peter a. steenkiste & dina papagiannaki 1 18-759: wireless networks l ecture 15: wifi...
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Peter A. Steenkiste & Dina Papagiannaki 1
18-759: Wireless NetworksLecture 15: WiFi Self-
Organization
Dina Papagiannaki & Peter Steenkiste
Departments of Computer Science and
Electrical and Computer EngineeringSpring Semester 2009
http://www.cs.cmu.edu/~prs/wireless09/
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Overview
Self-organization/Management of WiFi networks
Urban, cooperative environments (unmanaged)
» Frequency selection
» User association
» Power control
Enteprise WLANs (managed) New application domains
(unmanaged+managed)
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Design of the Wired Internet
Overprovisioning the solution of choice!
Overprovisioning the solution of choice!
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From a managed core to an unmanaged edge
•A large fraction of the Internet’s clients are going wireless (WiFi, 3G, WiMax)
•Client performance primarily determined by the edge network
•802.11 networks challenge our traditional thinking in network design and management
•The need for measurement paramount due to the unreliability and dynamics of the medium, the notion of contention domains and mobility
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What determines client performance?
Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA)
• Transmitter senses the medium, uses Clear Channel Assessment (CCA) threshold to determine state
• If medium idle, randomize access
• When backoff counter=0, transmit
• Upon ACK, reset backoff counter
• If no ACK, double contention window, adjust transmission rate and try again
Performance = f(access probability, retransmissions, link quality, hidden terminals)
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Client throughput
54 Mbps
Effective throughput~ 30 Mbps
11 Mbps
Effective throughput< 10 Mbps
1 Mbps
Effective throughput ~Kbps
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Solutions and operations
Minimize the number of transmitters in the same contention domain
Ensure high quality links between clients and APs
Minimize the effect of hidden terminals
Frequency selectionPower Control
User Association
RTS/CTSScheduling
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Minimize the number of transmitters in the same contention domain
Frequency selectionPower Control
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WiFi frequency selection
2.4 GHz band (802.11b/g) – 11 channels, 3 orthogonal
5 GHz band (802.11a) – 11/12 channels depending on continent
Question: Which frequency should each AP operate on for optimal performance?
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Cellular analogue
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3 802.11b/g frequencies not enough
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WiFi scanning tools: NetStumbler
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Frequency Selection
(1, 6, 11)
(1, 6, 11)
(1, 6, 11)
(1, 6, 11)
(1, 6, 11)
(1, 6, 11)
(1, 6, 11)
(1, 6, 11)
(1, 6, 11)(1, 6, 11)
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clientAP
Mbps/11g Client1 Client2 Client3
Before 11.81 6.86 14.37
After 30.51 (*3) 30 (*5) 29.45 (*2)
Effect of Frequency Selection
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Alternatives
Interference is the outcome of transmissions! Frequency selection could take workload into
account» Requires additional measurements
» Could lead to instability if based on variable measurements
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Partially overlapping channels
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Using partially overlapping channels
Partially overlapped channels not considered harmful, In ACM Sigmetrics 2006
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Minimize the number of transmitters in the same contention domain
Frequency selectionPower Control
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Power Control in 802.11
Heterogeneous transmit powers across nodes can lead to node starvation!
1st order starvation
We need to ensure that there is symmetry in the nodes’ contention domains.
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What is the benefit of power control?
Reducing transmission power can reduce interference in the network Increasing transmission power can improve client SINR thus allowing for higher transmission rates There is a tradeoff between the amount of interference we introduce in the network and the additional throughput benefit at the client
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Condition for starvation free power control
We need to ensure network symmetry We have proven that for starvation-free power control we need to keep the product of CCA threshold and transmission power constant CCA * P = C
The louder you are going to shout the more carefully you should listen for the nodes that whisper
Interference Mitigation through Power Control in High Density 802.11 WLANs, IEEE Infocom 2007
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clientAP
Mbps/11g Client1 Client2 Client3
Before 11.81 6.86 14.37
After 29.45 (*3) 22.59 (*4) 30.51 (*2)
Effect of Power Control
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Ensure high quality links between clients and APs
User Association
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User throughput
Internet
1. Channel access time2. Aggregated transmission delay3. Wireless channel quality
State of the art can lead to unnecessarily low throughput!
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User association
Balance the user associations for minimal potential delay fairness. Users take into account the personal and social cost of different association rules.
