Mitigating the Impact of Physical Layer Capture and ACK Interference in Wireless 802.11 Networks Wang Wei

Mitigating the Impact of Physical LayerCapture and ACK Interference inWireless 802.11 Networks

Wang Wei

School of Computing

WiFi is Ubiquitous

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The Trends of WiFi

Higher density

WiFi hotspot market: Annual growth at 84%

Global WiFi Hotspot Market 2012-2016, by Research and Markets

More traffic

WiFi

WiFi will make up 56% of Internet traffic in 2017

Cisco Visual Networking Index forecast, 2012-2017

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The Problems of WiFi

Higher density More traffic

Serious interference

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The Problems of WiFi

Serious interferenceUnfairness due to capture

Pitfall of Message in Message Mechanism

MAC ACK interference

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Thesis Outline

Unfairness due to capture



FairMesh

AdaptiveMIM

MinPACK

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Link Layer Unfairness due to Capture

Common in mesh networks, esp. near Gateway

Some nodes have little chance to TX successfully

Seriously affect user experience

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Link Layer Unfairness due to Capture

Common in mesh networks, esp. near Gateway

A GW NB

CE

F

D

X

Part of the network is un-usable!

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Design Challenges

The unfairness problem is complex• Multiple causes for unfairness

• Arbitrary topology/traffic

• Tradeoff between fairness and efficiency

• No central controller in mesh networks

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Three Canonical Scenarios

Asymmetric topology

Direct capture

Indirect capture

1 dB difference of RSSI can result in capture effect!

A B C D

A B C

A B C D

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Three Canonical Scenarios

Asymmetric topology

Direct capture

Indirect capture

A B C D

A B C

A B C D

OffenderVictim

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Degree of Unfairness

A B C

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A B C

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A B C

AB is nearly starved

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A B C

Slightly better fairness, but still quite unfair

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How to Improve Fairness?

Reduce inter-packet time: More TX opportunity for itself but less opportunity for others

Time

Adjust the inter-packet time!

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How to Improve Fairness?

Increase inter-packet time: Less TX opportunity for itself but more opportunity for others

Time

Adjust the inter-packet time!

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Design Decisions

Decision 1: Slow down offender or Speed up victim?

Slow down offender!

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Design Decisions

Decision 2: Increase CWmin or AIFS?

CWmin!

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Impact of CWmin

A B COrange: throughput of CB

Blue:throughput of AB

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Impact of CWmin

A B C

Large CWmin for both: Good for fairness, but bad for efficiency

Default CWmin: Good for efficiency, but bad for fairness

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Impact of CWmin

A B C

Need a dynamic CWmin adjustment algo that improves fairness without affecting much efficiency

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Key Observation

A B C

AB dominatesCB dominates

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Key Observation

A B C

Good fairnessWorse efficiency

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Design of FairMesh

Detect the existence of unfairness

Appropriately slow down the offenders by adjusting their CWmin

Every node runs FairMesh protocolEvery node knows local 2-hop topology

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Unfairness Detection

Estimate throughput

Prompt?

Accurate?

Use sliding window (1 s)

Use per-neighbor seq. no.

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Unfairness Detection

Identify victim link & offending link

Fairness notion?Victim link?

max-min fairness

The link with the lowest thruput

Offending link?

The link with the highest thruput that has “conflict” with victim

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Slow Down the Offender

Who Shall Take Action?

