1. MMSN: Multi-Frequency Media Access Control for Wireless Sensor Networks Cheoleun Moon Computer Science Div. at KAIST

2. 2/22 Contents  Motivation  Overhead Analysis  New Protocol Framework Frequency Assignment Media Access Design  Performance Evaluation  Conclusions

3. 3/22 Ad-hoc Wireless Sensor Networks Sensors & Actuators Limited CPU and memorys Limited radio bandwidth Self-organize

4. 4/22 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, … ) 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

5. 5/22 Multi-Channel MAC in MANET  Require more powerful hardware/multiple transceivers Listen to multiple channels simultaneously  Frequent Use of RTS/CTS Controls For frequency negotiation Due to using 802.11

6. 6/22 Basic Problems for WSN  Don’t use multiple transceivers Energy Cost  Packet Size 30 bytes versus 512 bytes in MANET  RTS/CTS Costly overhead

7. 7/22 RTS/CTS Overhead Analysis  RTS/CTS are too heavyweight for WSN: Mainly due to small packet size: 30~50 bytes in WSN vs. 512+ byte s 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. 8/22 Contributions  First multi-frequency MAC, specially designe d for WSN  Developed four frequency assignment sche mes Supports various tradeoffs  New toggle transmission and toggle snoopin g for media access control

9. 9/22 Frequency Assignment Complications - Not enough frequencies - Broadcast F1 F2 F3 F4 F5 F6 F7 F8 Reception Frequency

10. 10/22 Frequency Assignment Schemes 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 #frequ encies  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 communicati on overhead

11. 11/22 Media Access Design (1/4)  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 …...

12. 12/22 Media Access Design (2/4)  Case 1 When a node has no packet to transmit

13. 13/22 Media Access Design (3/4)  Case 2 When a node has a broadcast packet to transmit

14. 14/22 Media Access Design (4/4)  Case 3 When a node has a unicast packet to transmit

15. 15/22 Toggle Snooping  During “back off (f self, f dest )”, toggle snooping is used

16. 16/22 Toggle Transmission  When a node has unicast packet to send transmits a preamble f self so that no node sends to me f dest so that no node sends to destination

17. 17/22 Simulation Configuration ComponentsSetting SimulatorGloMoSim Terrain(200m X 200m) Square Node Number289 (17x17) Node PlacementUniform Payload Size32 Bytes ApplicationMany-to-Many/Gossip CBR Streams Routing LayerGF MAC LayerCSMA/MMSN Radio LayerRADIO-ACCNOISE Radio Bandwidth250Kbps Radio Range20m~45m Confidence IntervalsThe 90% confidence intervals are shown in each figure

18. 18/22 Performance Metrics  Aggregate MAC throughput Total amount of data successfully delivered in MAC per unit time  Packet delivery ratio (Total # of data packets delivered by MAC layer) (Total # of data packet the network layer requests MAC)  Channel access delay Delay data packet from the network layer waits for the channel  Energy consumption

19. 19/22 Performance with Different #Physical Frequencies – With Light Load ① Performance when delivery ratio > 93% ② Scalable performance improvement ③ Overhead observed when #frequency is small ④ More scalable performance with Gossip than many-to-many traffic

20. 20/22 Performance with Different # Physical Frequencies – With Higher Load ① When load is heavy, CSMA has 77% delivery ratio, while MMSN performs much better ② MMSN needs less channels to beat CSMA, when the load is heavier

21. 21/22 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. 22/22 Conclusions  First multi-frequency MAC, specially designed for WSN, where single-transceiver devices are used Explore tradeoffs in frequency assignment Design toggle transmission and toggle snooping MMSN demonstrated scalable performance in simulation