View
220
Download
0
Category
Tags:
Preview:
Citation preview
Mitigating the Impact of Physical LayerCapture and ACK Interference inWireless 802.11 Networks
Wang Wei
School of Computing
WiFi is Ubiquitous
School of Computing
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
School of Computing
The Problems of WiFi
Higher density More traffic
Serious interference
School of Computing
The Problems of WiFi
Serious interferenceUnfairness due to capture
Pitfall of Message in Message Mechanism
MAC ACK interference
School of Computing
Thesis Outline
Unfairness due to capture
Pitfall of Message in Message Mechanism
MAC ACK interference
FairMesh
AdaptiveMIM
MinPACK
School of Computing
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
School of Computing
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!
School of Computing
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
School of Computing
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
School of Computing
Three Canonical Scenarios
Asymmetric topology
Direct capture
Indirect capture
A B C D
A B C
A B C D
OffenderVictim
School of Computing
Degree of Unfairness
A B C
School of Computing
Degree of Unfairness
A B C
School of Computing
Degree of Unfairness
A B C
AB is nearly starved
School of Computing
Degree of Unfairness
A B C
Slightly better fairness, but still quite unfair
School of Computing
How to Improve Fairness?
Reduce inter-packet time: More TX opportunity for itself but less opportunity for others
Time
Adjust the inter-packet time!
School of Computing
How to Improve Fairness?
Increase inter-packet time: Less TX opportunity for itself but more opportunity for others
Time
Adjust the inter-packet time!
School of Computing
Design Decisions
Decision 1: Slow down offender or Speed up victim?
Slow down offender!
School of Computing
Design Decisions
Decision 2: Increase CWmin or AIFS?
CWmin!
School of Computing
Impact of CWmin
A B COrange: throughput of CB
Blue:throughput of AB
School of Computing
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
School of Computing
Impact of CWmin
A B C
Need a dynamic CWmin adjustment algo that improves fairness without affecting much efficiency
School of Computing
Key Observation
A B C
AB dominatesCB dominates
School of Computing
Key Observation
A B C
Good fairnessWorse efficiency
School of Computing
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
School of Computing
Unfairness Detection
Estimate throughput
Prompt?
Accurate?
Use sliding window (1 s)
Use per-neighbor seq. no.
School of Computing
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
School of Computing
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
School of Computing
Slow Down the Offender
CWmin adjustment algorithm
Propose water-discharging algo
Basic idea:Gradually slow down offender until the minimum thruput cannot be improved
School of Computing
Slow Down the Offender
A B C
An example for 2-link case
D4 4
School of Computing
45
Slow Down the Offender
A B C
An example for 2-link case
D4
MIN
Offender
School of Computing
45 45
Slow Down the Offender
A B C
An example for 2-link case
D
MIN better
Offender
School of Computing
54 5
Slow Down the Offender
A B C
An example for 2-link case
D
MIN worse
Revert
School of Computing
54 5
Slow Down the Offender
A B C
An example for 2-link case
D
School of Computing
Evaluation of FairMesh
Testbed implementation (No BEB)
ns-2 simulation(for BEB)
Use saturating UDP traffic
School of Computing
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
School of Computing
FairMesh vs. 802.11 & Prior Work
A B C D
With BEB
School of Computing
FairMesh vs. 802.11 & Prior Work
A B C D
With BEB
School of Computing
FairMesh vs. 802.11 & Prior Work
A B C
With BEB
School of Computing
FairMesh vs. 802.11 & Prior Work
A B C D
With BEB
School of Computing
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
School of Computing
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
School of Computing
Large-scale NetworksArbitrarily select 20 links, from 20-node testbed
802.11
School of Computing
Large-scale NetworksArbitrarily select 20 links, from 20-node testbed
802.11
School of Computing
Large-scale NetworksArbitrarily select 20 links, from 20-node testbed
FairMesh
School of Computing
Large-scale NetworksArbitrarily select 50 links, from 50-node simulation
802.11
School of Computing
Large-scale NetworksArbitrarily select 50 links, from 50-node simulation
FairMesh
School of Computing
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
School of Computing
Thesis Outline
Unfairness due to capture
Pitfall of Message in Message Mechanism
MAC ACK interference
FairMesh
AdaptiveMIM
MinPACK
In thesis
School of Computing
Dense AP Deployment
School of Computing
AP Density Measurement
War-walking
School of Computing
Each 1-sec duration is considered as a “sample”
War-walking
Low speed: 1 m/sIdentify an AP based on BSSID in BeaconWiFi sniffer
School of Computing
War-walking
Commercial area
University campus
Residential area
School of Computing
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
School of Computing
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
School of Computing
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
School of Computing
MAC ACK Interference
DATA
DATA
ACKACK
MAC ACK frames effectively extend the interference range of a hotspot
AP1
C1
AP2C2
School of Computing
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
School of Computing
Impact of MAC ACK Interference
11n vs. 11n, UDPAP1 AP2C1
C2
School of Computing
Impact of MAC ACK Interference
11n vs. 11n, UDPAP1 AP2C1
C2
School of Computing
11n vs. 11n, UDP
Impact of MAC ACK Interference
AP1 AP2C1
C2
School of Computing
11n vs. 11n, UDP
Impact of MAC ACK Interference
AP1 AP2C1
C2Better throughput
School of Computing
11n vs. 11n, TCP
Impact of MAC ACK Interference
AP1 AP2C1
C2
Better fairness
School of Computing
11a vs. 11n, UDP
11a11n
Impact of MAC ACK Interference
AP1 AP2C1
C2
School of Computing
11a vs. 11n, UDP
11n11a
Impact of MAC ACK Interference
AP1 AP2C1
C2No 802.11n starvation
School of Computing
Power Control of ACKDATA
SenderACK Sender
Default ACK power
ACK power reduced too much!
