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Multi-Channel Wireless Networks: Theory to Practice

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Title: Multi-Channel Wireless Networks: Theory to Practice


1
Multi-Channel Wireless NetworksTheory to
Practice
  • Nitin Vaidya
  • Electrical and Computer Engineering
  • University of Illinois at Urbana-Champaign

2
Multi-Channel Wireless NetworksAcknowledgements
  • Ph.D
  • Jungmin So (2006)
  • Pradeep Kyasanur (2006)
  • Vartika Bhandari (2008)
  • Vijay Raman ()
  • Post-docs
  • Wonyong Yoon
  • Cheolgi Kim
  • Sung-Hwa Lim
  • M.S.
  • Priya Ravichandran (2003)
  • Chandrakanth Chereddi (2006)
  • Rishi Bhardwaj (2007)
  • Thomas Shen (2008)
  • Vijay Raman (2008)

Funded in part byNSF, ARO, Motorola, Boeing
3
Preliminaries
4
Wireless Networks
  • Wireless paradigms
  • Single hop versus Multi-hop
  • Multi-hop networks
  • Mesh networks, ad hoc networks, sensor networks

5
What Makes Wireless Networks Interesting?
  • Significant path loss
  • - Signal deteriorates over space
  • Spatial re-use feasible

B
A
S
5
6
What Makes Wireless Networks Interesting?
  • Interference management non-trivial

D
B
C
A
S
I
6
7
What Makes Wireless Networks Interesting?
  • Many forms of diversity
  • Time
  • Route
  • Antenna
  • Path
  • Channel

8
What Makes Wireless Networks Interesting?
  • Antenna diversity

D
C
A
B
Sidelobes not shown
9
What Makes Wireless Networks Interesting?
  • Path diversity

10
What Makes Wireless Networks Interesting?
  • Channel diversity

Low gain
B
A
B
A
High gain
11
Wireless Capacity
  • Wireless capacity limited
  • In dense environments, performance suffers
  • How to improve performance ?

12
Improving Wireless Capacity
  • Exploit physical resources, diversity
  • Exploiting diversity requires appropriate
    protocols

13
This Talk
  • Utilizing multiple channels
  • in multi-hop wireless

14
Multi-Channel Environments
Available spectrum
Spectrum divided into channels
2
3
4

c
1
15
Multiple Channels
8 channels
4 channels
26 MHz
100 MHz
200 MHz
150 MHz
2.45 GHz
915 MHz
5.25 GHz
5.8 GHz
IEEE 802.11 in ISM Band
16
Shared Access Time Spectrum
D
B
C
A
One Channel
Two Channels
A
B
C
A
A
B
Spectrum
C
C
Time
Time
17
OutlineTheory to Practice
Linux box
18
Interfaces Channels
  • An interface can only use one channel at a time

W
cW
  • Switching between channels may incur delay

19
Multiple Interfaces
  • Decreasing hardware cost allows formultiple
    interfaces
  • m interfaces per node

20
Practical Scenario
  • m lt c A host can
    only be on
  • subset of channels

1
m
m1
cm unused channels at each node
c
21
Multi-Channel Mesh
  • How to best utilize multiple channelsin a mesh
    networkwith limited hardware ?

?
22
Need for New Protocols
  • m lt c

c 4 channels m 2 interfaces
1,2
23
Multi-Channel NetworksMany Inter-Dependent Issues
  • How to choose a channel for a transmission?
  • How to schedule transmissions?
  • How to measurechannel quality
  • - gain, load
  • How to select routes ?

B
A
C
24
Switchability
25
Channel Switching
  • Unconstrained An interface can tune to any
    available channel
  • Constrained
  • Restricted channel switching

26
Constrained Switchability
  • An interface may be constrained to use only
    asubset of channels
  • Motivation
  • Hardware limitations (untuned radio petrovic
    )
  • Hardware heterogeneity (802.11b/g versus
    802.11a/b/g)
  • Policy issues (cognitive radios)

27
Impact of Constrained Switching
Reduced Connectivity Detour Routing
28
Impact of Constrained Switching
1 relay on channel bZ
3 relays on channel aX,Y,Z
a
a
a, b
X
Y
a, b
D
S
Z
a, b
Coupling between channel selection relay choice
29
Cross-Channel Interference
30
Cross-Channel Interference
  • Orthogonal channels
  • Interference between nearby channels

2
3
4

c
1
31
Cross-Channel Interference
  • Options
  • Avoid using nearby channels ? Spectrum
    underutilized
  • More channels, but nearby channels assigned to
    nodes farther away
  • ? More complex channel management

32
Protocol Design Space
Orthogonal channels Overlapping channels
Unconstrained switching This talk
Constrained switching
33
OutlineTheory to Practice
Linux box
34
Capacity Analysis
  • How does capacity improve with more channels ?
  • How many interfaces necessary toefficiently
    utilize c channels ?

