1. 1 Crowded Spectrum in Wireless Sensor Networks Gang Zhou, John A. Stankovic, Sang H. Son Department of Computer Science University of Virginia May, 2006

2. University of Virginia 2/16 Spectrum Crisis – Single Network AA… Find objects; The position is (x,y); Persons with guns; What are they talking about? (Audio) What are they doing? (Video) Need Higher Throughput!

3. University of Virginia 3/16 Spectrum Crisis – Co-existing Health Care WSN Security WSN Other Devices Co-existing WSNs & Electric Appliances Need Frequency Management

4. University of Virginia 4/16 Outline The Spectrum Crisis Initial solutions in three dimensions  Single Network Throughput  Cooperative Networks  Non-cooperative Networks and Electric Appliances Open Challenges Summary

5. University of Virginia 5/16 Single Network Throughput Limited single-channel bandwidth in WSN  19.2kbps in MICA2, 250kbps in MICAz/Telos The bandwidth requirement is increasing  Support audio/video streams (assisted living, … ) Multi-channel design needed Hardware appearing Multi-channel support in MICAz/Telos More frequencies available in the future Collision-based: B-MAC Scheduling-based: TRAMA Hybrid: Z-MAC Software still lags behind

6. University of Virginia 6/16 State of the Art: Multi-Channel MAC in MANET  Require more powerful hardware/multiple transceivers  Listen to multiple channels simultaneously [Nasipuri 1999], [Wu 2000], [Nasipuri 2000], [Caccaco 2002]  Frequent Use of RTS/CTS Controls  For frequency negotiation  Due to using 802.11 Examples: [Jain 2001], [Tzamaloukas 2001], [Fitzek 2003], [Li 2003], [Bahl 2004], [So 2004], [Adya 2004], [Raniwala 2005]

7. University of Virginia 7/16 Basic Problems for WSN Don ’ t use multiple transceivers  Cost  Form factor Packet Size  30 bytes versus 512 bytes (or larger) in MANET RTS/CTS  Costly overhead

8. University of Virginia 8/16 RTS/CTS Overhead Analysis [Zhou INFOCOM ’ 06] MMAC: RTS/CTS frequency negotiation 802.11 for data communication RTS/CTS Controls are too heavyweight for WSN:  Mainly due to small packet size: 30~50 bytes in WSN vs. 512+ bytes in MANET  From 802.11: RTS-CTS-DATA-ACK  From frequency negotiation: case study with MMAC

9. University of Virginia 9/16 Design Consideration - Frequency Assignment F1 F2 F3 F4 F5 F6 F7 F8 Reception Frequency Complications Not enough frequencies Broadcast

10. University of Virginia 10/16 Design Consideration - Media Access F1 F2 F3 F4 F5 F6 F7 F8 Issues: Packet to Broadcast Receive Broadcast Send Unicast Receive Unicast No sending/no receiving See [Zhou,INFOCOM’06] for our solution

11. University of Virginia 11/16 Co-existing & Cooperative Networks The Challenges:  QoS Control Different priorities for different networks, different bandwidths Map to frequency decision  Space-Dimension Flexibility Frequency decision depends on node density & network density  Time-Dimension Flexibility Dynamic frequency adjustment See [Zhou, EmNets’06] for our solution

12. University of Virginia 12/16 Non-cooperative Networks and Devices IEEE Standards in 2.4GHz ISM Band 2.4 GHz Electronic Devices & Electric Appliances 802.11 (1997) 78 channels (1 MHz Distance) 802.11 b 14 channels (5 MHz Distance) 802.15.4 16 channels (5 MHz Distance) 802.15.1 (Bluetooth) 79 channels (1 MHz Distance)

13. University of Virginia 13/16 When MICAz operates on 2.45 GHz, 46%~81% PRR When MICAz operates on 2.42 GHz, PRR not impacted by presenter Measurement with Spectrum Analyzer

14. University of Virginia 14/16 Deal With the Crowded Spectrum New challenges:  Interference from a different radio Measurement & metrics  Interference from electric appliances Measurement & metrics  Incorporate these into: Static frequency assignment Dynamic frequency adjustment Media access

15. University of Virginia 15/16 More Open Challenges What is/are the best place/places to provide spectrum management in WSN communication stack? More unlicensed frequencies from the FCC? Tradeoff between #channels and bandwidth: static/dynamic? More sophisticated radio hardware? Take advantage of partially-overlapping channels? A service between MAC and PHY, supporting existing single-channel minded MACs?

16. University of Virginia 16/16 Summary Present a vision of crowded WSNs & the spectrum crisis Initial efforts in three complementary dimensions  Single WSN  Cooperative WSNs  Non-cooperative WSNs

17. University of Virginia 17/16 Backup Slides

18. University of Virginia 18/16 Frequency Assignment When #frequencies >= #nodes within two hops When #frequencies < #nodes within two hops Exclusive Frequency Assignment Implicit-ConsensusEven SelectionEavesdropping Both guarantee that nodes within two hops get different frequencies The left scheme needs smaller #frequencies The right one has less communication overhead Balance available frequencies within two hops The left scheme has fewer potential conflicts The right one has less communication overhead

19. University of Virginia 19 Media Access Design Different frequencies for unicast reception The same frequency for broadcast reception Time is divided into slots, each of which consists of a broadcast contention period and a transmission period. T b c T tran T b c T …...

20. University of Virginia 20 Media Access Design Case 1: When a node has no packet to transmit

21. University of Virginia 21 Media Access Design Case 2: When a node has a broadcast packet to transmit

22. University of Virginia 22 Media Access Design Case 3: When a node has a unicast packet to transmit

23. University of Virginia 23 Toggle Snooping During “ “, toggle snooping is used

24. University of Virginia 24 Toggle Transmission  When a node has unicast packet to send  Transmits a preamble  so that no node sends to me  so that no node compete for the same channel  We let

25. University of Virginia 25 Co-existing & Cooperative Networks The Challenges:  QoS Control  Space-Dimension Flexibility Node density & network density  Time-Dimension Flexibility More dynamics

26. University of Virginia 26 Co-existing & Cooperative Networks The Solutions:  Static frequency assignment Collect (ID, gID, ) from (two-hop) neighbors Chained frequency decision: (increasing gID & ID)  The candidate frequency range  Randomly choose one of the least chosen frequencies from the range  Dynamic frequency adjustment Reassign nodes from crowded frequencies to light ones Avoid pushing around “hot potatoes”