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Multi-Channel Wireless Networks:Capacity and Protocols

Nitin H. VaidyaUniversity of Illinois at Urbana-Champaign

Joint work with

Pradeep KyasanurChandrakanth Chereddi

Jungmin So

Computer Communications Workshop 2005© 2005

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Multi-hop Wireless Networks

Mesh networks

Ad hoc networks

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Wireless Capacity

Wireless capacity limited

In dense environments, performance suffers

How to improve performance ?

4

Improving Wireless Capacity

Exploit physical resources

Exploit diversity/multiplicity of resources

Examples …

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Exploit Infrastructure

Infrastructure provides a tunnel to forward packets

EA

B CD

BS1 BS2

X

Z

infrastructure

Ad hoc connectivity

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Exploit Antennas

Steered/switched directional antennas

A

D

CB A B

D

C

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Improve Spatial ReusePower/Rate/Carrier-Sense Control

A B C D

A B C D

Transmit SpatialPower Rate reuse

High High Low

Low Low High

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Exploiting Diversity

Exploiting diversity

requires suitable protocols

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This Talk

Utilizing multiple channels in wireless networks

Capacity bounds

Insights on protocol design

Implementation issues

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Multiple Channels

Available frequency spectrum often split into channels

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

802.11 in ISM Band

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Multiple Channels

Common practice in multi-hop networks:Tune all interfaces to the same channel

Channel 1

Channel 1

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Better capacity using multiple interfaces One interface per channel

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c c

1

c

Multiple Channels

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Multiple Channels with Interface Constraint

What if interfaces (m) < channels (c) ?

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c = mm = c

1

c

1

m m

Baseline Our system

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This Talk

Utilizing multiple channels in wireless networks

Capacity bounds

Insights on protocol design

Implementation issues

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Capacity = ?[Gupta-Kumar]

Random source-destination pairs among randomly positioned n hosts in unit area, with n ∞

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Capacity = ?

= minimum flow throughput Capacity = n

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Capacity Constraints

Capacity constrained by available spectrum bandwidth

Other factors further constrainwireless network capacity …

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Connectivity Constraint[Gupta-Kumar]

Need routes between source-destination pairs Places a lower bound on transmit range

Not connected Connected

AD A

D

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Interference Constraint [Gupta-Kumar]

Interference among simultaneous transmissions Limits spatial reuse

is aGuardparameter

dAB

(1+)dDC

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Capacity of Wireless Networks[Gupta-Kumar]

Bit rate for each transmission = W

Capacity increases with n as

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Capacity of Wireless Networks[Gupta-Kumar]

Result holds when m = c

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11

cm = c

W/c

W

W/c

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Capacity of Wireless Networks

What if fewer interfaces ?

Additional constraints on capacity become relevant

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Interface Constraint

Throughput is limited by number of interfaces in a neighborhood

•Interfaces, a resource

k nodes in the “neighborhood”

total throughput ≤ k * m * W/c

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Destination Bottleneck Constraint

Per-flow throughput limited bytotal number of flows at a host

Df incomi

ng

flows

If node throughput = T

Per-flow throughput = T / f

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Random network – Region 1

Capacity constrained by connectivity + interference

No dependence on m and

c

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Random network – Region 2

Capacity constrained by interfaces + interference

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Random network – Region 3

Capacity constrained by destination bottleneck

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This Talk

Utilizing multiple channels in wireless networks

Capacity bounds

Insights on protocol design

Implementation issues

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Insights from Capacity Analysis (1)

Static channel allocation does not yield optimal performance in general

Must dynamically switch channels

Need protocol mechanisms for channel switching

A

C

BChannel 1

2 D3

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Insights from Capacity Analysis (2)

Optimal transmission range function of

density of nodes and

number of channels

Goal: # of interfering nodes = # of channels

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Insights from Capacity Analysis (3)

Routes must be distributed within a neighborhood

This is not necessary in single channel networks

A

B

C

D

E

F

A

B

C

D

EF

Multi-Channel (m<c)Optimal strategy

Single Channel (m=c=1)Optimal strategy

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Insights from Capacity Analysis (4)

Channel switching delay potentially detrimental, but may be hidden with

careful scheduling, or

additional interfaces

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Example Configuration

IEEE 802.11a/b devices

2 interfaces per host

Soekris box

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This Talk

Utilizing multiple channels in wireless networks

Capacity bounds

Insights on protocol design

Implementation issues

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Which Layers to beMulti-Channel Aware?

Practical decision:

Above MAC layer

Allows use of unmodified 802.11

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Separation of Responsibility

Interface management: Shorter timescales

Dynamic channel assignmentto interfaces

Interface switching

Routing: Longer timescales

Multi-channel aware route selection metrics

Link

Network

Transport

PhysicalLayer

Upper layers

802.11

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Channel Assignment

2 interfaces much better than 1 Hybrid channel assignment: Static + Dynamic

A

Fixed (ch 1)

Switchable

B

Fixed (ch 2)

Switchable

C

Fixed (ch 3)

Switchable12 3 2

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Selecting Channel Diverse Routes

Most routing protocols use shortest-hop metric Not sufficient with multi-channel networks

Need to exploit channel diversity

A

B

C

D

1 1

2 1

Route A-C-D is better

Select routes with greater channel diversity

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Impact of Switching Cost

Interface switching cost has to be considered A node may be on different routes, requiring switching

A

B

C

D

2 1

2 1

Route A-B-D is better

E3

Prefer routes that do not require excessive switching

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Other Issues

Routing table entries need to store interface and channel identifiers

Packet buffering pending channel switch

Multi-channel broadcast

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Testbed Status

Interface and channel abstraction layer implemented

•Can run legacy routing protocol above this

Multi-channel routing implementation in progress

20+ node testbed to be deployed later in Fall 2005

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Conclusion

Capacity results hint at significant

performance benefits using

many channels with few interfaces

Need suitable protocols to exploit the channels

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Thanks!

www.crhc.uiuc.edu / wireless