35
Network Model
36
Network ModelGupta-Kumar
  • Random source-destination pairs amongrandomly
    positioned n node in unit area,with n ? 8

37
Capacity ?
  • l minimum flow throughput
  • Capacity n l

38
Capacity Constraints
  • Capacity constrained by available
  • Spectrum bandwidth
  • Interference

39
CapacityGupta-Kumar
  • c m

capacity a
1
1
m c
c m
Capacity scales linearly with channels
40
Capacity
  • What if fewer interfaces ?

1
m
m1
c
41
Interface Constraint
  • Throughput is limited by number of interfaces in
    a neighborhood

N nodes in the neighborhood ? total throughput
N m W
Interfaces as a resource in addition to spectrum,
time and space
42
Mutlti-Channel Capacity
Order O(.)
Channels (c/m)
43
Capacity with n ? 8
  • Are these results relevant ?
  • Yield insights on design of
  • good routing and scheduling protocols
  • Insights relevant in smaller networks too

44
OutlineTheory to Practice
Linux box
45
Insights from Analysis (1)
  • Channel Assignment
  • Need to balance load on channels
  • Local coordination in channel assignment helpful

46
Insights from Analysis (2)
  • Static channel allocation
  • not optimal performance
  • in general
  • Must dynamically switch channels

Channel 1
B
A
C
2
D
47
Insights from Analysis (3)
  • Optimal transmission range function ofnumber of
    channels
  • Intuition of interfering nodes of
    channels

48
Insights from Analysis (4)
  • Routes must be distributed within a neighborhood

D
D
F
F
B
B
E
A
A
E
C
C
m 1 c 1 , 2
49
Insights from Analysis (5)
  • Channel switching delay potentially detrimental,
    but may be hidden with
  • careful scheduling create idle time
    on interfaces between channel switches
  • additional interfaces

50
Protocol Design Timescale Separation
  • Routing Longer timescales
  • (Optional) Multi-channel awareroute selection
  • Interface management Shorter timescales
  • Dynamic channel assignment
  • Interface switching

51
Channel Management
  • Two interfaces much better than one
  • Hybrid channel assignment Static Dynamic

A
B
C
Fixed (ch 1)
Fixed (ch 2)
Fixed (ch 3)
Switchable
Switchable
Switchable
1
2
3
2
Channel assignment locally balanced
52
Selecting Channel Diverse Routes
1
3
4
4
3
B
C
A
4
4
D
E
F
2
4
2
A needs route to C Route A-B-C better ? More
channel diverse
53
Impact of Switching Coston Route Selection
1
4
3
B
C
A
2
Route A-B-C in use D needs route to F Route D-E-F
better
3
D
E
F
2
4
2
4
2
Prefer routes that do not require frequent
switching
54
CBR Random topology(50 nodes, 50 flows, 500m
x 500m area)
(m,c)
55
OutlineTheory to Practice
Linux box
56
Net-X Testbed
  • Linux 2.4
  • Two 802.11a radiosper mesh node (m 2)
  • Legacy clients with1 radio
  • c 5 channels

Soekris 4521
Net-X source available
57
Phy-Aware Support
  • Additional mechanisms needed to choose channels
    based on destination
  • Next hop not equivalent to a wireless interface
    id
  • Phy-aware forwarding not supported traditionally
  • In general, need a constraint specificationfor
    desired channel(s), antenna beamform,power/rate,
    to be used for the next hop

58
Phy-Aware Support
  • Multi-channel (phy-aware)broadcast
  • Channel switching from user space has high
    latency frequent switching from user space
    undesirable

59
New Kernel Support
  • Interface management needs to be hidden
    fromdata path
  • Buffering packets for different channels
  • Scheduling interface switching

Interface switchesto channel 1
Ch. 2
Packet to B
buffer packet
Ch. 1
Packet to C
Packet to C arrives
60
Net-X Architecture
Multi-Channel Routing, Channel Assignment
User Applications
  • Abstraction layer simplifies use of multiple
    interfaces
  • Implemented by extending Linux bonding driver

IP Stack
ARP
Interface and Channel Abstraction Layer
Interface Device Driver
Interface Device Driver
61
Recent Work
62
Impact of Channel Switching
  • Channel switching incurs delay
  • A multihop route may involve several channel
    switches along the route
  • High delays not be suitable for certain delay
    sensitive applications, e.g. VoIP

63
Impact of Channel Switching
  • An alternative
  • Do not switch interfaces when routing delay
    sensitive traffic

A
B
C
Fixed (ch 1)
Fixed (ch 2)
Fixed (ch 3)
2
1
Switchable
Fixed (ch 1)
Fixed (ch 2)
Switchable for normal traffic
64
Impact of Channel Switching
Delay experienced by a single VoIP flow over
multiple hops
Proposed approach Static channel
allocation Single channel allocation
Hybrid channel allocation
65
Wrap-up
Linux box
66
Current Status
  • 25 node network operational
  • Protocol improvements ongoing process
  • Further results for
  • Scheduling in multi-channel networks
  • Constrained channel assignment
  • Cross-channel interference