A set of rules to elect “coordinators”

Distributed Unique

A

B

DC

Instruct the offender to slow down

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CWmin adjustment algorithm

Propose water-discharging algo

Basic idea:Gradually slow down offender until the minimum thruput cannot be improved

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A B C

An example for 2-link case

D4 4

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45


A B C


D4

MIN

Offender

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45 45


A B C


D

MIN better

Offender

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54 5


A B C


D

MIN worse

Revert

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54 5


A B C


D

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Evaluation of FairMesh

Testbed implementation (No BEB)

ns-2 simulation(for BEB)

Use saturating UDP traffic

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Evaluation of FairMesh Outline• FairMesh vs. 802.11 & prior work• BEB vs. no-BEB• Lossy links & Proportional fairness• Higher data rate• Large-scale networks• Multi-hop TCP flows

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FairMesh vs. 802.11 & Prior Work

A B C D

With BEB

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A B C D

With BEB

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A B C

With BEB

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A B C D

With BEB

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proportional fairness: instead of MIN, try to improve

Lossy Links & Proportional Fairness

A B C42% loss

max-min fairness: achieve equality, but small total thruput

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Lossy Links & Proportional Fairness

A B C42% loss

max-min fairness: achieve equality, but small total thruput

proportional fairness: instead of MIN, try to improve

No equality, but better total thruput

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Large-scale NetworksArbitrarily select 20 links, from 20-node testbed

802.11

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802.11

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FairMesh

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Large-scale NetworksArbitrarily select 50 links, from 50-node simulation

802.11

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Large-scale NetworksArbitrarily select 50 links, from 50-node simulation

FairMesh

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Evaluation Summary of FairMesh

• FairMesh achieves better fairness for the three canonical scenarios, as compared with 802.11 and prior work

• FairMesh can be easily modified to support proportional fairness

• FairMesh can be applied for large-scale networks and achieves near-optimal max-min throughput allocation

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Thesis Outline




FairMesh

AdaptiveMIM

MinPACK

In thesis

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Dense AP Deployment

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AP Density Measurement

War-walking

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Each 1-sec duration is considered as a “sample”

War-walking

Low speed: 1 m/sIdentify an AP based on BSSID in BeaconWiFi sniffer

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War-walking

Commercial area

University campus

Residential area

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AP Density Results

ScenariosMedian number of APs

Channel 1 Channel 6 Channel 11 Others

Commercial 6 6 9 < 1

University 8 6 5 < 1

Residential 9 15 10 < 4

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Interference Mitigation

Current approaches:

• Regulate the tx power of the MAC Data frames from AP

Our key observation:

• MAC Acknowledgment frames from clients could also cause serious interference to neighbor cells

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Interference Mitigation

Power control of Data frames cannot be applied for ACK frames, because:Data frames have tx status feedback, but ACK frames don’t have

Also,• ACK frames are sent at low data rate• ACK frames are small

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MAC ACK Interference

DATA

DATA

ACKACK

MAC ACK frames effectively extend the interference range of a hotspot

AP1

C1

AP2C2

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Measure the Impact of ACK Interference

AP1 AP2C1

C2

• Campus WLAN- Cisco AP (1140 series)

• Clients with Atheros adapters- 802.11a and 802.11n

DATA

DATA

Experiment Setup

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Impact of MAC ACK Interference

11n vs. 11n, UDPAP1 AP2C1

C2

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11n vs. 11n, UDPAP1 AP2C1

C2

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11n vs. 11n, UDP


AP1 AP2C1

C2

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11n vs. 11n, UDP


AP1 AP2C1

C2Better throughput

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11n vs. 11n, TCP


AP1 AP2C1

C2

Better fairness

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11a vs. 11n, UDP

11a11n


AP1 AP2C1

C2

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11a vs. 11n, UDP

11n11a


AP1 AP2C1

C2No 802.11n starvation

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Power Control of ACKDATA

SenderACK Sender

Default ACK power

ACK power reduced too much!

Sender has to retx!

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Key idea Gradually reduce the power of ACK, until the point just before the success rate of ACK starts decreasing.

Called Minimum Power for ACK (MinPACK)

ChallengeHow can the ACK sender accurately estimate the success rate of ACK?

Power Control of ACK

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Estimation of ACK Success Rate

DATA Sender

ACK Sender Feedback-based method

1 ACK tx1 ACK rx

2 ACK tx

3 ACK tx2 ACK rx

Inform ACK sender 2/3 = 67%

Accurate, but need to modify DATA sender!

