ashu sabharwalrice university capacity and fairness in multihop wireless backhaul networks ashu...

Ashu Sabharwal Rice University

Capacity and Fairness in Multihop Wireless Backhaul Networks

Ashu SabharwalECE, Rice University

Ashu Sabharwal Rice University

Wireless Utopia:Mobile Broadband

• WiFi Hot-spots– Reasonable speeds – Expensive + poor coverage low subscriber rates,

failing companies,…

• 3G– Ubiquitous, allows mobility but low data rates– Expensive to deploy slow deployments

• Major costs– Wired connection to backbone– Spectral fees– Uneasy “on-demand” growth

Ashu Sabharwal Rice University

Transit Access Points:Multi-hop Backbone

• Few wires– Most TAPs multi-hop to wired gateways– Add wires to TAPs as demand grows

• Use both licensed and unlicensed spectrum– Licensed spectrum: protected, allows guarantees– Unlicensed spectrum: free, more, less interference

outdoors

Multiple radios& MIMO

Ashu Sabharwal Rice University

Major Challenges

• High information density around wires– Capacity per gateway log(n)

• Service quality transparent to user location– Users close to wire can win big– TCP on RTT time-scale, too slow

Ashu Sabharwal Rice University

Characteristics of TAP Networks

• No mobility in backbone– TAPs don’t move static topology

• Slow variability can be used at all time-scales– Physical layer can use fast feedback – Medium access could be topology aware– Qos routing can be reliably done

Opportunity for optimization based on topologyvia feedback at multiple time-scales

Ashu Sabharwal Rice University

Outline

• Opportunistic Cooperative Relaying [Sadeghi,Chawathe,Khoshnevis,Sabharwal]

– Route diversity– Cooperative PHY– OCR

• TAP Fairness [Gambiroza,Sadeghi,Knightly]

– Performance of current protocols– Inter-TAP fairness model

• Rice TAP Testbed

Ashu Sabharwal Rice University

Multi-hop Networks

• Multiple routes to destination– Many routes exist to destination– Route quality function of time

• Coherence time – Time for which channel SNR remains constant– For low mobility channels, several packets long

Route diversity

0

1

2

3

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Cooperative PHY

• Why use only one route every time ?– Carrier sense will shut off many TAPs– Use their power and antenna resources

0

1

2

3

Ashu Sabharwal Rice University

Cooperative PHY

• Send packet(s) to other TAPs

0

1

2

3

Ashu Sabharwal Rice University

Cooperative PHY

• Send packet(s) to other TAPs• All TAPs together “forward” the packet

– Acts like a 3M x M antenna system (in above picture)– Simplest form of network coding

0

1

2

3

Ashu Sabharwal Rice University

Throughput Gains

• Rule: Choose best “k-out-of-m” routes leading to minimum total delay

• Substantial gains for moderate network size

Maximum Available Routes

Th

roughput

(Mbit

s/s)

~60%

~70%

Ashu Sabharwal Rice University

Challenges in Realizing Route Diversity

• Quality of routes unknown– Use of a route depends on its current condition– Thus, routes have to measured before every use

• Multiple TAP coordination– Medium access has to coordinate multiple TAPs

• Knowledge of routes– Many routes exist– Which subset to actively monitor ?

Ashu Sabharwal Rice University

Opportunistic Cooperative Relaying

• 4-way multi-node handshake– Allows source (TAP 0) to know all channel qualities– AND coordinate participating TAPs– TAP 0 chooses the smallest delay route

• Multi-hop MAC– Forwarded packets do not contend again– Slot reservation ensures safe passage to destination

Ashu Sabharwal Rice University

Throughput Performance

• Throughput gains (20-30%) outweigh spatial reuse loss

• 2-4 routes give max gain due to handshake overhead

Distance from source (d)

Th

roughput

(Mbit

s/s)

0 12

3

200 m

d

2-hop 802.11

2-route OCR3-route OCR

4-route OCR

Ashu Sabharwal Rice University

Outline

• Opportunistic Cooperative Relaying [Sadeghi,Chawathe,Khoshnevis,Sabharwal]

– Route diversity– Cooperative PHY– OCR

• TAP Fairness [Gambiroza,Sadeghi,Knightly]

– Performance of current protocols– Inter-TAP fairness model

• Rice TAP Testbed

Ashu Sabharwal Rice University

Unfairness in Current Protocol

• IEEE 802.11, 5 MUs/TAP • TAP1 completely starved

– Same for TCP– Caused mainly by information assymetry

• In general, closest to the wire TAP wins

TAP1 TAP2 TAP3

TA(1)

TAP4

TA(2)TA(3)

MU1 MUn1 MUn4MU1MUn3MU1MUn2MU1

Internet

...

...

...

...

0

399

518

917

0

400

800

1200

TAP1 TAP2 TAP3 Total

Goodput [kb/sec]

UDP/CSMA

Ashu Sabharwal Rice University

Inter-TAP Fairness

• Ingress Aggregation– Flows originating from a TAP treated as one– TAPs implement inter-flow fairness

• Temporal fairness– Different links have different throughputs– Throughput fairness hurts good links

• Removal of Spatial Bias– Equal temporal share not sufficient– More hop flows get lesser bandwidth

Ashu Sabharwal Rice University

Throughput with Temporal Fairness

• Temporal Fairness– Equal time shares to all flows– Flow receives 1/F of the throughput of the case it was the

only flow

• Shares: 18%, 21%, 61%

• Increase in number of hops decrease in throughput

TAP1 TAP2 TAP3

TA(1)

TAP4

TA(2)TA(3)

Internet

∑=

=fh

i i

f

CF

T

1

1

1

20Mbps 5Mbps 10Mbps

Ashu Sabharwal Rice University

Removing Spatial Bias

• Spatial Bias Removal (SBR)– Find the bottleneck link of each flow– Share of all flows traversing bottleneck equal

• SBR+Temporal Fair = Equal temporal share in bottleneck links

• SBR + Throughput Fair = Equal throughput for all flows regardless of their paths

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Throughput Comparisons

20Mbps 5Mbps 10Mbps

Example

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Outline

• Opportunistic Cooperative Relaying [Sadeghi,Chawathe,Khoshnevis,Sabharwal]

– Route diversity– Cooperative PHY– OCR

• TAP Fairness [Gambiroza,Sadeghi,Knightly]

– Performance of current protocols– Inter-TAP fairness model

• Rice TAP Testbed

Ashu Sabharwal Rice University

TAP Hardware Design

• Platform for new PHY + Protocol Design• Generous compute resources

– High-end FPGAs with fast interconnects– Simulink GUI environment for development

• 2.4 GHz ISM band radios– 4x4 MIMO system

• Open-source design– Both hardware and software

QuickTime™ and aTIFF (Uncompressed) decompressor

are needed to see this picture.

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TAP Testbed Goals

• Prototype network on and around Rice campus• Measurement studies from channel conditions

to traffic patterns

Ashu Sabharwal Rice University

Summary

• Transit Access Points– WiFi “footprint” is dismal– 3G too slow and too expensive– Removing wires is the key for economic viability

• Challenges– Enabling high capacity backbone– Multi-hop fairness