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CENTAUR: Realizing the Full Potential of Centralized WLANs Through a Hybrid
Data Path
Vivek Shrivastava* , Shravan Rayanchu, Suman BanerjeeUniversity of Wisconsin-Madison
Nabeel Ahmed, Srinivasan KeshavUniversity of Waterloo, Ontario
Konstantina PapagiannakiIntel Labs, Pittsburgh
Arunesh MishraGoogle Inc.
* [email protected]
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Centralization of Enterprise WLANs
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Wireless controller
Access Point
Clients
Internet
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Centralization of Enterprise WLANs
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Power controlChannel assignment
Common control plane functions
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Centralization of Enterprise WLANs
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What about data plane functions?
Power controlChannel assignmentData scheduling ?
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Can centralized scheduling help?
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Can centralized scheduling help?
Hidden terminals
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Hidden terminals
Can centralized scheduling help?
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Hidden terminals
Can centralized scheduling help?
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1. Carrier sense
Hidden terminals
Can centralized scheduling help?
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2. Channel free, transmit
Hidden terminals
Can centralized scheduling help?
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• Collision!• Backoffs• Low throughputs
3. Collision !
Can centralized scheduling help?
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How bad is it ?
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Experiments on production building-wide WLANs •W1: 5 floors
9 APs, 45 clients
• W2: 1 floor 21 APs, 51 clients
10% links suffer more than 70% throughput
reduction
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A lost opportunity ?
Suppose infrastructure can gather conflict data
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A lost opportunity ?
And when packets arrive …
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A lost opportunity ?
… realize interference will happen …
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A lost opportunity ?
1. Transmit first packet
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A lost opportunity ?
1. Transmit first packet2. Transmit second packet
with delay
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Use an in-band scheduler
Simple FIFO schedule with interference
avoidance
Scheduling functionality
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What about exposed terminals ?
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• In our experiments, about 41%of link pairs suffer from exposed terminal interference
• Disabling carrier sense to solve the problem can be dangerous for uplink, non-enterprise traffic
• We will show how centralization can help even here
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Challenge
•Centralization has obvious overheads
•How to make this feasible and useful under
• Real applications
• Common large-scale wireless environments
• Presence of uplink and non-enterprise traffic
• Requirements of no client modifications
20Vivek Shrivastava
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Contributions
1.Design CENTAUR, a hybrid (partly centralized, partly distributed) scheduling approach
• Resolves hidden and exposed terminals
• Requires no client modifications
• Requires no carrier sense disabling
2.Evaluate CENTAUR on two WLAN testbeds with real-world traffic traces
• 1.48x greater throughput for bulk data traffic
• 1.38x reduction in web-transaction times
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Outline
• A naïve attempt at centralized scheduling (DET)
•Our hybrid centralized scheduler (CENTAUR)
•Evaluation
•Related Work
•Summary
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Outline
• A naïve attempt at centralized scheduling (DET)
•Our hybrid centralized scheduler (CENTAUR)
•Evaluation
•Related Work
•Summary
23Vivek Shrivastava
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DET: A Simple Deterministic Scheduler
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DET: A Simple Deterministic Scheduler
Input: Conflict graph
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DET: A Simple Deterministic Scheduler
Goal: Schedule each incoming downlink packet
Input: Conflict graph
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DET: A Simple Deterministic Scheduler
1 2 3 4Transmission
slots
Goal: Schedule each incoming downlink packet
Input: Conflict graph
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DET: A Simple Deterministic Scheduler
1 2 3 4Transmission
slots
1. New packet arrives
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DET: A Simple Deterministic Scheduler
1 2 3 4Transmission
slots
1. New packet arrives2. Find the earliest conflict
free slot
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DET: A Simple Deterministic Scheduler
1 2 3 4Transmission
slots
1. New packet arrives2. Find the earliest conflict
free slot
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DET: A Simple Deterministic Scheduler
1 2 3 4Transmission
slots
1. New packet arrives2. Find the earliest conflict
free slot3. Schedule the packet in that
slot
5
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Performance of DET
4xNo gains for exposed terminals; Non-
conflicting links perform worse under load
