MMSN: Multi-Frequency Media Access Control for Wireless Sensor Networks

1. MMSN: Multi-Frequency Media Access Control for Wireless Sensor Networks Gang Zhou, Chengdu Huang, Ting Yan, Tian He John. A. Stankovic, Tarek F. Abdelzaher Department of Computer Science University of Virginia

2. Outline • Motivation • State of the Art • Overhead Analysis • Contribution – New Protocol Framework • Frequency Assignment • Media Access Design • Performance Evaluation • Conclusions

3. Ad Hoc Wireless Sensor Networks • Sensors • Actuators • CPUs/Memory • Radio • Minimal capacity Self-organize

4. Multi-channel design needed Hardware appearing Software still lags behind • Multi-channel support in MICAz/Telos • More frequencies available in the future • Collision-based: B-MAC • Scheduling-based: TRAMA • Hybrid: Z-MAC Motivation • 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, …)

5. 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]

6. 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

7. RTS/CTS Overhead Analysis • RTS/CTS 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 MMAC: • RTS/CTS frequency negotiation • 802.11 for data communication

8. Contributions • A new multi-frequency MAC, specially designed forWSN; • Single half-duplex radio transceiver; • Small packets sizes; • Developed four frequency assignment schemes • Supports various tradeoffs • Toggle transmission and toggle snooping techniques for media access control; • An optimal non-uniform backoff algorithm, and a lightweight approximation;

9. Frequency Assignment Reception Frequency F8 F7 • Complications • Not enough frequencies • Broadcast F6 F5 F1 F4 F2 F3

10. Frequency Assignment

11. Media Access Design • Issues: • Packet to Broadcast • Receive Broadcast • Send Unicast • Receive Unicast • No sending/no receiving F8 F7 F6 F5 F1 F4 F2 F3

12. … ... T T T T c b tran b c tran 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.

13. Media Access Design Case 1: When a node has no packet to transmit

14. Media Access Design Case 2: When a node has a broadcast packet to transmit

15. Media Access Design Case 3: When a node has a unicast packet to transmit

16. Toggle Snooping • During “ “, toggle snooping is used

17. Toggle Transmission • When a node has unicast packet to send • Transmits a preamble • so that no node sends to me • so that no node sends to destination • We let

18. Simulation Configuration

19. Performance with Different #Physical Frequencies- With Light Load • Performance when delivery ratio > 93% • Scalable performance improvement • Overhead observed when #frequency is small • More scalable performance with Gossip than many-to-many traffic

20. Performance with Different #Physical Frequencies– With Higher Load • When load is heavy, CSMA has 77% delivery ratio, while MMSN performs much better • MMSN needs less channels to beat CSMA, when the load is heavier

21. Performance with Different System Load Observation: CSMA has a sharp decrease of packet delivery ratio, while MMSN does not. Reason: The non-uniform backoff in time-slotted MMSN is tolerant to system load variation, while the uniform backoff in CSMA is not.

22. Conclusions • First multi-frequency MAC,speciallydesigned for WSN, where single-transceiver devices are used • Explore tradeoffs in frequency assignment • Design toggle transmission and toggle snooping • Theoretical analysis of an non-uniform back-off algorithm • MMSN demonstrated scalable performance in simulation

23. Thanks to anonymous reviewers for their valuable comments! The End!

24. Performance with Different Node Densities

25. Backup Slides: Optimal Non-Uniform Backoff

26. Even Selection Frequency Assignment • Beacon (multiple times) to collect nodes’ IDs within two hops • Frequency decision is made sequentially in the increasing order of nodes’ IDs • When making a decision, randomly choose one of the least chosen frequencies (once no unique ones left) • Notify neighbors of decision • NOTE: Frequency assignment happens once (or a few times)

27. Back Off Period - Slotted Backoff into a slot Transmit at end of a slot

28. Let 34 slices of length TTS; 68 nodes compete for the channel --- a timer fires Uniform backoff Non-uniform backoff Non-Uniform Backoff: Motivation & an Optimal Solution • An optimal distribution is presented in the paper • Uses recursive computation • Distribution depends on node density • A simple approximation is needed

29. implementation Non-uniform Backoff: A Simple Approximation