67
Summary
  • Significant performance benefits usingmany
    channels despite limited hardware
  • Insights from analysis useful in protocol design
  • Conversely, implementation experience helps
    formulate new to theoretical problems

Important to complete the loop from theory to
practice
68
Thanks!
  • www.crhc.uiuc.edu/wireless

69
Thanks!
  • www.crhc.uiuc.edu/wireless

70
Thanks!
  • www.crhc.uiuc.edu/wireless

71
Thanks!
  • www.crhc.uiuc.edu/wireless

72
Scenario 1
  • m c One interface per channel

73
Constrained Switchability
  • An interface may be constrained to use only
    asubset of channels
  • Motivation
  • Hardware limitations (untuned radio petrovic
    )
  • Hardware heterogeneity (802.11b/g versus
    802.11a/b/g)
  • Policy issues (cognitive radios)

74
Impact of Constrained Switching
Reduced Connectivity Detour Routing
75
Impact of Constrained Switching
1 relay on channel bZ
3 relays on channel aX,Y,Z
a
a
a, b
X
Y
a, b
D
S
Z
a, b
Coupling between channel selection relay choice
76
Impact of Constrained Switching
b, d
a, c
a, b
b
a
G
H
X
d
c
c, d
Y
d, f
d
c, f
c
Q
P
6 channels a, b, c, d, e, f
Bottleneck formed at Y
77
Destination Bottleneck Constraint
  • A node may be destination of multiple flows
  • Node throughput shared by all the incident flows

P
Node throughput T mW Per-flow throughput
T / f
f incoming flows
D
78
Mutlti-Channel Network Capacity
Interface anddestination bottlenecks
Interference andinterface bottleneck
Connectivity and interference
Ratio c/m
79
Routing Approach
  • Legacy routing protocols can be operated over our
    interface management layer
  • Does yield significant benefits from multiple
    channel
  • Does not consider cost of channel switching
  • An alternative
  • Develop a channel-aware metric(aware of channel
    diversity and switching costs)

80
Impact of Channel Switching
  • Channel switching incurs delay
  • Mainly software delays
  • Also time spent on a channel before switching to
    another
  • A multihop route may involve several channel
    switches along the route
  • Higher switching cost for longer routes
  • High delays may not be suitable for certain delay
    sensitive applications, e.g. VoIP

81
Impact of Channel Switching
  • An alternative
  • Do not switch interfaces when routing a delay
    sensitive traffic
  • Allow switching after finished routing delay
    sensitive traffic

A
B
C
Fixed (ch 1)
Fixed (ch 2)
Fixed (ch 3)
2
1
Switchable
Fixed (ch 1)
Fixed (ch 2)
Switchable for normal traffic
82
Impact of Channel Switching
Delay experienced by a single VoIP flow over
multiple hops
Proposed approach Static channel
allocation Single channel allocation
Hybrid channel allocation
83
Cross-Channel Interference
84
Cross-Channel Interference
  • Options
  • Avoid using nearby channels ? Spectrum
    underutilized
  • More channels, but nearby channels assigned to
    nodes farther away
  • ? More complex channel management

85
Cross-Channel Interference
D
A
C
B
86
Cross Channel Interference
  • Cross channel interference significant when two
    radios in a node use neighboring channels
  • A possible approach
  • Dynamically assign well separated channels for
    other radios in a node based on current
    transmission channel

87
Cross Channel Interference
Result for ten 6 Mbps multihop flows in a 20 node
network
Using only 5 non-adjacent channels
Using all 12 802.11a channels
Improvement up to 32.18 when using all channels
88
Research Opportunities
  • Significant effort in protocol design needed to
    exploit available physical resources
  • Examples
  • MIMO (multi-antenna)
  • Cooperative relaying
  • Dense wireless infrastructure

89
Thanks!
  • www.crhc.uiuc.edu/wireless

90
Thanks!
  • www.crhc.uiuc.edu/wireless

91
What Makes Wireless Networks Interesting?
  • Time diversity

D
C
gain
Time
92
What Makes Wireless Networks Interesting?
  • Route diversity

infrastructure
AP1
AP2
Access point
B
C
D
E
A
F
Z
X
93
Why Divide Spectrum into Channels ?
  • Manageability
  • Different networks on different channelsavoids
    interference between networks
  • Contention mitigation
  • Fewer nodes on a channel reduces channel
    contention

94
Why Divide Spectrum into Channels ?
  • Lower transmission rate per channel
  • Slower hardware (simpler, cheaper)
  • Reducing impact of bandwidth-independent overhead

95
Connectivity ConstraintGupta-Kumar
  • Need routes between source-destination pairs
  • Places a lower bound on transmit power

A
A
D
D
Not connected
Connected
96
Interference Constraint Gupta-Kumar
  • Interference among simultaneous transmissions
  • Limits spatial reuse

D
C
gt r
A
r
B
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