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DATA Sender

ACK Sender

Passive estimation method

1

2

1 ACK tx

2 ACK tx

2

3 ACK tx

2/3 = 67%

Not perfect due to retx limit, but good enough in practice

3

Prev ACK success

Prev ACK fail

Prev ACK success

Estimation of ACK Success Rate

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MinPACK Protocol

200 ms

Time

ACK tx power

Initially at max power

Get the max ACK success rate Φmax

max

Reduce if Φ > Φmax- δ

Periodically set to max power to get new Φmax

Revert to previous level otherwise

Repeat power adjustment

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Evaluation of MinPACK

Outline• Gain of MinPACK

- 11a vs. 11a in 20-node testbed- 11n vs. 11n in campus WLAN- 11a vs. 11n in campus WLAN

• Interaction with DATA power control

• Adaptation to client mobility

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Gain of MinPACK

• 20-node outdoor 802.11a testbed

• Arbitrarily select 38 pairs of competing links, with UDP traffic

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Throughput Gain

Equal20% gain50% gain

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Throughput Gain

• MinPACK does no harm

• Median gain is 31%

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Throughput Gain

• MinPACK does no harm

• Median gain is 31%

• Passive method achieves

similar performance to

Feedback method

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Power Control of Data Frames is not Sufficient

AP1 AP2

C1

C2

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AP1 AP2C1

C2

Default ACK power

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AP1 AP2C1

C2

Default ACK power

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AP1 AP2C1

C2

Default ACK power

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AP1 AP2C1

C2

Default ACK power

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AP1 AP2C1

C2

Default ACK power

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AP1 AP2C1

C2

Default ACK power

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AP1 AP2C1

C2

Default ACK powervs. MinPACK

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AP1 AP2C1

C2


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AP1 AP2C1

C2


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AP1 AP2C1

C2


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AP1 AP2C1

C2


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AP1 AP2C1

C2


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Mobility

AP1C1 AP2

C2

Default ACK powerHigh throughput for both C1 and C2

Low throughput for C2Low throughput for C1

60 m40 m

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Mobility

AP1C1 AP2

C260 m

40 m

High throughput for both C1 and C2 Better fairness, slightly

higher total throughputDefault ACK power

MinPACK

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Evaluation Summary of MinPACK

• MinPACK is able to improve the throughput of 2 competing flows that suffer from ACK interference

• MinPACK is complementary to existing Data power control protocol

• MinPACK is adaptive to node mobility

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Thesis Outline




FairMesh

AdaptiveMIM

MinPACK

In thesis

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Thesis Contribution Summary

• FairMesh- Accurate assessment method for unfairness- Distributed election of coordinators- Water-discharging algorithm for max-min fairness

• MinPACK- Two simple and accurate methods for ACK success

rate estimation- Distributed ACK power control protocol

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Thesis Conclusion• Studied link-layer performance of dense WiFi

networks with heavy traffic• Investigated two important problems that

received little attention previously- Physical-layer capture effect- MAC ACK interference

• Proposed effective solutions that can be immediately applied in practical deployment- FairMesh- MinPACK

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Open Issues & Future Work

• Open issues:- FairMesh with routing protocol and traffic priority?- FairMesh with mobility?- MinPACK with mixed downlink/uplink traffic?

• Future work:- Early discard of unwanted MAC frames- Aerial WiFi networks using UAV

Thank YouWISH YOU A HAPPY AND PROSPEROUS NEW YEAR!

Backup Slides

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WiFi Testbed

• 20 nodes• 3D topology

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WiFi Testbed

Alix board

802.11abg cards

500 MHz CPU

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Improvement of Fairness

Default ACK power

MinPACK

MinPACK achieves better fairness for this link pair

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MinPACK achieves better efficiency for this link pair


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• Fairness is improved for most link pairs.• Some link pairs have

fairness and efficiency both improved.


Documents

Mitigating the Impact of Physical Layer Capture and ACK Interference in Wireless 802.11 Networks Wang Wei