Mbps/11g Client1
Before ~ 5
After ~ 8
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Overall network fairness improved
Mean:1428, variance:4378031
Mean:1559, variance: 627638
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Implementation and Experimental set-up
The 3 algorithms are implemented» for both APs and clients
» on Intel 2915 prototype driver and firmware
Testbed A : U Cambridge, UK» 21 APs, 30 client
Technical Characteristics» Nodes: Soekris net4826,
» Wireless cards: Intel 2915 a/b/g
– 5-dBi omnidirectional antennae
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Impact of different algorithms
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Minimize the effect of hidden terminals
RTS/CTSScheduling
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The transformation of enterprise WLANs
Centralization of the control – increased security, opportunity for optimal configuration
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Hidden and Exposed Terminals
WLANs HP Labs Seoul National University
Our Testbed
Exposed Terminals
39% 9% 39%
Hidden Terminals
43% 70% 35%
In 30% of the hidden terminals (300 cases) performance degradation was greater than 90%!
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Conflict Graph and its measurement
•Micro-probing can measure the conflict graph of a 20 node network in 20 seconds!
•No client modifications
•As accurate as state of the art with ~400 times less overheadCarrier-sense
Interference
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Centralized Scheduling
(Y,C2)
(Y,C3)
(X,C4)
(X,C1)
Scheduler1 2 3 4
C1
C4
C3
C2
XY
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Implementation Issues
• Need for a conflict graph• Tight synchronization among APs• Precise knowledge of when a transmission
will be over (not easy due to retransmission and changes in transmission rate)
• Speculation is required!
DCF performs better in the general case!
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Hybrid Scheduling
• Use speculative centralized scheduling only for hidden terminals
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The network design space
Network density
man
agem
ent
Frequency selection, power control
Centralized control and scheduling
Mor
e de
vice
s on
the
sam
e fr
eque
ncy
overhearing
PHY coding
conscious choice reachability, minimal handoff time
power control, user association
No single design will fit-all – understanding constraints and solution space is essential. Applications will be the main drivers
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The FON model – large scale WiFi collaboration
home EnterpriseNeighborhood Network
Management degree
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Next-generation community networks
• Community networks today seen as infrastructure for Internet access
• … and services on the move [Cabernet, ViFi, Dome]
• What if such networks formed the new Internet edge with the ability to provide better and new types of services?
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Community WiFi
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New Types of Services in Neighborhood WiFi
• Reliability through broadband provider diversity• Higher uplink capacity through wireless-assisted
broadband link aggregation• Services beyond the limitations of one home’s
resources
Most broadband connections underutilizedWireless speeds far exceed last mile
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Today’s home use
InternetInternet
“broadband”
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Large uploads are very slow!
InternetInternet
!?!
zzz..
zzz..
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Burstable broadband service
InternetInternet
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Burstable broadband service
InternetInternet
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Burstable broadband service
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Aggregation through wireless
What is the best strategy to get efficient aggregation of bandwidth?
Problem: Individual losses can significantly harm performance.
Wireless unicast
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Opportunistic wireless reception
Wireless unicast
Opportunistic broadcast
??
?
Solution needs to minimize redundancy in wired transmissions while making optimal use of the wireless medium
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Link-alike offers significant gains through
opportunism
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Open Questions
• How could such a mechanism be adjusted for real-time content, such as high resolution video conferencing?
• Automated frequency selection restricts the number of APs in the same frequency, limiting the potential for overhearing
• What are the mechanisms needed for optimal performance of neighborhood networks?
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US Spectrum Allocation
WiFi
700 MHz and white spaces
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References
Partially-overlapped Channels not considered harmful, Arunesh Mishra, Vivek Shrivastava, Suman Banerjee, William Arbaugh. In ACM Sigmetrics, St. Malo, France, June 2006.
MDG: Measurement-Driven Guidelines for 802.11 WLAN Design, I. Broustis, K. Papagiannaki, S. Krishnamurthy, M. Faloutsos, and V. Mhatre. In ACM Mobicom, Montreal, Canada, September 2007.
Interference Mitigation through Power Control in High Density 802.11 WLANs, V. Mhatre, K. Papagiannaki and F. Baccelli. In IEEE Infocom, Anchorage, Alaska, May, 2007.
Measurement-Based Self Organization of Interfering 802.11 Wireless Access Networks, B. Kaufmann, F. Baccelli, A. Chaintreau, V. Mhatre, K. Papagiannaki and C. Diot. In IEEE Infocom, Anchorage, Alaska, May, 2007.
Link-alike: Using Wireless to Share Network Resources in a Neighborhood, S. Jakubczak, D. Andersen, M. Kaminsky, K. Papagiannaki, and S. Seshan. To appear in ACM Sigmobile Mobile Computing and Communications Review (MC2R)