doc.: ieee 802.11-05/0168r0 submission march 2005 violeta gambiroza, rice universityslide 1...

March 2005

Violeta Gambiroza, Rice UniversitySlide 1

doc.: IEEE 802.11-05/0168r0

Submission

End-to-End Performance and Fairness in Multihop Wireless Backhaul Networks

Notice: This document has been prepared to assist IEEE 802.11. It is offered as a basis for discussion and is not binding on the contributing individual(s) or organization(s). The material in this document is subject to change in form and content after further study. The contributor(s) reserve(s) the right to add, amend or withdraw material contained herein.

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Date: 2005-03-16

Name Company Address Phone email V. Gambiroza Rice University 6100 Main,

Houston, TX 77005 (713) 348-2371 [email protected]

B. Sadeghi Intel Corp. 2111 NE 25th Ave, JF3-206, Hillsboro, OR 97124

(503) 712-8367 [email protected]

E. Knightly Rice University 6100 Main, Houston, TX 77005

(713) 348-5748 [email protected]

Authors:

March 2005


doc.: IEEE 802.11-05/0168r0

Submission

Abstract

Wireless IEEE 802.11 networks in residences, small businesses, and public “hot spots” typically encounter the wireline access link (DSL, cable modem, T1, etc.) as the slowest and most expensive part of the end-to-end path. Consequently, network architectures have been proposed that employ multiple wireless hops in route to and from the wired Internet. Unfortunately, use of current media access and transport protocols for such systems can result in severe unfairness and even starvation for flows that are an increasing number of hops away from a wired Internet entry point. Our objective is to study fairness and end-to-end performance in multihop wireless backhaul networks via the following methodology. First, we develop a formal reference model that characterizes objectives such as removing spatial bias (i.e., providing performance that is independent of the number of wireless hops to a wire) and maximizing spatial reuse. Second, we perform an extensive set of simulation experiments to quantify the impact of the key performance factors towards achieving these goals. For example, we study the roles of the MAC protocol, end-to-end congestion control, antenna technology, and traffic types. Next, we develop and study a distributed layer 2 fairness algorithm which targets to achieve the fairness of the reference model without modification to TCP. Finally, we study the critical relationship between fairness and aggregate throughput and in particular study the fairness-constrained system capacity of multihop wireless backhaul networks.

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doc.: IEEE 802.11-05/0168r0

Submission

TAPs: Multihop Wireless Infrastructure

• Transit Access Points (TAPs) are APs with – MIMO antennas

– multiple air interfaces

– enhanced MAC/scheduling/routing protocols

• Form wireless backbone with limited wired gateways

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Research Challenges

1. Physical layer– Achieve 400 Mb/s among TAPs

2. Media access– Target multi-hop and exploit PHY capabilities

3. Fairness and traffic control– With TCP/WiFi, nodes farther away from wires get

much less bandwidth and can starve

4. Prototypes, Testbeds, and Measurement Studies– Platforms for experimentation and proof-of-concept

3. Fairness and traffic control

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Submission

Technology For All DeploymentTechnology For All – Houston, Texas (non-profit)

• Empower low income communities through technology– $10,326 per-capita income

• Applications– Education and work-at-home (“Learn-and-Earn” and Job-Tech)

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Submission

Technology For All Deployment• Multi-hop IEEE 802.11 wireless network covering 40,000 residents

– Single wireline Internet backhaul

– Long-haul directional links

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Submission

Rice TAP Platform• 400 Mb/sec via 4x4 MIMO custom design

– Single 20 MHz WiFi channel at 2.4 GHz and 20 bits/sec/Hz efficiency– Feedback-based algorithms for beam-forming MIMO

• Custom MAC design and FPGA implementation

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doc.: IEEE 802.11-05/0168r0

Submission

Prototype and Testbed Deployment• FPGA implementation of enhanced MAC (opportunistic,

MIMO, multi-channel, QoS)