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Outline
• A naïve attempt at centralized scheduling (DET)
•Our hybrid centralized scheduler (CENTAUR)
•Evaluation
•Related Work
•Summary
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Overview of CENTAUR
• Incorporate basic DET scheduler
• Tackle DET’s shortcomings:
• Amortize scheduling overhead
• Improve performance for exposed links
• Avoid degrading normal links
• Coexist with non-enterprise and uplink traffic
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(1) Avoid Scheduling Overheads
• Problem: Per-packet scheduling performs poorly under high network loads
• Solution:
• Schedule packets in batches (or epochs)
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(1) Avoid Scheduling Overheads
• Problem: Per-packet scheduling performs poorly under high network loads
• Solution:
• Schedule packets in batches (or epochs)
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(1) Avoid Scheduling Overheads
• Problem: Per-packet scheduling performs poorly under high network loads
• Solution:
• Schedule packets in batches (or epochs)
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Wired ack
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(1) Avoid Scheduling Overheads
• Problem: Per-packet scheduling performs poorly under high network loads
• Solution:
• Schedule packets in batches (or epochs)
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(1) Avoid Scheduling Overheads
• Problem: Per-packet scheduling performs poorly under high network loads
• Solution:
• Schedule packets in batches (or epochs)
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(1) Avoid Scheduling Overheads
• Problem: Per-packet scheduling performs poorly under high network loads
• Solution:
• Schedule packets in batches (or epochs)
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(1) Avoid Scheduling Overheads
• Problem: Per-packet scheduling performs poorly under high network loads
• Solution:
• Schedule packets in batches (or epochs)
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(1) Avoid Scheduling Overheads
• Problem: Per-packet scheduling performs poorly under high network loads
• Solution:
• Schedule packets in batches (or epochs)
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Wired ack
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(1) Avoid Scheduling Overheads
• Problem: Per-packet scheduling performs poorly under high network loads
• Solution:
• Schedule packets in batches (or epochs)
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(2) Improve Exposed Terminals
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• Problem: Exposed links can operate in parallel but don’t due to carrier-sensing
• Solution:
• Schedule batch of packets, fix backoff periods
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(2) Improve Exposed Terminals
• Problem: Exposed links can operate in parallel but don’t due to carrier-sensing
• Solution:
• Schedule batch of packets, fix backoff periods
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Variable wired delay
(2) Improve Exposed Terminals
• Problem: Exposed links can operate in parallel but don’t due to carrier-sensing
• Solution:
• Schedule batch of packets, fix backoff periods
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(2) Improve Exposed Terminals
• Problem: Exposed links can operate in parallel but don’t due to carrier-sensing
• Solution:
• Schedule batch of packets, fix backoff periods
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Carrier sense, deferral
(2) Improve Exposed Terminals
• Problem: Exposed links can operate in parallel but don’t due to carrier-sensing
• Solution:
• Schedule batch of packets, fix backoff periods
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(2) Improve Exposed Terminals
• Problem: Exposed links can operate in parallel but don’t due to carrier-sensing
• Solution:
• Schedule batch of packets, fix backoff periods
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(2) Improve Exposed Terminals
• Problem: Exposed links can operate in parallel but don’t due to carrier-sensing
• Solution:
• Schedule batch of packets, fix backoff periods
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Schedule packets in batches
(2) Improve Exposed Terminals
• Problem: Exposed links can operate in parallel but don’t due to carrier-sensing
• Solution:
• Schedule batch of packets, fix backoff periods
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(2) Improve Exposed Terminals
• Problem: Exposed links can operate in parallel but don’t due to carrier-sensing
• Solution:
• Schedule batch of packets, fix backoff periods
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(2) Improve Exposed Terminals
• Problem: Exposed links can operate in parallel but don’t due to carrier-sensing
• Solution:
• Schedule batch of packets, fix backoff periods
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First packets can be out of sync
(2) Improve Exposed Terminals
• Problem: Exposed links can operate in parallel but don’t due to carrier-sensing
• Solution:
• Schedule batch of packets, fix backoff periods
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Waiting packets synchronized by carrier sense !
(2) Improve Exposed Terminals
• Problem: Exposed links can operate in parallel but don’t due to carrier-sensing
• Solution:
• Schedule batch of packets, fix backoff periods
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(2) Improve Exposed Terminals
• Problem: Exposed links can operate in parallel but don’t due to carrier-sensing
• Solution:
• Schedule batch of packets, fix backoff periods
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After first packet, both APs transmit
simultaneously.
