1. Can Theory Meet Practice: The Case of Multi-Channel Wireless Networks
Nitin Vaidya
Electrical and Computer Engineering
University of Illinois at Urbana-Champaign
Sept

2. Multi-Channel Wireless Networks Acknowledgements
Ph.D
Jungmin So (2006)
Pradeep Kyasanur (2006)
Vartika Bhandari (2008)
Post-docs
Wonyong Yoon
Cheolgi Kim
M.S.
Priya Ravichandran (2003)
Chandrakanth Chereddi (2006)
Rishi Bhardwaj (2007)
Thomas Shen (2008)
Vijay Raman
Funded in part by: NSF, ARO, Motorola, Boeing

3. Preliminaries …

4. Wireless Networks Wireless paradigms: Multi-hop networks:
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
distance
power S 5 5

6. What Makes Wireless Networks Interesting?
Interference management non-trivial D B C A
distance
power S I 6 6

7. What Makes Wireless Networks Interesting?
Many forms of diversity
Time
Route
Antenna
Path
Channel 7

8. What Makes Wireless Networks Interesting?
Time diversity D C
gain
Time 8

9. What Makes Wireless Networks Interesting?
Route diversity
infrastructure
AP1
AP2
Access
point B C D E A F Z X

10. What Makes Wireless Networks Interesting?
Antenna diversity D C A B
Sidelobes not shown

11. What Makes Wireless Networks Interesting?
Path diversity

12. What Makes Wireless Networks Interesting?
Channel diversity A B C D
Low interference
High interference
Low gain B A B A
High gain

13. Wireless Capacity Wireless capacity limited
In dense environments, performance suffers
How to improve performance ?

14. Improving Wireless Capacity
Exploit physical resources, diversity
Exploiting diversity requires appropriate protocols

15. State of Multi-Hop Wireless
Very large volume of activity
Beautiful theory
Asymptotic Capacity
Throughput-optimal scheduling
Network utility optimization
Network coding
Cooperative relaying

16. State of Multi-Hop Wireless
Very large volume of activity
Practical protocols & deployments
Many wireless standards
And many more MAC & routing protocols
Many experimental deployments
Mesh devices
Sensor devices
Start-ups

17. State of Multi-Hop Wireless
Despite the volume of activity
Theoretical developments haven’t been translated to practice
Theory often ignores realities of wireless networks
Greater success in cellular environments

18. Picture from Wikipedia
What is Lacking ?
Meaningful contact between
Practice
Networking
Theory
Comm
Picture from Wikipedia

19. This Talk
Utilizing multiple channels
in multi-hop wireless

20. Multi-Channel Environments
Available spectrum
Spectrum divided into channels 1 2 3 4 … c

21. Multiple Channels 3 channels 8 channels 4 channels
26 MHz
100 MHz
200 MHz
150 MHz
915 MHz
2.45 GHz
5.25 GHz
5.8 GHz
IEEE in ISM Band

22. Outline Theory to Practice
Capacity
bounds
channels
capacity A B C D E F
Fixed
Switchable
Insights on
protocol design
Multi-channel
protocol
Channel Abstraction Module
IP Stack
Interface
Device Driver
User
Applications
ARP
OS improvements
Software architecture
Net-X
testbed
CSL
Linux box

23. Interfaces & Channels An interface can only use one channel at a time
Channel c W cW
Switching between channels may incur delay

24. Multiple Interfaces
Reducing hardware cost allows for multiple interfaces
m interfaces per node 1 m

25. Practical Scenario m < c A host can only be on subset of channels 1
c–m unused channels
at each node c

26. Multi-Channel Mesh
How to best utilize multiple channels in a mesh network with limited hardware ? ?

27. Need for New Protocols m < c c = 4 channels m = 2 interfaces
Some channels not used A B C D
1,2
1,2
Network poorly connected A B C D
1,3
2,4
1,2
3,4

28. Multi-Channel Networks Many Inter-Dependent Issues
How to choose a channel for a transmission?
How to schedule transmissions?
How to measure “channel quality”
- gain, load
How to select routes ? B A C

29. Outline Theory to Practice
Capacity
bounds
channels
capacity A B C D E F
Fixed
Switchable
Insights on
protocol design
Multi-channel
protocol
Channel Abstraction Module
IP Stack
Interface
Device Driver
User
Applications
ARP
OS improvements
Software architecture
Net-X
testbed
CSL
Linux box

30. Capacity Analysis How does capacity improve with more channels ?
How many interfaces necessary to efficiently utilize c channels ?

