Multi-Channel Wireless Networks: Theory to Practice

1. Multi-Channel Wireless Networks:Theory to Practice Nitin Vaidya Electrical and Computer Engineering University of Illinois at Urbana-Champaign Sept. 8. 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 Performance • Exploit physical resources, diversity • Exploiting diversity requires appropriate protocols

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

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

17. 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

18. Shared Access : Time & Spectrum D B C A One Channel Two Channels A B A C A B Spectrum C C Time Time

19. Why Divide Spectrum into Channels ? • Manageability: • Different networks on different channelsavoids interference between networks • Contention mitigation: • Fewer nodes on a channel reduces channel contention

20. data size/rate fixed time Why Divide Spectrum into Channels ? • Lower transmission rate per channel • Slower hardware (simpler, cheaper) • Reducing impact of bandwidth-independent overhead

21. 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

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

23. Channel Switching • Unconstrained : An interface can tune to any available channel • Constrained : Restricted channel switching

24. This Talk • Assume unconstrained switching • Constrained switching results elsewhere

25. 1 m Multiple Interfaces • Reducing hardware cost allows formultiple interfaces • m interfaces per node

26. 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

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

28. 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

29. 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

30. 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

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

32. Network Model

33. Network Model[Gupta-Kumar] • Random source-destination pairs amongrandomly positioned n node in unit area,with n  ∞

34. Capacity = ? • l = minimum flow throughput • Capacity = n l

35. Capacity Constraints • Capacity constrained by availablespectrum bandwidth • Other factors further constrainwireless network capacity …

36. Connectivity Constraint[Gupta-Kumar] • Need routes between source-destination pairs Places a lower bound on transmit power A A D D Not connected Connected

37. Interference Constraint[Gupta-Kumar] • Interference among simultaneous transmissions • Limits spatial reuse D C > r A r B

38. Capacity[Gupta-Kumar] • c = m capacity a 1 1 m = c c = m Capacity scales linearly with channels

39. 1 m Capacity • What if fewer interfaces ? 1 m m+1 c

40. 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

41. Mutlti-Channel Capacity Order of Channels (c/m)

42. Capacity with n  ∞ Are these results relevant ? • Yield insights on design of good routing and scheduling protocols • Insights relevant in smaller networks too

43. 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

44. Insights from Analysis (1) Channel Assignment • Need to balance load on channels • Local coordination in channel assignment helpful

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

46. Insights from Analysis (3) • Optimal transmission range function ofnumber of channels Intuition: # of interfering nodes ≈ # of channels

47. 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 interface c = 1 , 2 channels

48. 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

49. 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

50. 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 2 1 3 2 Channel assignment locally balanced