(2) Improve Exposed Terminals
• Problem: Exposed links can operate in parallel but don’t due to carrier-sensing
• Solution:
• Schedule batch of packets, fix backoff periods
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(3) Avoid Degrading Normal Links
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A B
C D
Hidden terminals
Non-hidden/Non-exposed
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(3) Avoid Degrading Normal Links
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A B
C D
Scheduler
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(3) Avoid Degrading Normal Links
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A B
C D
Scheduler
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(3) Avoid Degrading Normal Links
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A B
C D
Scheduler
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(3) Avoid Degrading Normal Links
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A B
C D
Scheduler
Hybrid scheduling
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Outline
• A naïve attempt at centralized scheduling (DET)
•Our hybrid centralized scheduler (CENTAUR)
•Evaluation
•Related Work
•Summary
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Large-Scale Experiments
•Platform:
• Two WLAN testbeds in separate buildings
•Topology
• Representative: 7 APs, 12 clients
•Traffic and metrics
• UDP, TCP, VoIP, HTTP (real traces)
• Throughput, delay, MOS, web transaction delay
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Result 1: UDP/TCP Performance
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Result 1: UDP/TCP Performance
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DCF
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Result 1: UDP/TCP Performance
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DCF
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Result 1: UDP/TCP Performance
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DCF Per Packet
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Result 1: UDP/TCP Performance
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DCF Per Packet
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Result 1: UDP/TCP Performance
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Epoch BasedDCF Per Packet
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Result 1: UDP/TCP Performance
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Epoch BasedDCF Per Packet
Hidden terminal starves some clients
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Result 1: UDP/TCP Performance
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Epoch BasedDCF Per Packet
Better fairness
than DCF
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Result 1: UDP/TCP Performance
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Epoch BasedDCF Per Packet
Exploits exposed
terminals, higher system
throughput
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Result 1: UDP/TCP Performance
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Epoch BasedDCF Per Packet
Avg. delay is smallest for epoch
based scheduling
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Result 1: UDP/TCP Performance
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Epoch BasedDCF Per Packet
90th percentile delays are higher for
epoch based scheduling
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Result 1: UDP/TCP Performance
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Epoch BasedDCF Per Packet
CENTAUR yields up to 60% higher total throughput and 50% lower per-packet delay
compared to DCF
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More Results in Paper
•Centaur micro-benchmarks: performance for exposed and hidden terminals under centaur
•Uplink traffic: coexistence and persistent gains with different fractions of uplink traffic
•Data rate: robust to changes in data rate and ARF
•Realistic HTTP traces: significant reduction in web transaction delay
•VoIP traffic: better performance (MOS) for voice traffic with small epoch duration
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Related Work
•Commercial WLAN offerings (Aruba, Meru)
•Theoretical formulations (Vaidya ‘00, Kanodia ‘01)
•Epoch based scheduling (Kompella ‘05, 802.11n/e)
• Interference mitigation (CMAP, SIC, Shuffle)
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Summary
• Interference a growing problem in enterprises
•Careful design of a centralized data plane provides substantial performance gains
•CENTAUR implements a hybrid data path to improve aggregate performance without client modifications
•CENTAUR does not disable carrier sense and co-exists with non-enterprise and uplink traffic
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Future work
•Even more efficient conflict graph generation
•What if we were allowed client modifications ?
Questions ?
