centaur: realizing the full potential of centralized wlans through a hybrid data path vivek...

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]

2

Centralization of Enterprise WLANs

2Vivek Shrivastava

Wireless controller

Access Point

Clients

Internet

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3Vivek Shrivastava

Power controlChannel assignment

Common control plane functions

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4Vivek Shrivastava

What about data plane functions?

Power controlChannel assignmentData scheduling ?

5 5Vivek Shrivastava

Can centralized scheduling help?



Hidden terminals


1. Carrier sense

Hidden terminals



2. Channel free, transmit

Hidden terminals



• Collision!• Backoffs• Low throughputs

3. Collision !


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How bad is it ?

12Vivek Shrivastava

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


A lost opportunity ?

Suppose infrastructure can gather conflict data



And when packets arrive …



… realize interference will happen …



1. Transmit first packet



1. Transmit first packet2. Transmit second packet

with delay


Use an in-band scheduler

Simple FIFO schedule with interference

avoidance

Scheduling functionality

19

What about exposed terminals ?

19Vivek Shrivastava

• 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

20

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

21

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

21Vivek Shrivastava

22

Outline

• A naïve attempt at centralized scheduling (DET)

•Our hybrid centralized scheduler (CENTAUR)

•Evaluation

•Related Work

•Summary

22Vivek Shrivastava

23

Outline



•Evaluation

•Related Work

•Summary

23Vivek Shrivastava


DET: A Simple Deterministic Scheduler



Input: Conflict graph



Goal: Schedule each incoming downlink packet




1 2 3 4Transmission

slots

Goal: Schedule each incoming downlink packet




1 2 3 4Transmission

slots

1. New packet arrives



1 2 3 4Transmission

slots

1. New packet arrives2. Find the earliest conflict

free slot



1 2 3 4Transmission

slots


free slot



1 2 3 4Transmission

slots


free slot3. Schedule the packet in that

slot

5

Performance of DET

4xNo gains for exposed terminals; Non-

conflicting links perform worse under load

32Vivek Shrivastava

33

Outline



•Evaluation

•Related Work

•Summary

33Vivek Shrivastava

34

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

34Vivek Shrivastava

35

(1) Avoid Scheduling Overheads

• Problem: Per-packet scheduling performs poorly under high network loads

• Solution:

• Schedule packets in batches (or epochs)

35Vivek Shrivastava

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• Solution:


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• Solution:


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Wired ack

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• Solution:


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• Solution:


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• Solution:


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• Solution:


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• Solution:


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Wired ack

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• Solution:


43Vivek Shrivastava

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(2) Improve Exposed Terminals

44Vivek Shrivastava

• Problem: Exposed links can operate in parallel but don’t due to carrier-sensing

• Solution:

• Schedule batch of packets, fix backoff periods




• Solution:



Variable wired delay



• Solution:





• Solution:



Carrier sense, deferral



• Solution:





• Solution:



Schedule packets in batches



• Solution:





• Solution:



First packets can be out of sync



• Solution:



Waiting packets synchronized by carrier sense !



• Solution:





• Solution:



After first packet, both APs transmit

simultaneously.



• Solution:


58

(3) Avoid Degrading Normal Links

58Vivek Shrivastava

A B

C D

Hidden terminals

Non-hidden/Non-exposed

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A B

C D

Scheduler

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60Vivek Shrivastava

A B

C D

Scheduler

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61Vivek Shrivastava

A B

C D

Scheduler

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62Vivek Shrivastava

A B

C D

Scheduler

Hybrid scheduling

63

Outline



•Evaluation

•Related Work

•Summary

63Vivek Shrivastava

64

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

64Vivek Shrivastava

65

Result 1: UDP/TCP Performance

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DCF

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DCF

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DCF Per Packet

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DCF Per Packet

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Epoch BasedDCF Per Packet

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Hidden terminal starves some clients

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Better fairness

than DCF

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Exploits exposed

terminals, higher system

throughput

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Avg. delay is smallest for epoch

based scheduling

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90th percentile delays are higher for

epoch based scheduling

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76Vivek Shrivastava


CENTAUR yields up to 60% higher total throughput and 50% lower per-packet delay

compared to DCF

77

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

77Vivek Shrivastava

78

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)

78Vivek Shrivastava

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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

79Vivek Shrivastava

Vivek Shrivastava 80

Future work

•Even more efficient conflict graph generation

•What if we were allowed client modifications ?