• Build prototypes and deploy on Rice campus and nearby neighborhoods

• Measurement study from channel conditions to traffic patterns

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doc.: IEEE 802.11-05/0168r0

Submission

Wireless Backhaul Networks TAP Networks

Residential user

or small business

Ethernet

Ethernet

Ethernet

Ethernet

Wireless Backhaul

Network

Internet

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Submission

Fundamental Scenario

Ethernet

Ethernet

Ethernet

Ethernet

One branch of the access tree

Ethernet

Ethernet

Ethernet

Ethernet

Internet

TAP1 TAP2 TAP3 TAP4

• Traffic matrix

– Traffic to and from Internet

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Submission

Parking Lot Scenario

• Similar to parking lot with one exit

Ethernet

Ethernet

Ethernet

Ethernet

Internet

TAP1 TAP2 TAP3 TAP4

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Fairness Problem

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doc.: IEEE 802.11-05/0168r0

Submission

Fairness Problem

Goal Ensure equal shares independent of spatial location

We need multihop fairness

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Contributions

• Fairness reference model– Objective: Remove spatial bias and max spatial reuse– Granularity: Ingress-Aggregated flows

• vs. source-destination pair– Resource: space and time

• vs. bandwidth and buffers– Precise definition in [GSK04]

• Performance study– TCP – Inter-TAP fairness algorithm

• Capacity and fairness

Ethernet

Ethernet

Ethernet

Ethernet

Wireless Backhaul

Network

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Submission

Problem Statement

• Fairness reference model defined

• Distributed algorithm – Targeted at achieving shares defined by reference model

• Solution space

– Local solution – insufficient

• Example: Parking lot

– Multihop solution

• Flow e2e – TCP

• Multihop wireless network e2e – Inter-TAP Fairness Algorithm (IFA)

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Performance Factors (1/2)

Factors investigated

• Fairness algorithms– Uncontrolled UDP, TCP, IFA

• Media access control– 802.11 with two-way and four-way handshake

• Antenna technologies– Omni directional, sector

• Carrier sense range, multiple topologies and flow scenarios…

Other simulation specs

• Channel rate constant 2 Mb/sec, 1000 byte packets

Goal

• Study end-to-end performance and fairness

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Submission


Well understood topologies

Increased no. of hops from destination

Reduced throughput

Increased no. of source-dest. pairs

Reduced throughput

Topology

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Parking lot

MU-TAP and TAP-TAP transmissions on orthogonal channels

Ethernet

Ethernet

Ethernet

Ethernet

Internet

TAP1 TAP2 TAP3 TAP4

TA(1)TA(2)

TA(3)

Topology

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Submission

Fairness with TCP MAC, Hidden Terminals and Information Asymmetry

320.5 320.5 320.5

1000

38.5

0

400

800

1200

1600

TA(1) TA(2) TA(3) TA(4) Total

Goo

dp

ut

[kb

/sec

]

Obj. Basic RTS/CTS

• Idealized objective– Assumes perfect collision-free

MAC

ACK Traffic

MUs generate long lived TCP-Sack flows

Carrier sense range = transmission range

Ethernet

Ethernet

Ethernet

Ethernet

TAP1 TAP2 TAP3 TAP4

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doc.: IEEE 802.11-05/0168r0

Submission

320.5 320.5 320.5

2 20

1247

1000

38.5 48

1177

0

400

800

1200

1600


Goo

dp

ut

[kb

/sec

]

Obj. Basic RTS/CTS

ACK Traffic

• TAP1 and TAP2 traffic starved– Both are hidden terminals

– Timeouts – significant throughput penalty• TCP generates bursts of

packets



Ethernet

Ethernet

Ethernet

Ethernet

TAP1 TAP2 TAP3 TAP4


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doc.: IEEE 802.11-05/0168r0

Submission

320.5 320.5 320.5

2 20

1247

3 27 40.7

1000

38.5 48

1177

1058.7988

0

400

800

1200

1600


Goo

dp

ut

[kb

/sec

]

Obj. Basic RTS/CTS

• RTS/CTS exchange introduces information asymmetry [KSSK02]– TAP1 has no information of

TAP3-TAP4 trans.ACK Traffic

• Capacity and fairness need to be considered jointly– Total is up to 125% of

objective while two flows are starved




Ethernet

Ethernet

Ethernet

Ethernet

TAP1 TAP2 TAP3 TAP4

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Submission

TCP and Sector Antennas

Ethernet

Ethernet

Ethernet

Ethernet

TAP1 TAP2 TAP3 TAP4

641 641 641

2000

247313

4777

356

1386.3

730

53.3

1219

692

167

0

400

800

1200

1600

2000

2400


Goo

dp

ut

[kb

/sec

]

