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Wireless Networks:Things I Wish I Had Learned in

Kindergarten

Nitin Vaidya

Illinois Center for Wireless Systems (ICWS)University of Illinois at Urbana-Champaign

www.icws.uiuc.edu© 2007 Vaidya

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Mesh Networks

Multi-hop wireless networks

B

A

E

D

FC

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Outline

A few obvious observations

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Those who cannot learn from historyare doomed to repeat it

With apologies to George Santayana

outgrow

1

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Relaying : Multi-hop routes (store-and-forward)

Pre-History of Wireless Communications:Smoke Signals, Fires, Semaphore

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Pre-History of Wireless Communications:Homing Pigeons

Exploiting mobility

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Pre-History of Wireless Communications:Perimeter Guards

Aggregating knowledge

overcome

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Reusing Ideas Reasonable,but Need to Explore Better Alternatives

No wired-equivalentfor wireless networks No links !

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Wireless Channel Offers Rich Diversity

Current mesh protocolsexploit diversityonly to a limited extent

The vanishing link :Diversity confusesthe notion of a link

Layer 1 : 2+ gap

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Interference is Information

2

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Interference is Information

A

B

D

C

SignalInterference

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Interference is Information

Discard interference

Per-flow capacity decreases with network size

Utilize information in “interference”

Per-flow capacity independent of network size

Requires network scale cooperation

[Gupta-Kumar,Ozgur et al.]

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Interference is Information

Cooperation is already used in wireless networks Routing, medium access, data caching, …

Need to design protocols that facilitatefundamentally better cooperation

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Bits Are Not Automobiles

3

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Bits Are Not Automobiles

We treat information networks same asphysical transportation networks

•Planes, Trains and Automobiles

Bits can be combined (encoded) andseparated, unlike physical objects

Network coding:Deliberate (reversible) injection of interference

[Ahlswede et al.]

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Network Coding

A CBP

P QQ

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Network Coding

A CBP Q

P +QQ+

Q P

[Katabi,Medard]

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Physics Does Not Know Layers

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Physics Does Not Know Layers

Layering is an abstraction, not a theorem

Backpressure scheduler ( “ throughput optimal ” ) spans traditional layers 1 through 3:

arg max ∑ W(l) r(l)r Є Rate l Region

[Tassiulas]

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Physics Does Not Know Layers

Layering is useful, but need a principled approach to identifying appropriate cross-layer exchange

Great start towards this: Network utility optimization» Queue as price

Shortcomings:» Not all requirements easy to capture as concave

utility» Framework does not (yet) yield enough insight on

practical “scheduling/routing”[Kelley,Srikant,Shroff]

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Opportunism Pays

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Opportunism Pays

Channel variations make it difficult to predict short-term optimal in advance

Late binding can work better

– Opportunistic beamforming– Opportunistic routing (network layer)– MAC-Layer anycasting (MAC layer)– …

[Viswanath,Morris,RoyChoudhury]

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Theory and Practice:The Twain Must Meet

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Theory and Practice

(Phy) Theory has had a significant impact on cellular system design

Little impact so far on multi-hop wireless networks

Difficulty arises from capturing essential system characteristics in a tractable abstraction

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Net-XMulti-Interface Multi-Channel Mesh

26 MHz 100 MHz 200 MHz 150 MHz

2.45 GHz 915 MHz 5.25 GHz 5.8 GHz

3 channels

8 channels

4 channels

250 MHz 500 MHz 1000 MHz

61.25 GHz

24.125 GHz 122.5 GHz

[Kyasanur-Vaidya]

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Channel-Interface Scenario 1

One interface per channel used in the network

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m c = m

number of interfaces m = number of channels used c

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Channel-Interface Scenario 2

number of interfaces m < number of channels c

1

c

1

m m

This is the likely scenario

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Net-XTheory

to Practice

A

B

C

D

E F

Fixed

Switchable

Insights onprotocol design

Multi-channelprotocol

Channel Abstraction Module

IP Stack

InterfaceDevice Driver

User Applications

ARP

InterfaceDevice Driver

OS improvementsSoftware architecture

Net-Xtestbed

Linux boxes

Capacitybounds

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Net-X : Main Lessons

Interesting research at the intersection oftheory and protocols for real systems

» Many opportunities remain untapped

Physical layer capabilities provide the promise of higher performance

Practical protocols needed to realize these gains

– MIMO, Beamforming– Adaptive power/rate/carrier sensing– Channel diversity– Multi-user diversity

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Accuracy and Repeatability:One Without the Other

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Repeatable (Experimental) Evaluation of Wireless Networks

Electromagnetic isolation difficult in typical operating environments

EM isolation feasible in an anechoic chamber

Illinois Wireless Wind Tunnel

[Bernhard et al.]

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Illinois Wireless Wind Tunnel

Objectives

Controlled interference

Controlled mobility and environment

Accurate Scaling

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Network Scaling

Size of the anechoic chamber often smaller than the real network

Need to scale the wireless network

Scale power to scale network “size” Scale speed Scale large scale path loss variations (shadowing) Scale small scale fading:

Scaling of speed affects Doppler

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Reality Strikes Back

Can’t scale speed of light (adequately)

Hard to scale for path delay, or delay spread

Can’t evaluate accurately if PHY exploits delay spread

Trade-off:•EM isolation, but limited delay-spread•Non-isolation, but true-scale otherwise

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What You Don't Know Can Hurt You

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What You Don't Know Can Hurt You Channel variations make timely accurate

channel state dissemination impossible

Non-identical channel state observations can lead to conflicting actions

» Difficulty in distributed rate/power control» Difficulty in diagnosing attacks» Unique address assignment problem» Inaccurate topology estimates» Obfuscation of cause of packet loss» Hidden / exposed terminals

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Divide and Conquer

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Divide and Conquer

At higher rates, rate-independent overheads become significant

Partitioning the resources can

Reduce the impact of rate-independent overheads Improve contention resolution

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Conclusion

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Conclusion

Diversity in wireless networks provides many opportunities for improving performance

Networking researchers need to understand PHY better, and vice-versa

A truly cross-layer approach,collaborations between EE and CS/CE researchers, likely to be more successful

•In design and evaluation both GENI

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Thanks!

www.crhc.uiuc.edu / wireless

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Incomplete List of Other Researchers Whose Work Influenced the Observations

R. Ahlswede Mung Chiang Dina Katabi Frank Kelly P. R. Kumar Ralph Koetter Muriel Medard P. Larsson Robert Morris A. Ozgur

Ness Shroff R. Srikant Sasha Stolyar Leandros Tassiulas David Tse Terry Todd Pramod Viswanath R. W. Yeung