Questions ?

Characterizing System Latencies

System delays are high and variable, leading to inaccuracies for per-packet

scheduling

81

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Evaluation of Micro-Probing

82Vivek Shrivastava

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

83Vivek Shrivastava

•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

84

• 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

84Vivek Shrivastava

Improvements from using CENTAUR can be seen across many different network

topologies

85

Result 2: Impact of Uplink Traffic

85Vivek Shrivastava

CENTAUR provides persistent gains for different proportions of uplink and downlink

traffic load

86

Result 3: Impact of Topology

86Vivek Shrivastava

Improvements from using CENTAUR can be seen across many different network

topologies

87

CENTAUR Micro-Benchmarks (1)

87Vivek Shrivastava

Exposed Terminals

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CENTAUR Micro-Benchmarks (II)

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Hidden Terminals

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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

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

March,30 2009Prelim 2009


Mechanism

Target Proble

m

Approach


CMAP [NSDI ‘07]


Centaur

Exposed

Hidden

ExposedHidde

n

Disable CS

Signal manipulatio

n


Yes

Yes

No

802.11

GNU Radio

802.11


HiddenExtra

SignalingYes 802.1

1

Solve both hidden/expos

ed

March,30 2009Prelim 2009


Mechanism

Target Proble

m

Approach


CMAP [NSDI ‘07]


Centaur

Exposed

Hidden

ExposedHidde

n

Disable CS

Signal manipulatio

n


Yes

Yes

No

802.11

GNU Radio

802.11


HiddenExtra

SignalingYes 802.1

1

No client side changes for

Centaur

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.

94

WLANs HP LabsSeoul

National University

Our Testbed

Exposed Terminals

39% 9% 50%

Hidden Terminals

43% 70% 39%

Growth of Interference in Enterprise Wireless LANs

94

Interference an increasing problem according to leading enterprise WLAN

vendor

Vivek ShrivastavaVivek Shrivastava

95


95

Centralized control for better network security and manageability

Vivek Shrivastava

96

Can Centralized Data Plane Scheduling Help?

96Vivek Shrivastava

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97Vivek Shrivastava


• Solution:

• Schedule batch of packets to each exposed AP

• Fix back-off periods and use carrier-sensing to align transmissions -- double throughput!

98

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)

98Vivek Shrivastava

99

Types of Interference

Hidden Terminals

Collis

ions

CarrierSense

Exposed Terminals

Focus on downlink conflicts

Vivek Shrivastava 11

100


100Vivek Shrivastava

X Y

101



X Y

102



X Y


Can Centralized Data Plane Scheduling Help?Hidden

terminals

104

Quantifying Downlink Hidden Terminals


10% links suffer severe hidden terminal interference

105

Quantifying Downlink Exposed Terminals

105

41% links can obtain double the throughput with CS disabled, indicating

exposed terminal interference

Vivek Shrivastava

106



107



108



109



110




• Solution:


• Fix back-off periods and use carrier sensing to align transmissions -- double throughput!

111




• Solution:


• Fix back-off periods and use carrier sensing to align transmissions -- double throughput!

112



113

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


Can Centralized Data Plane Help?

114

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


115

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

116



Can centralized data plane be useful for improving performance in

WLANs ?• About 70-80% of enterprise traffic

is downlink in nature

117



A B

C D

Scheduler

118



A B

C D

Scheduler

119



A B

C D

Scheduler

120



A B

C D

Scheduler

Hybrid scheduling

121

Outline

•DET - A deterministic scheduler

•CENTAUR - A hybrid centralized scheduler

•Evaluation

•Related Work

•Summary


122

Outline



•Evaluation

•Related Work

•Summary


Performance of DET

4xNo gains for exposed terminals; Non-

conflicting links perform worse under load


124

Outline



•Evaluation

•Related Work

•Summary


125


• 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


126

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





Packets arriving

• Schedule one packet at a time• Consider packets in order of arrival

1 2 3 4Transmission

slots

129

What about exposed terminals ?

129

41% links can obtain double the throughput with CS disabled, indicating exposed terminal

interference

Vivek Shrivastava


Use an in-band scheduler

Simple FIFO schedule with interference

avoidance

scheduler

centaur: realizing the full potential of centralized wlans through a hybrid data path vivek...

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