Sender has to retx!
School of Computing
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
School of Computing
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!
School of Computing
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
School of Computing
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
School of Computing
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
School of Computing
Gain of MinPACK
• 20-node outdoor 802.11a testbed
• Arbitrarily select 38 pairs of competing links, with UDP traffic
School of Computing
Throughput Gain
Equal20% gain50% gain
School of Computing
Throughput Gain
• MinPACK does no harm
• Median gain is 31%
School of Computing
Throughput Gain
• MinPACK does no harm
• Median gain is 31%
• Passive method achieves
similar performance to
Feedback method
School of Computing
Power Control of Data Frames is not Sufficient
AP1 AP2
C1
C2
School of Computing
Power Control of Data Frames is not Sufficient
AP1 AP2C1
C2
Default ACK power
School of Computing
Power Control of Data Frames is not Sufficient
AP1 AP2C1
C2
Default ACK power
School of Computing
Power Control of Data Frames is not Sufficient
AP1 AP2C1
C2
Default ACK power
School of Computing
Power Control of Data Frames is not Sufficient
AP1 AP2C1
C2
Default ACK power
School of Computing
Power Control of Data Frames is not Sufficient
AP1 AP2C1
C2
Default ACK power
School of Computing
Power Control of Data Frames is not Sufficient
AP1 AP2C1
C2
Default ACK power
School of Computing
Power Control of Data Frames is not Sufficient
AP1 AP2C1
C2
Default ACK powervs. MinPACK
School of Computing
Power Control of Data Frames is not Sufficient
AP1 AP2C1
C2
Default ACK powervs. MinPACK
School of Computing
Power Control of Data Frames is not Sufficient
AP1 AP2C1
C2
Default ACK powervs. MinPACK
School of Computing
Power Control of Data Frames is not Sufficient
AP1 AP2C1
C2
Default ACK powervs. MinPACK
School of Computing
Power Control of Data Frames is not Sufficient
AP1 AP2C1
C2
Default ACK powervs. MinPACK
School of Computing
Power Control of Data Frames is not Sufficient
AP1 AP2C1
C2
Default ACK powervs. MinPACK
School of Computing
Mobility
AP1C1 AP2
C2
Default ACK powerHigh throughput for both C1 and C2
Low throughput for C2Low throughput for C1
60 m40 m
School of Computing
Mobility
AP1C1 AP2
C260 m
40 m
High throughput for both C1 and C2 Better fairness, slightly
higher total throughputDefault ACK power
MinPACK
School of Computing
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
School of Computing
Thesis Outline
Unfairness due to capture
Pitfall of Message in Message Mechanism
MAC ACK interference
FairMesh
AdaptiveMIM
MinPACK
In thesis
School of Computing
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
School of Computing
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
School of Computing
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
School of Computing
WiFi Testbed
• 20 nodes• 3D topology
School of Computing
WiFi Testbed
Alix board
802.11abg cards
500 MHz CPU
School of Computing
Improvement of Fairness
Default ACK power
MinPACK
MinPACK achieves better fairness for this link pair
School of Computing
MinPACK achieves better efficiency for this link pair
Improvement of Fairness
School of Computing
• Fairness is improved for most link pairs.• Some link pairs have
fairness and efficiency both improved.
Improvement of Fairness
Recommended