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Characterizing System Latencies
System delays are high and variable, leading to inaccuracies for per-packet
scheduling
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Evaluation of Micro-Probing
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Topologies 20 node 30 nodeBandwidth
Tests16.2 mins 1hr 11 mins
Micro-Probing
~4 secs ~11 secs
Can be computed in stages with each instance taking ~
2.5ms
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Result 2: Impact of Uplink Traffic
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•Vary proportion of downlink/uplink traffic
•6 different configurations
•80/20 ➔ 40/60 (downlink/uplink)
•Results:
•Downlink: 1.6x ➔ 6.8x gain in throughput
•Uplink: 1x ➔ 1.18x gain in throughput
CENTAUR provides persistent gains for different proportions of uplink and downlink
traffic load
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• Three topologies
• Hidden Heavy topology ➔ 10 links
• Exposed Heavy topology ➔ 6 links
• Mixed Topology ➔ 12 links
• Results: Up to 50% gain in overall system throughput
• Up to 6x gain for HT; Up to 1.7x gain for ET
Result 3: Impact of Topology
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Improvements from using CENTAUR can be seen across many different network
topologies
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Result 2: Impact of Uplink Traffic
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CENTAUR provides persistent gains for different proportions of uplink and downlink
traffic load
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Result 3: Impact of Topology
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Improvements from using CENTAUR can be seen across many different network
topologies
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CENTAUR Micro-Benchmarks (1)
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Exposed Terminals
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CENTAUR Micro-Benchmarks (II)
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Hidden Terminals
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Result 1: UDP/TCP Performance
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Client Index (1-12)
Client Index (1-12)
CENTAUR yields up to 60% higher total throughput and 50% lower per-packet delay
compared to DCF
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Other approaches to Hidden/Exposed Terminals
Mechanism
Target Proble
m
Approach
Client Changes Evaluation
CMAP [NSDI ‘07]
ZigZag [Sigcomm ’08] SIC [Mobicom ’08]
Centaur
Exposed
Hidden
ExposedHidde
n
Disable CS
Signal manipulatio
n
Centralized Scheduling
Yes
Yes
No
802.11
GNU Radio
802.11
Adaptive RTS/CTS [VTC ’03]
HiddenExtra
SignalingYes 802.1
1
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March,30 2009Prelim 2009
Other approaches to Hidden/Exposed Terminals
Mechanism
Target Proble
m
Approach
Client Changes Evaluation
CMAP [NSDI ‘07]
ZigZag [Sigcomm ’08] SIC [Mobicom ’08]
Centaur
Exposed
Hidden
ExposedHidde
n
Disable CS
Signal manipulatio
n
Centralized Scheduling
Yes
Yes
No
802.11
GNU Radio
802.11
Adaptive RTS/CTS [VTC ’03]
HiddenExtra
SignalingYes 802.1
1
Solve both hidden/expos
ed
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March,30 2009Prelim 2009
Other approaches to Hidden/Exposed Terminals
Mechanism
Target Proble
m
Approach
Client Changes Evaluation
CMAP [NSDI ‘07]
ZigZag [Sigcomm ’08] SIC [Mobicom ’08]
Centaur
Exposed
Hidden
ExposedHidde
n
Disable CS
Signal manipulatio
n
Centralized Scheduling
Yes
Yes
No
802.11
GNU Radio
802.11
Adaptive RTS/CTS [VTC ’03]
HiddenExtra
SignalingYes 802.1
1
No client side changes for
Centaur
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CENTAUR: Realizing the Full Potential of Centralized WLANs
Through a Hybrid Data Path
• Vivek Shrivastava* , Shravan Rayanchu, Suman Banerjee
• University of Wisconsin-Madison
Nabeel Ahmed, Srinivasan KeshavUniversity of Waterloo, Ontario
Konstantina PapagiannakiIntel Labs, Pittsburgh
Arunesh MishraGoogle Inc.
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WLANs HP LabsSeoul
National University
Our Testbed
Exposed Terminals
39% 9% 50%
Hidden Terminals
43% 70% 39%
Growth of Interference in Enterprise Wireless LANs
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Interference an increasing problem according to leading enterprise WLAN
vendor
Vivek ShrivastavaVivek Shrivastava
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Centralization of Enterprise WLANs
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Centralized control for better network security and manageability
Vivek Shrivastava
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Can Centralized Data Plane Scheduling Help?
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(2) Improve Exposed Terminals
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• Problem: Exposed links can operate in parallel but don’t due to carrier-sensing
• Solution:
• Schedule batch of packets to each exposed AP
• Fix back-off periods and use carrier-sensing to align transmissions -- double throughput!
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Related Work
•Commercial WLAN offerings (Aruba, Meru)
•Research proposals (MiFi, DenseAP, Smarta)
•TXOP in 802.11e/802.11n packet aggregation
• Interference Mitigation (CMAPs, ZigZag, SIC)
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Types of Interference
Hidden Terminals
Collis
ions
CarrierSense
Exposed Terminals
Focus on downlink conflicts
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Can Centralized Data Plane Scheduling Help?
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X Y
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Can Centralized Data Plane Scheduling Help?
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X Y
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Can Centralized Data Plane Scheduling Help?
