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Exploiting Diversity in Wireless Networks

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5. Diversity / Heterogeneity. Many dimensions: Physical layer. Architecture. Upper layer ... Exploit diversity to choose channel with best gain ... – PowerPoint PPT presentation

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Title: Exploiting Diversity in Wireless Networks


1
Exploiting Diversity in Wireless Networks
  • Nitin H. Vaidya
  • University of Illinois at Urbana-Champaign
  • www.crhc.uiuc.edu/wireless
  • Presentation at Mesh Networking Summit
  • Snoqualmie, WA, June 23-24, 2004

2
Capacity of Wireless Networks
  • Limited by
  • Interference
  • Available spectrum
  • Need to find ways to get most out of available
  • spectrum

3
Diversity / Multiplicity / Heterogeneity
  • Diversity provides flexibility in using available
    resources
  • Can help improve performance

4
Diversity / Multiplicity / Heterogeneity
  • Research Agenda
  • Abstractions that capture diversity
  • Protocols that exploit diversity

5
Diversity / Heterogeneity
  • Many dimensions
  • Physical layer
  • Architecture
  • Upper layer

6
Channel Diversity
7
Channel Diversity
  • Multiple channels can help improve performance
  • Obvious approaches
  • Exploit diversity to choose channel with best
    gain
  • Use multiple channels simultaneously to improve
    capacity
  • Developing practical protocols for the obvious
    approaches is still a challenge

8
Alternative Approach
  • Exploit protocol characteristics to benefit from
    the diversity
  • Examples
  • Pipelining
  • Backup routes

9
IEEE 802.11
  • Channel contention resolved using backoff(and
    optional RTS/CTS)

Backoff
Data / ACK
RTS/CTS
10
Simple Observation
  • Backoff keeps channel idle ? unproductive
  • Most protocols have such idle contention periods

Unproductive
Backoff
Data / ACK
RTS/CTS
11
Pipelining Using Multiple Channels
  • Control Channel Backoff and RTS/CTS
  • Data Channel Data and ACK

Backoff
RTS/CTS
Backoff
RTS/CTS
RTS/CTS
Backoff
Stage 1
Data / ACK
Data / ACK
Stage 2
12
Pipelining works well only if pipeline stages are
balanced !
Control Channel
Data / ACK
Data / ACK
Data Channel
13
Solution Partial Pipelining
  • Only partially resolve channel contention in the
    pipelined stage

14
Partial Pipelining
  • Stage 1 Narrow-Band Busy Tone Channel
  • Stage 2 Data channel

Control Channel
Data / ACK
Data / ACK
Data Channel
15
Partial Pipelining
  • No packets transmitted on busy tone channel
  • Bandwidth can be small

16
Partial Pipelining
  • By migrating backoff to a narrow-band channel,
    cost of backoff is reduced

Data Channel Bandwidth
Area cost of backoff
Busy Tone Channel Bandwidth
Backoff Duration
17
Moral of the Story
  • Looking beyond physical layerdiversity
    exploitation schemes helps
  • Protocol characteristics can be exploited

18
Another Example
19
Multiple Interfaces
  • Consider devices equipped with both 802.11a and b

802.11a 802.11b
Higher max rate Lower max rate
Lower range Higher range
20
Channel Diversity
  • 802.11b network
  • denser than the 802.11a network
  • but provides lower rate
  • Example approach
  • Use 802.11a as primary network
  • Use 802.11b network to provide backup routes when
    802.11a routes fail
  • The 802.11b network could be used for other
    things too

21
Protocol Interactions
  • For TCP, route failure more painful than a
    degradation in available capacity
  • The backup routes can avoid a route failure
  • Benefits of added capacity can be magnified by
    exploiting protocol behavior

22
Research Agenda
  • Develop practical protocols that can exploit
    diversity
  • Pay attention to protocol characteristics

23
Antenna Heterogeneity
24
Antenna Heterogeneity
  • Fixed beam antennas prevalent on mobile devices
  • Omnidirectional antennas (often with diversity)
  • Other antennas likely to become more prevalent
  • 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, but not necessarily
    narrow beams

25
Antenna Heterogeneity
  • Beamforms All antennas are not made equal
  • Timescale Can beamforms be changed at packet
    timescales?

26
Protocol Design
  • Protocols designed for fixed beam antennas
    inadequate with movable beam antennas
  • State of the art
  • MAC Protocols for specific antenna
    capabilities

27
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 beamforms a more
    fundamental property than directionality

28
Diversity / Heterogeneity
  • Many dimensions
  • Physical layer
  • Architecture
  • Upper layer

29
Pure Ad Hoc Networks
  • No infrastructure
  • All communication over (one or more) wireless hops

B
C
D
E
A
Z
Ad hoc connectivity
Y
X
30
Hybrid Networks
  • Infrastructure Ad hoc connectivity

infrastructure
AP1
AP2
B
C
D
E
A
Z
Ad hoc connectivity
Y
X
31
Hybrid Networks
R
  • Infrastructure may include wireless relays

infrastructure
AP1
AP2
R
P
C
B
R
D
A
Z
Ad hoc connectivity
Y
X
32
Hybrid Networks
  • Heterogeneity
  • Some hosts connected to a backbone, most are not
  • Access points/relays may have more processing
    capacity, energy

33
Heterogeneity Beneficial
  • Infrastructure provides a frame of reference
  • Provide location-aware services
  • Reduce route discovery overhead

AP0
AP1
AP2
AP3
R3
R2
R1
D
B
A
34
Heterogeneity Beneficial
  • Reduce diameter of the network
  • Lower delay
  • Potentially greater per-flow throughput

35
Infrastructure Facilitates New Trade-Offs
(hypothetical curves)
overhead
connectivity
User density distribution affects the trade-off
Ad hoc-ness
36
Research Issues
  • How to trade complexity with performance ?
  • Parameterize ad hoc-ness ?
  • Should the spectrum be divided between
    infrastructure and ad hoc components?
  • What functionality for relays / access points?

37
Misbehavior
38
Misbehavior
  • Misbehavior occurs with limited resources
  • Violating protocol specifications benefits
    misbehaving hosts
  • Example Small backoffs in 802.11 ? higher
    throughput

39
Research Agenda
  • Protocols that maximize performance while
    discouraging/penalizing misbehavior
  • Challenge
  • Wireless channel prone to temporal and spatial
    variations
  • Different players see different channel state
  • Impossible to detect misbehavior 100 reliably

40
Conclusions
41
Conclusions
  • Diversity/Heterogeneity natural to wireless
    networks
  • Need better abstractions to capture the diversity
  • Need protocols that can exploit available
    diversity
  • Need to be able to survive misbehavior

42
Other Research
  • Distributed algorithms for multi-hop wireless
    networks
  • Clock synchronization
  • Message ordering
  • Leader election
  • Mutual exclusion

43
Thanks! www.crhc.uiuc.edu/wireless
  • Advertisement
  • National Summit for Community Wireless Networks
  • Urbana-Champaign, Illinois
  • August 20-22, 2004
  • http//www.cuwireless.net
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