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Opportunistic Use of Client Opportunistic Use of Client Repeaters to Improve Repeaters to Improve
Performance of WLANsPerformance of WLANs
Victor Bahl1, Ranveer Chandra1, Patrick P. C. Lee2, Vishal Misra2,Jitendra Padhye1, Dan Rubenstein2, Yan Yu3
1Microsoft Research2Dept of Computer Science, Columbia University
3Google Inc.
Dec 12, 2008
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OutlineOutline
Rate anomaly problem
SoftRepeater design
Fairness requirements
Experimental results
Conclusions
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Rate Anomaly of 802.11Rate Anomaly of 802.11
Rate anomaly is well-known in WiFi 802.11 networks
Low-rate stations degrade throughput of high-rate stations
Why does rate anomaly exist? Stations reduce data rates when
signal strength is poor (auto-rate) Low-rate stations’ packets
consume more airtime 802.11 arbitrates
transmissions on per-packet basis
High-rate stations receive limited airtime throughput degrades
54Mbps
AP
A
54Mbps
B
Thro
ugh
put
(Mbps)
10
20
30
0
A, B near AP A far from AP
B
A
BA
18Mbps
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Limitations of Prior Solutions Limitations of Prior Solutions
What’s new? Rate anomaly is well-known, with many solutions proposed.
Assumptions of prior solutions: Require dedicated hardware (e.g., Cisco Aironet 1200 series APs) Change MAC layer (e.g., Lee et al., Infocom ’04; Liu et al., JSAC ’05) Construct ad-hoc mesh networks (e.g., Draves et al., Mobicom ’04)
Drawbacks of prior solutions: More cost for hardware change Not compatible with widely deployed infrastructure networks Inflexible – solutions cannot be activated on demand
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Our Solution: SoftRepeaterOur Solution: SoftRepeater SoftRepeater: A practical, deployable system that addresses
rate anomaly
Main idea: High-rate station (repeater) relays traffic for low-rate station
(client)
Key features: Repeater is opportunistic - activated only when both repeater and
client receive “beneficial” throughput No changes to 802.11 MAC and AP Deployable in infrastructure and adhoc networks
AP
A
B client
repeater
traffic for A and B traffic for A
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Design IssuesDesign Issues
How can we detect existence of rate anomaly occurring?
How do we formally define “beneficial” throughput?
How do we support multiple interfaces on a wireless card? We need managed mode for communication between AP and
repeater We need adhoc mode for communication between repeater and
client
What fractions of time should we give to managed/adhoc modes to ensure “beneficial throughput”?
AP
A
B client
repeater
traffic for A and B traffic for A
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Our ContributionOur Contribution
Propose a handshaking protocol for detecting rate anomaly and reaching consensus on using SoftRepeater
Formalize a set of utility maximization problems for different fairness requirements
Implement SoftRepeater on Windows XP; conduct extensive testbed experiments and QualNet simulations
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SoftRepeater ArchitectureSoftRepeater Architecture Built on VirtualWifi – allowing two virtual
interfaces for a wireless card: Primary Virtual Interface – communication
between AP and repeater in managed mode Repeater Virtual Interface – communication
between repeater and client in adhoc mode
Repeater Virtual Interface activated only when beneficial to both repeater and client
Alternate between primary and repeater interfaces with switching overhead < 40ms
Optional Network Coding Engine that further boosts throughput, with slight modifications to AP
Multiple radios can be supported (not in our current experiments)
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Detecting Rate AnomalyDetecting Rate Anomaly
Goal: Determine When SoftRepeater is beneficial
Key steps: Collect information from nearby stations in promiscuous mode:
Number of packets transmitted Average size of packets RSSI Data transmission rate BSSID
Check utilization of medium. If neighbors send about the same number of packets, but at a low rate, rate anomaly may exist.
