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

2

OutlineOutline

Rate anomaly problem

SoftRepeater design

Fairness requirements

Experimental results

Conclusions

3

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

4

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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Questions: Questions: pclee@cs.columbia.edupclee@cs.columbia.edu

23

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