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Heterogeneity inMulti-Hop Wireless Networks

Nitin H. Vaidya

University of Illinois at Urbana-Champaign

www.crhc.uiuc.edu/~nhv

© 2003 Vaidya

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Summary

Heterogeneity is essential Heterogeneity is beneficial

Research Agenda Develop protocols that exploit the heterogeneity Develop mechanisms to better evaluate wireless systems

Proof by example …

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Heterogeneity

Many dimensions of heterogeneity:

Architecture

Physical capability of hosts

Higher layers

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Architecture

Multi-hop wireless networks

Pure ad hoc networks

Hybrid networks

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Pure Ad Hoc Networks

No “infrastructure” All communication over (one or more) wireless hops

EA

B CD

X

Z

Ad hoc connectivity

Y

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Hybrid Networks

Infrastructure + Ad hoc connectivity

EA

B CD

AP1 AP2

X

Z

infrastructure

Ad hoc connectivity

Y

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Hybrid Networks

Infrastructure may include wireless relays

EA

CD

AP1 AP2

X

Z

infrastructure

Ad hoc connectivity

Y

B

RP

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Hybrid Networks

Heterogeneity Some hosts connected to a backbone, most are not Access points may have more processing capacity, energy

EA

B CD

AP1 AP2

X

Z

infrastructure

Ad hoc connectivity

Y

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Hybrid Networks

Heterogeneous wireless technologies

EA

B CD

AP1 AP2

X

Z

infrastructure

Y

Type 1(3G)

Type 2(802.11)

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Hybrid Networks

Heterogeneity is essential

Pure ad hoc or pure infrastructure networks inadequate for many environments

Heterogeneity is beneficial …

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Benefit over Pure Ad Hoc Networks

Infrastructure provides a frame of reference Can assign approximate locations to the mobiles

– Provide location-aware services

– Reduce route discovery overhead

AP0 AP1 AP2 AP3

A

B DR2R1 R3

A

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Benefit over Pure Ad Hoc Networks

Infrastructure can reduce diameter of the network Lower delay Potentially greater per-flow throughput

EA

CD

AP1 AP2

X

Z

infrastructure

Ad hoc connectivity

Y

B

RP

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Infrastructure Facilitates New Trade-Offs (hypothetical curves)

User density distributionaffects the trade-offAd hoc-ness = K

co

nn

ec

tiv

ity

ov

erh

ea

d

Poor Man’s Ad Hoc Network

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Infrastructure Helps in Resource AllocationAddress Assignment

Unique IP addresses need to be assigned to hosts in a network

DHCP used in traditional networks

Difficult to use DHCP in pure ad hoc networks

But Can also be deployed on the infrastructure in a hybrid network

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Infrastructure Helps in Resource AllocationAddress Assignment

Impossible to detect address duplication in networks that can get partitioned

• Unbounded delays cause difficulty

Clusters of hosts may partition from the infrastructure, rejoin, over time

Need a mechanism to assign unique addresses despite partitions

• Impossible with unbounded message delays

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If a problem cannot be solved

Change the problem

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Weak Duplicate Address Detection

Packets from a given host to a given address

should be routed to the same destination,

despite duplication of the address

Achievable despite unbounded delay, but incurs overhead

Infrastructure to the rescue: Use weak DAD only for nodes partitioned from the infrastructure

Can this extend to other resource allocation problems?

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Benefit over Pure Infrastructure Networks

Ad hoc routing increases the “reach” of the infrastructure

Connectivity can be traded with overhead

Example: Limit “ad hoc-ness” to K hops

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Hybrid Networks: Research Issues

How to implement infrastructure?

How to deploy relays/access points?

• Density, distribution

What functionality should be given to relays and access points?

• Should they cooperate? With each other? With mobiles?

Are relays an optimization or necessary components?

Should the spectrum be divided between the infrastructure and ad hoc components?

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Hybrid Networks: Research IssuesHow to design protocols?

How to trade “complexity” with “performance” ?

• Parameterize ad hoc-ness ?

How to design protocols that maximize “performance” for a given complexity?

• Power control: How should the heterogeneity affect power control?

• MAC: Should the infrastructure do more work?

• Routing: Reduce overhead using infrastructure

• Transport: How to approach theoretical capacity bounds?

• How to deal with potentially unbounded delays?

The answers to the above questions are inter-dependent

• Power control, MAC, routing, transport protocols affect each other’s behavior

• Cross-layer design needed

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Heterogeneity

Many dimensions of heterogeneity:

Architecture

Physical capability of hosts– Antennas

– Topology control mechanisms

– Processing capability

– Energy availability

Higher layers

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Antenna Capabilities

“Fixed beam” antennas prevalent on mobile devices Omnidirectional antennas

“Movable beam” antennas likely to become more prevalent over time Switched, steered, adaptive, smart …

– Can form narrow beamforms, which may be changed over time

Re-configurable antennas– Beamforms can be changed over time by reconfiguring the

antenna

Different devices may incorporate different antennas

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Antenna Heterogeneity

All antennas are not made equal

Beamforms: Only directional, or omni too?

Timescale: Can beams be “moved” at packet timescales?

Single beam or multiple beams?

Variations with time?

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Antenna Capabilities

Protocols designed for omnidirectional (fixed beam) antennas inadequate with movable beam antennas

State of the art: MAC Protocols designed for specific antenna capabilities

Need “antenna-adaptive” MAC and routing protocols that allow for antenna heterogeneity

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Antenna Heterogeneity

Heterogeneity is essential

Enforcing homogeneity will limit benefits from antenna improvements

Heterogeneity is beneficial

Devices can employ best antennas that they can “afford”– Device constraints: energy, processing, size, weight, $$– Access points may use more capable antenna than mobiles

Antenna-adaptive protocols allow separation of the antenna as a “layer” in the protocol stack

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Antenna Heterogeneity: Research Challenge

How to design “antenna-adaptive” protocols ?

Need to develop suitable antenna abstractions that span a range of antenna designs

Forces us to think about essential characteristics of antennas

– Example: Variability of beam patterns a more fundamental property than directionality

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Evaluation of Wireless Networks

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Capacity

Capacity analysis:

• Capacity results useful to determine the gap between actual performance and the best case scenario

• Significant progress in recent years

• Need further work to model heterogeneous environments

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Evaluation of Wireless Protocols

Benchmarks: Need benchmarks for comparison of different protocols

• State of the art: Toy benchmarks, almost no real data (for evaluating multi-hop wireless networks)

Simulations

• Commonly used simulation models are poor

• Need better physical layer models accessible to protocol community

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Evaluation of Wireless Protocols

Experimentation:

“Full scale” experiments not always practical

Need mechanisms to build and experimentally evaluate “scaled models” of the network

– Physical dimensions

– Mobility

– Number of hosts

– Traffic density

How to “scale down” the network, and still maintain essential behaviors?

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Conclusions

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Conclusions

Heterogeneity essential, and beneficial

Heterogeneity Complexity ? Not necessarily Thinking about heterogeneity useful in arriving at better abstractions

Need protocols that can exploit heterogeneity

Need approaches for realistic comparative evaluation of protocols

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Thanks!