify performance bottlenecks
• Understand sensitivity of performance to
  different workload assumptions
• Understand impacts of wireless network channel
  quality and error rates

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Prototype Experiments

• Wireless Classroom Measurements
• CPSC 641 grad class (14 students)
• February 2003
• Wireless Web Server Benchmarking
• Laboratory experiments
• Stress-testing, workload sensitivities
• Wireless Media Server Experiments
• French 217 cinematography class (Mar 04)
• Video and audio streaming (8 students)

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Prototype Experiments 1

• Wireless Classroom Measurements
• CPSC 641 grad class (14 students)
• February 2003
• Wireless Web Server Benchmarking
• Laboratory experiments
• Stress-testing, workload sensitivities
• Wireless Media Server Experiments
• French 217 cinematography class (Mar 04)
• Video and audio streaming (8 students)

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Experimental Setup
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Classroom Experiments (Feb03)
0
1400
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(No Transcript)
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TCP Number of Packets
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Prototype Experiments 2

• Wireless Classroom Measurements
• CPSC 641 grad class (14 students)
• February 2003
• Wireless Web Server Benchmarking
• Laboratory experiments
• Stress-testing, workload sensitivities
• Wireless Media Server Experiments
• French 217 cinematography class (Mar 04)
• Video and audio streaming (8 students)

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Experimental Setup
SnifferPro 4.6
wireless sniffer
Apache 1.3.23
httperf
clients
server
...

• Empirical measurement methodology
• Network In-building 11 Mbps IEEE 802.11b
  wireless LAN, operating in ad hoc mode (single
  hop no mobility)
• Hardware Compaq Evo N600c notebooks (1.2 GHz
  Pentium III, 128 MB RAM, 512 KB L2 Cache, Cisco
  Aironet 350 network cards
• Software Redhat Linux 7.3 (kernel 2.4.18-3),
  Apache, SnifferPro, httperf for Web workload
  generation

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Benchmarking Experiments
Experimental Factors and Levels
Performance Metrics HTTP response time, network
throughput
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Tutorial HTTP and TCP

• TCP is a connection-oriented protocol

YOUR DATA HERE
Web Client
Web Server
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Example Web Page
Harry Potter Movies
The new HP book will be out soon and then there
will be a new movie shortly after that. Title is
still secret, but my guess is... Harry Potter
and the Bathtub Ring
hpface.jpg
page.html
castle.gif
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Server
Client
The classic approach in HTTP/1.0 is to use
one HTTP request per TCP connection, serially.
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Server
Concurrent (parallel) TCP connections can be
used to make things faster.
Client
C
C
S
S
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Server
Client
The persistent HTTP approach can re-use
the same TCP connection for Multiple HTTP
transfers, one after another, serially. Amortizes
TCP overhead, but maintains TCP state longer at
server.
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Research Question

• What is the range of sustainable load?
• Design
• Number of Client 1
• HTTP request rate (req/sec) 10, 20, , 160
• HTTP transfer size 1 KB
• Non-persistent HTTP
• Client-server distance 1 meter (same desk)

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Request Rate Results
Transmit Queue Length
Transaction Rate (req/sec)
Transmitted Packet ID (b) Client Transmit Queue
Request Rate (req/sec) (a) Successful HTTP
Transaction Rate
- Maximum sustainable 1 KB HTTP transaction rate
for 1 client is about 85 reqs/sec
(throughput about 0.9 Mbps) - Beyond this request
rate, the client link-layer transmit queue
builds up and overflows, losing packets even
before they get onto the wireless LAN! - Wireless
LAN is bottleneck (802.11b channel access
protocol)
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HTTP/1.0 Transaction (1 KB)
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CSMA-CA Acknowledgement
Carrier Sense Multiple Access with Collision
Avoidance
others
source
destination
Air is free for DIFS time period (128 usec)
DIFS
send frame
data
All other devices must defer while air is busy
NAV defer access
SIFS
Receive ACK back that frame was received intact!
ack

• Every frame is acked - except broadcast and
  multicast!

• SIFS - Short Inter-Frame Space (approx 28 µs)

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Research Question

• Does persistent HTTP help?
• Design
• Number of Clients 1, 2
• HTTP request rate 10 req/sec
• HTTP transfer size 1 KB
• Persistent HTTP
• Client-server distance 1 m (same desk)

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Results for Persistent Connections
YES!
Throughput (Mbps)
HTTP Req/Connection

• Peak throughput 3.2 Mbps, 3.5x improvement over
  non-persistent connection case (0.9 Mbps) for 1
  KB transfers
• Typically 2 TCP packets per HTTP transaction (vs
  10)

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HTTP/1.1 Transactions (1 KB)
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Research Question

• What is maximum throughput achievable?
• Design
• Number of Client 1
• HTTP request rate 10 req/sec
• HTTP transfer size (KB) 1, 2, 4, 8,
• Non-persistent HTTP
• Client-server distance 1 m (same desk)

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Transfer Size Results
TCP Connection Duration (sec)
Frequency ()

• bottleneck shifts to the servers link
• layer transmit queue

Connection ID
Duration (Sec)
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Summary of Results

• Wireless Web servers can work!
• Wireless LAN is the bottleneck
• Bottleneck manifests itself differently,
  depending on the Web workload
• client side, for small HTTP transfers
• server side, for large HTTP transfers
• unfairness amongst clients if TCP SYN losses
• network thrashing in some scenarios
• Persistent HTTP helps a lot!

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Prototype Experiments 3

• Wireless Classroom Measurements
• CPSC 641 grad class (14 students)
• February 2003
• Wireless Web Server Benchmarking
• Laboratory experiments
• Stress-testing, workload sensitivities
• Wireless Media Server Experiments
• French 217 cinematography class (Mar 04)
• Video and audio streaming (8 students)

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Experimental Setup
Wireless Sniffer
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Video Frame Size Distribution
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Video Frame Sizes (I-frames)
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UDP Packet Traffic on WLAN
Done
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TCP Packet Traffic on WLAN
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Summary of Results

• Wireless media servers can work!
• Classroom experiment worked perfectly
• Students loved it!
• Wireless LAN is the bottleneck
• Bottleneck at server WLAN interface
• Works beautifully with up to 8 clients (400
  kbps video, 128 kbps audio each)
• Complete disaster with 9 or more clients!
• Queue overflow, packet loss, late, etc.
• Need to use multicast to make this scalable

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Mobility Experiment
Wireless Sniffer
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Mobility Experiment
Wireless Sniffer
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(No Transcript)
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The Bad Apple Phenomenon

• What? One user with poor or transient wireless
  connectivity in the WLAN disrupts performance for
  everyone
• Why? Shared broadcast WLAN lots of MAC-layer
  retransmissions FIFO server queue Head of Line
  (HOL) blocking
• Solutions?
• Disable MAC-layer retransmissions (yuck!)
• Multiple queues and packet scheduling

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Conclusions

• Portable networks a novel paradigm for the use
  of wireless ad hoc networks
• Reasonable performance with existing
  off-the-shelf hardware and software
• Performance bottleneck at the WLAN manifests
  itself in interesting ways
• IEEE 802.11a (55 Mbps) may help
• Need to explore novel scenarios for the use of
  this networking paradigm

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The End

• Thanks for your attention!
• Credits
• Guangwei Bai, Jean Cao, Kenny Oladosu
• My iCORE research team
• More info carey_at_cpsc.ucalgary.ca
• Questions?