A Wireless Mesh Network Based on Multichannel Ad Hoc Connections

1. A Wireless Mesh Network Based on Multichannel Ad Hoc Connections Jingyi He Gary Chan HKUST Oct. 22, 2003

2. Outline • Introduction • System Architecture • Channel Requirement • Illustrative System Performance • Conclusions

3. Current Wireless LAN Deployment • Attached to the wired network • Operates in the IEEE 802.11 Infrastructure Mode

4. IEEE 802.11 Operational Modes • Infrastructure Mode (WLAN) • Ad Hoc Mode (MANET)

5. Limitations of Current WLAN Deployment • Depending on the existence of the wired infrastructure • Limited coverage: only one hop away from the wired network (~ 250m)

6. To Extend WLAN Coverage – Wireless Mesh Network

7. Next … • Introduction • System Architecture • Channel Requirement • Illustrative System Performance • Conclusions

8. Major Building Blocks • The Air Interface • PHY layer • MAC layer • The Routing Strategy (the network layer)

9. PHY Layer - Multichannel Operation • All the WLANs can use the same channel with good planning • Communications between APs use different channels than that used by the WLANs  at least two wireless interfaces are needed at each AP • Multiple channels can be used in the mesh network

10. Transmission Range (TR) and Interference Range (IR) • TR: the maximum distance between two mobile nodes which can communicate with each other. • IR: the maximum distance between two mobile nodes which disable each other from being active (transmitting or receiving) simultaneously.

11. TR and IR – cont’d • Typical values: • TR: 250m • IR: 550m • For mesh communication: larger values TR IR

12. Power Control • Since the APs are stationary, power control is easier than in ad hoc networks • Power control can be used to control the connectivity of the mesh network • Power control can be used to reduce the interferences and hence channel requirement for a given connectivity

13. MAC Layer - Options • IEEE 802.11 • CSMA/CA • IEEE 802.16a Mesh Mode • Scheduling Based (No collision) • Other names of IEEE 802.16: • WirelessMAN • Broadband Wireless Access (BWA) system

14. Our choice – 802.11 • Under the same framework of 802.11, adding the ad hoc functionality to the APs would require minor modifications • 802.11 AP has been a mature market product with wide-spread deployment, while 802.16 hasn’t seen the same commercial success.

15. Network Layer - Routing in the Wireless Mesh Network • Fixed routing is not desirable as channel conditions could vary drastically even between two fixed points (as observed from MIT Roofnet testbed) • Ad hoc routing protocols can be used, with reduced control overhead (e.g., less frequent routing information exchange)

16. Modifications needed for Multichannel Operation • A wireless interface field should be added to each routing table entry • Broadcast packets (e.g., routing information updates) should be sent to all interfaces • A mechanism which handles multiple channels between two APs is needed

17. Next … • Introduction • System Architecture • Channel Requirement • Illustrative System Performance • Conclusions

18. Connectivity Patterns of WMesh • All links active (ALA) • Spanning tree (ST) • Spanning tree with power control (ST-Pctrl)

19. All Links Active (ALA) 8 channels

20. Spanning Tree (ST) 5 channels

21. Spanning Tree with Power Control (ST-Pctrl) 4 channels

22. Determining the Channel Requirement • Given: the connectivity of the wireless mesh network • Approaches: • Step 1: Graph tranformation • Step 2: Graph coloring 2 4 2 5 1 3 1 3 5 4

23. Topologies Considered • Random • Grid a a

24. Number of APs Needed • To set up a connected mesh of APs for the square region of width a (m)

25. Number of Channels Needed - Random

26. Number of Channels Needed - Grid

27. Next … • Introduction • System Architecture • Channel Requirement • Illustrative System Performance • Conclusions

28. Performance Metrics • Throughput • Overall • Per-channel • Fairness among the connections • Jain’s Fairness Index • f (0, 1], with f =1 meaning perfect fairness

29. Simulation Setup • Simulator: ns-2 (2.1b9a) • Mesh topology: • 16 APs in grid manner with a=1250m • ST-Pctrl connectivity 2 9 1 2 6 7 8 3 3 9 4 5 1 2 6

30. Simulation Setup – cont’d • Traffic pattern • Skewed: a small number of APs (attached to the infrastructure) are the destinations of the connections from other infrastructureless APs • Routing Protocol • Modified AODV

31. Single infrastructure AP 2 9 1 2 6 7 8 3 3 9 4 5 1 2 6

32. Throughput ~ UDP traffic load • Overall • Per-channel

33. Packet loss rate ~ traffic load

34. System throughput ~ packet loss rate

35. Fairness index ~ traffic load

36. Unfairness at high loads

37. Multiple infrastructure APs • 1 channel

38. System throughput ~ # of Infra-APs

39. Fairness index ~ # of Infra-APs

40. Multiple infrastructure APs • 9 channels

41. System throughput ~ # of Infra-APs

42. Fairness index ~ # of Infra-APs

43. Next … • Introduction • System Architecture • Channel Requirement • Illustrative System Performance • Conclusions

44. Conclusions • A framework of the proposed multichannel wireless mesh network has been presented • Channel requirement in such networks has been studied. Results show that a sparse connectivity can reduce the channel requirement • System performance has been studied. Results show that multichannel network achieves higher throughput and better fairness than single-channel ad hoc network. However, with sparse connectivity, the channel efficiency is not as high.

45. Future Work • Better connectivity and channel assignment, to improve the channel efficiency • Infrastructure node placement