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X Y
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Can Centralized Data Plane Scheduling Help?Hidden
terminals
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Quantifying Downlink Hidden Terminals
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10% links suffer severe hidden terminal interference
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Quantifying Downlink Exposed Terminals
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41% links can obtain double the throughput with CS disabled, indicating
exposed terminal interference
Vivek Shrivastava
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Result 1: UDP/TCP Performance
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Result 1: UDP/TCP Performance
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Result 1: UDP/TCP Performance
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Centralization of Enterprise WLANs
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(2) Improve Exposed Terminals
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• Problem: Exposed links can operate in parallel but don’t due to carrier-sensing
• Solution:
• Schedule batch of packets to each exposed AP
• Fix back-off periods and use carrier sensing to align transmissions -- double throughput!
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(2) Improve Exposed Terminals
111Vivek Shrivastava
• Problem: Exposed links can operate in parallel but don’t due to carrier-sensing
• Solution:
• Schedule batch of packets to each exposed AP
• Fix back-off periods and use carrier sensing to align transmissions -- double throughput!
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(3) Avoid Degrading Normal Links
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1. Can it solve any interference problems prevalent in enterprise WLANs
• Hidden and exposed terminals
• 88% of links suffer some losses due to co-channel interference (Jigsaw, Sigcomm 2007)
2. If so, how can we implement it efficiently for practical WLAN deployments
• No client modifications, support legacy clients
• Coexistence with non-enterprise, uplink traffic
113Vivek Shrivastava
Can Centralized Data Plane Help?
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Quantifying Downlink Interference
•Prior Work: Jigsaw [Sigcomm2006] analysis reveals 56% of all interference traffic is downlink in nature.
•Our Work: Two production WLANs
• W1: 5 floors, 9 APs, 45 clients
• W2: 1 floor, 21 APs, 51 clients
•Download ‘bulk’ traffic from the Internet
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WLANsUW-
MadisonUW-
Ontario
Exposed Terminals
39% 9%
Hidden Terminals
43% 70%
Growth of Interference in Enterprise Wireless LANs
115
Interference an increasing problem according to leading enterprise WLAN
vendor
Vivek ShrivastavaVivek Shrivastava
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Centralization of Enterprise WLANs
116Vivek Shrivastava
Can centralized data plane be useful for improving performance in
WLANs ?• About 70-80% of enterprise traffic
is downlink in nature
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(3) Avoid Degrading Normal Links
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A B
C D
Scheduler
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(3) Avoid Degrading Normal Links
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A B
C D
Scheduler
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(3) Avoid Degrading Normal Links
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A B
C D
Scheduler
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(3) Avoid Degrading Normal Links
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A B
C D
Scheduler
Hybrid scheduling
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Outline
•DET - A deterministic scheduler
•CENTAUR - A hybrid centralized scheduler
•Evaluation
•Related Work
•Summary
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Outline
•DET - A deterministic scheduler
•CENTAUR - A hybrid centralized scheduler
•Evaluation
•Related Work
•Summary
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Performance of DET
4xNo gains for exposed terminals; Non-
conflicting links perform worse under load
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Outline
•DET - A deterministic scheduler
•CENTAUR - A hybrid centralized scheduler
•Evaluation
•Related Work
•Summary
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DET: A Simple Deterministic Scheduler
• Key Idea: Perform per-packet scheduling
• Given
• Conflict graph G = (L,E)
• Scheduled packets {P1, P2, ..., Pr}; Unscheduled Pr+1
• Objective
• Minimize t(Pr+1)
• Constraint: No two packets on interfering links are scheduled together
• Schedule downlink packets only
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1. What are the problems it can solve ?
• Hidden and exposed terminals
2. Can we implement it efficiently ?
• No client modifications
• Coexistence with non-enterprise & uplink traffic
• Don’t disable carrier sensing
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Can centralized scheduling help?
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Can centralized scheduling help?
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DET: A Simple Deterministic Scheduler
Packets arriving
• Schedule one packet at a time• Consider packets in order of arrival
1 2 3 4Transmission
slots
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What about exposed terminals ?
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41% links can obtain double the throughput with CS disabled, indicating exposed terminal
interference
Vivek Shrivastava
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130 130Vivek Shrivastava
Use an in-band scheduler
Simple FIFO schedule with interference
avoidance
scheduler