31. Network Model

32. Network Model [Gupta-Kumar]
Random source-destination pairs among randomly positioned n node in unit area, with n  ∞

33. Capacity = ?
l = minimum flow throughput
Capacity = n l

34. Capacity [Gupta-Kumar]
c = m
capacity a 1 1
m = c
c = m
Capacity scales linearly with channels
IF # interfaces also scaled

35. Capacity
What if fewer interfaces ? 1 m 1 m
m+1 c

36. Mutlti-Channel Capacity
Order O(.)
Channels (c/m)

37. Capacity with n  ∞ Are these results relevant ?
Yield insights on design of
good routing and scheduling protocols

38. Outline Theory to Practice
Capacity
bounds
channels
capacity A B C D E F
Fixed
Switchable
Insights on
protocol design
Multi-channel
protocol
Channel Abstraction Module
IP Stack
Interface
Device Driver
User
Applications
ARP
OS improvements
Software architecture
Net-X
testbed
CSL
Linux box

39. Insights from Analysis (1)
Channel Usage
Need to balance load on channels
Local channel assignment schemes helpful in some large scale scenarios
 Local mechanisms with some hints from nearby nodes

40. Insights from Analysis (2)
Static channel allocation
not optimal performance
in general
Must dynamically switch channels
Channel 1 B A 2 C D

41. Insights from Analysis (3)
Small number of switchable interfaces suffice
How to use a larger number of interfaces ?

42. Channel Management Hybrid channel assignment: Static + Dynamic A B C 2
Fixed (ch 1)
Fixed (ch 2)
Fixed (ch 3)
Switchable 2 1
Switchable 3 2
Switchable

43. Insights from Analysis (4)
Interface bottleneck can constrain performance
Interfaces as a resource
in addition to spectrum, time and space

44. Alleviating Interface Bottleneck
Routes must be distributed within a neighborhood D D F B F B E A E A C C
m = 1
c = 1 , 2

45. 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

46. Insights from Analysis (6)
Optimal transmission range function of number of channels
Intuition: # of interfering nodes ≈ # of channels

47. Protocol Design: Timescale Separation
Routing: Longer timescales
(Optional) Multi-channel aware route selection
Interface management: Shorter timescales
Dynamic channel assignment
Interface switching
Link
Network
Transport
Physical
Layer
Upper layers
802.11

48. CBR – Random topology (50 nodes, 50 flows, 500m x 500m area)
(m,c)

49. Outline Theory to Practice
Capacity
bounds
channels
capacity A B C D E F
Fixed
Switchable
Insights on
protocol design
Multi-channel
protocol
Channel Abstraction Module
IP Stack
Interface
Device Driver
User
Applications
ARP
OS improvements
Software architecture
Net-X
testbed
CSL
Linux box

50. Net-X Testbed Linux 2.4 Two 802.11a radios per mesh node (m = 2)
Legacy clients with 1 radio
c = 5 channels
Soekris 4521
Net-X source
available

51. Phy-Aware Support A B C
Ch. 1
Ch. 2
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” specification for desired channel(s), antenna beamform, power/rate, … to be used for the next hop

52. Phy-Aware Support Multi-channel (phy-aware) broadcast B Ch. 2 D A
Channel switching from user space has high latency: frequent switching from user space undesirable

53. Interface switches to channel 1
New Kernel Support
Interface management needs to be hidden from “data path”
Buffering packets for different channels
Scheduling interface switching
Interface switches to channel 1
Ch. 2
Packet to B
Ch. 1
buffer packet
Packet to C
Packet to C arrives

54. Asymmetry A B C 2 1 3 2 Fixed (ch 1) Fixed (ch 2) Fixed (ch 3)
Switchable 2 1
Switchable 3 2
Switchable

55. Shortcomings

56. Shortcomings
Scheduling using legacy protocol (802.11), ignorant of multi-channel nature of the system
Ignores interface heterogeneity
All nodes assumed to switch to all available channel
/ abg versus /bg
Does not explicitly handle channel heterogeneity
Channel gains and external interference vary

57. CSL Capacity bounds Insights on protocol design OS improvements
channels
capacity A B C D E F
Fixed
Switchable
Insights on
protocol design
Multi-channel
protocol
Channel Abstraction Module
IP Stack
Interface
Device Driver
User
Applications
ARP
OS improvements
Software architecture
Net-X
testbed
CSL
Linux box

58. Modeling Interface Heterogeneity
Each radio can operate only on a subset of channels

59. Adjacent (c, f) Assignment
Each node assigned a contiguous block of f channels
This model also subsumes untuned radios [Petrovic et al] as special cases
Example: f=2, c=8

60. Random (c, f) Assignment
Each node is assigned a random f-subset of channels
More freedom in choices; no adjacency constraint
Example: f=2, c=8

61. Impact of Constrained Channel Switching
Connectivity
(4, 5)
A node cannot communicate will all “in range” nodes
(2, 3)
(5, 6)
(1, 2)
(1, 3)
(6, 7)
(3, 6)
Bottleneck Formation
(7, 8)
(2, 5)
Some channels may be scarce in certain network regions, leading to overload on some channels/nodes

62. Channel Heterogeneity
Not all channels are made equal
 Distributed scheduling with heterogeneous channels

63. Status Capacity + Scheduling Protocol stack G Channel-binding
Channel Queues
Neighbor Queues
Interface Queues
IF #2
IF #m
Channel-binding
Interface-binding
IF #1
Capacity + Scheduling
Protocol stack

64. Wrap-up CSL Capacity bounds Insights on protocol design
channels
capacity A B C D E F
Fixed
Switchable
Insights on
protocol design
Wrap-up
Multi-channel
protocol
Channel Abstraction Module
IP Stack
Interface
Device Driver
User
Applications
ARP
OS improvements
Software architecture
Net-X
testbed
CSL
Linux box

65. Summary
Significant performance benefits using many 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

66. Research Opportunities
Significant effort in protocol design needed to exploit available physical resources
Examples:
MIMO (multi-antenna)
Cooperative relaying
Dense wireless infrastructure

67. Thanks!