Obj. Basic RTS/ CTS

ACK Traffic

• Impact of hidden terminals and information asymmetry mitigated

• Severe spatial bias – TAP1 traffic obtains 26% of objective

• Total goodput increased

• Total goodput is 67% of the objective


TAPs use sector antennas

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doc.: IEEE 802.11-05/0168r0

Submission

Inter-TAP Fairness Algorithm (IFA)

• Idealized version of algorithm– Omniscient calculation of fair rates

• Practical algorithm needs messaging and incurs delays

• Limit traffic rate at ingress

• Outcome– Approximates objectives despite MAC anomalies

– Independent of transport protocols (TCP/UDP/…)

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Submission

TCP and IFA



Ethernet

Ethernet

Ethernet

Ethernet

TAP1 TAP2 TAP3 TAP4

• End-to-end performance considerably improved– TAP-aggregated throughput is 59% to

75% of the objective

• Hidden terminal problem mitigated– Contention considerably decreased– TCP cannot inject bursts of packets

320.5 320.5 320.5

1000

190 223

679

38.5 26

240

0

400

800

1200


Goo

dput

[kb

/sec

]

Obj. Basic 802.11/IFA

ACK Traffic

• Spatial bias– IFA alone cannot eliminate it

• Rates lower than the objective

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Submission

Inter-TAP Performance Isolation Provide inter-TAP performance isolation

independent of traffic types

ACK Traffic

326.8 326.8 326.8

993.4

209246

709.4

13 8.4

246

0

200

400

600

800

1000


Goo

dput

[kb

/sec

]

Obj. Basic 802.11

• TCP achieves 64% of idealized objective, while UDP obtains 75%

• Even with balanced contention TCP reduces its rate– Having more MUs per TAP TCP

performance degraded

Each TAP has one MUTAP1: MU transmits TCP trafficTAP2 and TAP3: MU transmits UDP traffic

Ethernet

Ethernet

Ethernet

Ethernet

TAP1 TAP2 TAP3 TAP4

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doc.: IEEE 802.11-05/0168r0

Submission

Summary of Findings (1/2)

• Starvation of upstream flows (UDP, TCP, with or w/o RTS/CTS)– “Parking Lot” scenario results in hidden terminals and information

asymmetry

• Sector antennas and carrier sense range mitigate the hidden terminal problem– Severe spatial bias

• SA: Throughput as low as 26% of targeted values • CSR: Throughput as low as 34% of targeted values

• TCP able to exploit spatial reuse

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doc.: IEEE 802.11-05/0168r0

Submission

Summary of Findings (2/2)

• IFA approximates reference model performance

• The impact of hidden terminal problem and information asymmetry mitigated– Without any modifications to CSMA/CA

• TCP over IFA achieves 59% to 75% of idealized objective– Without any modifications to TCP

• Inter-TAP performance isolation

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doc.: IEEE 802.11-05/0168r0

Submission

Conclusions

Fairness• Fairness reference model formally defined• Designed for multihop wireless access networks

Performance study• Starvation of upstream flows• Sector antennas, larger carrier sense range, IFA mitigate the

problem• IFA approximates performance of reference model

Capacity and fairness• Consider joint effects

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doc.: IEEE 802.11-05/0168r0

Submission

References

• [GSK04] V. Gambiroza, B. Sadeghi, and E. Knightly, "End-to-End Performance and Fairness in Multihop Wireless Backhaul Networks,'' in Proceedings of ACM MobiCom 2004, Philadelphia, PA, September 2004.

• TAPs project: http://taps.rice.edu/

• TFA: http://www-old.ece.rice.edu/networks/TFA.pdf

• Rice Networks Group: http://www-old.ece.rice.edu/networks/

March 2005


doc.: IEEE 802.11-05/0168r0

Submission

End-to-End Performance and Fairness in Multihop Wireless Backhaul Networks

V. Gambiroza and E. Knightly, Rice University

B. Sadeghi, Intel Corporation

doc.: ieee 802.11-05/0168r0 submission march 2005 violeta gambiroza, rice universityslide 1...

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