Can Theory Meet Practice : The Case of Multi-Channel Wireless Networks

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. 19. 2008

2. 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 Multi-Channel Wireless NetworksAcknowledgements Funded in part by:NSF, 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 power distance B A S 5

6. What Makes Wireless Networks Interesting? Interference management non-trivial power distance 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? Time diversity D C gain Time

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. High interference D B C A D B C A Low interference What Makes Wireless Networks Interesting? Channel diversity 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. 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 802.11 in ISM Band

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

23. Channel 1 Channel c Interfaces & Channels • An interface can only use one channel at a time • Switching between channels may incur delay W cW

24. 1 m Multiple Interfaces • Reducing hardware cost allows formultiple interfaces • m interfaces per node

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

26. Multi-Channel Mesh • How to best utilize multiple channelsin a mesh networkwith limited hardware ? ?

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

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

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

31. Network Model

32. Network Model[Gupta-Kumar] • Random source-destination pairs amongrandomly 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. 1 m Capacity • What if fewer interfaces ? 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. capacity User Applications Multi-channel protocol channels Capacity bounds Insights on protocol design Fixed D IP Stack OS improvements Software architecture Net-X testbed F B ARP E Switchable A Channel Abstraction Module C Interface Device Driver Interface Device Driver CSL OutlineTheory to Practice 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 C 2 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 Fixed (ch 1) Fixed (ch 2) Fixed (ch 3) Switchable Switchable Switchable 2 1 3 2

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 F B B E A A E 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 ofnumber of channels Intuition: # of interfering nodes ≈ # of channels

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

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

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

50. 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