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