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Repeater Utility FunctionRepeater Utility Function Goal: capture throughput gain of both repeater and client
Define α: fraction of time spent in managed mode
Assumptions: Stations always have backlogged data to send (i.e., saturated case)
Implying equal channel access Good approximation for file-transfer applications
Zero switching overhead 1 - α = fraction of time spent in adhoc mode Can easily account for non-zero switching overhead
Intuition: if utility improved for both repeater and client, activate SoftRepeater
APA
B clientrepeater
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Repeater Utility FunctionRepeater Utility Function
Without SoftRepeater: B’s throughput: TB
A’s throughput: TA
With SoftRepeater: B’s Throughput: αTB/ 2 A’s throughput: min(αTB/ 2 , (1- α)TA,B)
TA,B = inferred throughput between A and B from RSSI measurement
If max-min fairness is used, repeater utility function becomes
T* = maxα min{αTB/ 2, min(αTB/ 2 , (1- α)TA,B)}
If T* > TA and T* > TB (better for both) activate SoftRepeater
AP
A
B
client
repeater
TA
TBTA,B
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Generalizing Repeater Utility Generalizing Repeater Utility FunctionFunction
For different objectives: Maximizing total throughput: starve client (bad) Max-min fairness Proportional fairness
For different settings: In presence of interfering nodes In presence of multiple clients Multiple radios Multiple wireless cards
Details in paper and tech report
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Repeater Initiation ProtocolRepeater Initiation Protocol
Goal: confirm and reach consensus on activating SoftRepeater
For now: simple 4-way handshake: B broadcasts SoftRepeater offer A infers data rate from A to B (from RSSI) and unicasts response B picks clients to serve (if utility improved) and broadcasts final
“Take it or leave it” offer A unicasts accept/reject
AP
A
B
client
repeater
1. broadcast offer
3. broadcast new offer
2. unicast response
4. unicast accept/reject
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Testbed ExperimentsTestbed Experiments
SoftRepeater is implemented on Windows XP
Testbed experiments in office building
AP located at X Repeater (node R) fixed at
Y Client (node C) moved
between Y, T, Z
Use 802.11a, with auto-rate feature enabled
Focus on Max-Min fairness
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Experiment 1: Downlink UDPExperiment 1: Downlink UDP
UDP throughput improved by 200% with SoftRepeater when rate anomaly exists
AP R C
rate anomaly scenario:
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Experiment 2: Downlink TCPExperiment 2: Downlink TCP
TCP throughput improved by 50% with SoftRepeater when rate anomaly exists, even communication alternates between managed and adhoc modes
AP R C
rate anomaly scenario:
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Experiment 3: UDP with 2 Experiment 3: UDP with 2 clientsclients
UDP throughput improved with SoftRepeater when two clients served
AP R C1 C2
rate anomaly scenario:
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Qualnet Simulation: Effectiveness of Qualnet Simulation: Effectiveness of Repeater Initiation ProtocolRepeater Initiation Protocol
SoftRepeater activated only when there is throughput gain
AP in office 0 Client in office 9 Downlink UDP for
both repeater and client
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Qualnet Simulation: Multiple Qualnet Simulation: Multiple ClientsClients
SoftRepeater improves the baseline throughput by more than 65%.
AP in office 0 Repeater in office 3 N clients in office 9 Downlink UDP
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Summary of Experimental Summary of Experimental ResultsResults
Main observation: throughput significantly improved for UDP/TCP flows when rate anomaly exists
More experiments in paper/tech. report Correctness of repeater initiation protocol Extension with network coding Various traffic scenarios Qualnet simulation for more “complicated” scenarios (e.g.,
interfering nodes, multiple repeaters/clients)
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ConclusionsConclusions
Propose SoftRepeater, a practical, deployable system that addresses rate anomaly problem
Formulate different utility maximization problems for SoftRepeater
Implement a prototype that demonstrates the improvement of SoftRepeater
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Security IssuesSecurity Issues
Security concerns: Privacy
End-to-end encryption (e.g., IPsec) can be used Greedy/malicious repeaters
Client monitors channel; quits if performance becomes worse after SoftRepeater is used
Conclusion: Security is no worse than SoftRepeater-free networks