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1
Portable Networks:Prototype and Performance
Carey WilliamsonGuangwei Bai and Kehinde
OladosuDept of Computer Science
University of Calgary
2
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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8
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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11
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
12
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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#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
16
Research Objectives Assess feasibility of portable networks Benchmark the capabilities and
limitations of wireless Web servers in an IEEE 802.11b Wireless LAN
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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Experimental Setupwireless sniffer
clients server
...
SnifferPro 4.6
Apache1.3.23
httperf
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
18
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 bookwill be out in Juneand then there willbe a new movieshortly 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 Concurrent (parallel) TCPconnections can be usedto make things faster.TCP SYN
TCP FIN
page.htmlGet
castle.gif
Get
F
S
Get
hpface.jpg
S
F
C S C S
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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Benchmarking Experiments
Factor Levels Number of Clients 1, 2, 4
HTTP Transaction Rate (per-client) 10, 20, 30, …, 160
HTTP Transfer Size (KB) 1, 2, 4, 8, …, 100
Persistent Connections no, yes
HTTP Requests per Connection 1, 5, 10, 15, …, 60
Transmit Power (mW) 1, 5, 20, 30, 50, 100
Client-Server Distance (m) 1, 10, 100
Experimental Factors and Levels
Performance Metrics: HTTP response time, network throughput
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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)
Research Question
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Request Rate Results
Tra
nsm
it Q
ueu
e L
engt
h
Transmitted Packet ID
(b) Client Transmit Queue
Tra
nsacti
on
Rate
(re
q/s
ec)
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)
CSMA-CA + AcknowledgementCarrier Sense Multiple Access with Collision Avoidance
* SIFS - Short Inter-Frame Space (approx 28 µs)
• Every frame is ack’ed - except broadcast and multicast!
“Air” is freefor DIFS time
period(128 usec)
Receive ACK backthat frame was received intact!
send frame
source destination others
DIFS
SIFS
All other devicesmust defer while
“air” is busy
data
ack
NA
V:
defe
r access
IEEE 802.11b Frame Format
Long Preamble = 144 bits
• Interoperable with older 802.11 devices
• Entire Preamble and 48 bit PLCP Header sent at 1 Mbps
128-bit Preamble
(Long)
16 bitStart ofFrame
Delimiter
Signal Speed
1,2,5.5,11
Mbps
Lengthof
Payload
16 bitCRC
Payload0-2312 bytes
Transmitted at 1 Mbps Transmitted at X Mbps
DLL HDR (IP TCP HTTP Data)
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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
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)
Th
rou
ghp
ut
(Mb
ps)
HTTP Req/Connection
YES!
30
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 ResultsT
CP
Con
nec
tion
Du
rati
on (
sec)
Connection ID
Fre
qu
ency
(%
)
Duration (Sec)
48 KB:Mean Duration: 97 msThroughput: 4.1 Mbps
bottleneck shifts to the server’s link layer transmit queue
8 KB:
Mean Duration: 24 ms
Throughput: 2.9 Mbps
1 KB:
Mean Duration: 9.7 ms
Throughput: 0.9 Mbps
32
Classroom Experiments (Feb’03)
Aggregate Packets Transmitted on WLAN
0
100
200
300
400
500
600
700
800
Time in Seconds
Pa
cke
ts p
er
1 se
c In
terv
al
0 1400
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Packets Transmitted by Client IP 200
0
100
200
300
400
500
600
700
Time in Seconds
Pa
cke
ts p
er
1 s
ec I
nte
rva
l
Packets Transmitted by Client IP 204
0
100
200
300
400
500
600
700
Time in Seconds
Pa
cke
ts p
er
1 s
ec I
nte
rva
l
Packets Transmitted by Client IP 207
0
100
200
300
400
500
600
700
Time in Seconds
Pa
cke
ts p
er
1 s
ec I
nte
rva
l
Packets Transmitted by Client IP 212
0
100
200
300
400
500
600
700
Time in Seconds
Pa
cke
ts p
er
1 s
ec I
nte
rva
l
Packets Transmitted by Client IP 220
0
100
200
300
400
500
600
700
Time in Seconds
Pa
cke
ts p
er
1 s
ec I
nte
rva
l
Packets Transmitted by Client IP 208
0
100
200
300
400
500
600
700
Time in Seconds
Pa
cke
ts p
er
1 s
ec I
nte
rva
l
34
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!
35
Conclusions Portable networks: a novel paradigm
for the use of 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
36
The End
Thanks for your attention! Credits:
Guangwei Bai Kehinde (Kenny) Oladosu
More info: [email protected]
Questions?