Ashu SabharwalRice University Capacity and Fairness in Multihop Wireless Backhaul Networks Ashu Sabharwal ECE, Rice University

Ashu Sabharwal Rice University

Capacity and Fairness in Multihop Wireless Backhaul Networks

Ashu SabharwalECE, 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


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


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


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


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


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


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


Cooperative PHY

• Send packet(s) to other TAPs

0

1

2

3


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


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%


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 ?


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


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


Outline







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


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


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


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


Throughput Comparisons

20Mbps 5Mbps 10Mbps

Example


Outline







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.


TAP Testbed Goals

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

to traffic patterns


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

Documents

Ashu SabharwalRice University Capacity and Fairness in Multihop Wireless Backhaul Networks Ashu Sabharwal ECE, Rice University