1. 1 MAC Layer Protocols for Sensor Networks Prasun Sinha Department of Computer Science and Engineering Ohio State University April 25 th, 2007 (some slides adapted from authors presentations found on the Internet)

2. 2 Introduction Wireless sensor network  Special ad hoc wireless network  Large number of nodes w/ sensors & actuators  Battery-powered nodes energy efficiency  Unplanned deployment self-organization  Node density & topology change robustness  Sensor-net applications Nodes cooperate for a common task In-network data processing

3. 3 Some Applications of Sensor Networks Data Collection Networks  Sensing Movement of Glaciers  Environment Monitoring  Habitat Monitoring Habitat Monitoring of Storm Petrels in Great Duck Island  Microsoft’s Effort to put every sensor on the web Event Triggered Networks  Structural Monitoring Golden Gate Bridge  Precision Agriculture Oregon and British Columbia Vineyards  Condition based Maintenance Hardware Manufacturing facilities  Military Applications  Environment Monitoring Poisonous gas, pollutants etc.  National Asset Protection Coastline, Border Patrol, Roadways, Oil/gas pipelines, Secure facilities

4. 4 Talk Outline SMAC: http://www.isi.edu/~weiye/pub/smac_ton.pdfhttp://www.isi.edu/~weiye/pub/smac_ton.pdf  “Medium Access Control With Coordinated Adaptive Sleeping for Wireless Sensor Networks”, Wei Ye, John Heidemann, and Deborah Estrin, Transactions on Networking, 2004, (also Infocom 2002) BMAC: http://www.polastre.com/papers/sensys04-bmac.pdfhttp://www.polastre.com/papers/sensys04-bmac.pdf  “Versatile Low Power Media Access for Wireless Sensor Networks”, Joseph Polastre, Jason Hill and David Culler, ACM SENSYS 2004 CMAC: http://www.cse.ohio-state.edu/~prasun/publications/conf/secon07-cmac.pdfhttp://www.cse.ohio-state.edu/~prasun/publications/conf/secon07-cmac.pdf  “CMAC: An Energy Efficient MAC Layer Protocol Using Convergent Packet Forwarding for Wireless Sensor Networks”, Sha Liu, Kai-Wei Fan and Prasun Sinha, IEEE SECON 2007

5. 5 Medium Access Control in Sensor Nets Important attributes of MAC protocols 1. Collision avoidance 2. Energy efficiency 3. Scalability in node density 4. Latency 5. Fairness 6. Throughput 7. Bandwidth utilization Primary Secondary

6. 6 Major sources of energy waste (cont.)  Idle listening Long idle time when no sensing event happens Collisions Control overhead Overhearing  We try to reduce energy consumption from all above sources  Combine benefits of TDMA + contention protocols Energy Efficiency in MAC Common to all wireless networks Dominant in sensor nets

7. 7 Sensor-MAC (S-MAC) Design Tradeoffs  Major components in S-MAC Periodic listen and sleep Collision avoidance Overhearing avoidance Massage passing Latency Fairness Energy

8. 8 Periodic Listen and Sleep Problem: Idle listening consumes significant energy Solution: Periodic listen and sleep Turn off radio when sleeping Reduce duty cycle to ~ 10% (200ms on/2s off) sleep listen sleep Latency Energy

9. 9 Periodic Listen and Sleep Schedules can differ Prefer neighboring nodes have same schedule — easy broadcast & low control overhead Border nodes: two schedules broadcast twice Node 1 Node 2 sleep listen sleep listen sleep Schedule 2 Schedule 1

10. 10 Periodic Listen and Sleep Schedule Synchronization  Remember neighbors’ schedules — to know when to send to them  Each node broadcasts its schedule every few periods of sleeping and listening  Re-sync when receiving a schedule update  Schedule packets also serve as beacons for new nodes to join a neighborhood

11. 11 Collision Avoidance Problem: Multiple senders want to talk Options: Contention vs. TDMA Solution: Similar to IEEE 802.11 ad hoc mode (DCF)  Physical and virtual carrier sense  Randomized backoff time  RTS/CTS for hidden terminal problem  RTS/CTS/DATA/ACK sequence

12. 12 Overhearing Avoidance Problem: Receive packets destined to others Solution: Sleep when neighbors talk  Basic idea from PAMAS (Singh, Raghavendra 1998)  But we only use in-channel signaling Who should sleep? All immediate neighbors of sender and receiver  How long to sleep? The duration field in each packet informs other nodes the sleep interval

13. 13 Message Passing Problem: Sensor net in-network processing requires entire message Solution: Don’t interleave different messages  Long message is fragmented & sent in burst  RTS/CTS reserve medium for entire message  Fragment-level error recovery — ACK — extend Tx time and re-transmit immediately Other nodes sleep for whole message time Fairness Energy Msg-level latency

14. 14 Msg Passing vs. 802.11 fragmentation S-MAC message passing RTS 21... Data 19 ACK 18 CTS 20 Data 17 ACK 16 Data 1 ACK 0 RTS 3... Data 3 ACK 2 CTS 2 Data 3 ACK 2 Data 1 ACK 0  Fragmentation in IEEE 802.11 No indication of entire time — other nodes keep listening If ACK is not received, give up Tx — fairness

15. 15 Implementation on Testbed Nodes  Platform Motes (UC Berkeley) 8-bit CPU at 4MHz, 8KB flash, 512B RAM 916MHz radio TinyOS: event-driven  Compared MAC modules 1.IEEE 802.11-like protocol w/o sleeping 2.Message passing with overhearing avoidance 3.S-MAC (2 + periodic listen/sleep)

16. 16 Experiments Topology and measured energy consumption on source nodes Source 1 Source 2 Sink 1 Sink 2 Each source node sends 10 messages — Each message has 400B in 10 fragments Measure total energy over time to send all messages

17. 17 S-MAC Conclusions S-MAC offers significant energy efficiency over always-listening MAC protocols S-MAC can function at 10% duty cycle

18. 18 Talk Outline SMAC: http://www.isi.edu/~weiye/pub/smac_ton.pdfhttp://www.isi.edu/~weiye/pub/smac_ton.pdf  “Medium Access Control With Coordinated Adaptive Sleeping for Wireless Sensor Networks”, Wei Ye, John Heidemann, and Deborah Estrin, Transactions on Networking, 2004, (also Infocom 2002) BMAC: http://www.polastre.com/papers/sensys04-bmac.pdfhttp://www.polastre.com/papers/sensys04-bmac.pdf  “Versatile Low Power Media Access for Wireless Sensor Networks”, Joseph Polastre, Jason Hill and David Culler, ACM SENSYS 2004 CMAC: http://www.cse.ohio-state.edu/~prasun/publications/conf/secon07-cmac.pdfhttp://www.cse.ohio-state.edu/~prasun/publications/conf/secon07-cmac.pdf  “CMAC: An Energy Efficient MAC Layer Protocol Using Convergent Packet Forwarding for Wireless Sensor Networks”, Sha Liu, Kai-Wei Fan and Prasun Sinha, IEEE SECON 2007

19. 19 BMAC Objectives Information sharing with higher layers Control and reconfiguration of link protocol Tradeoffs in link protocols

20. 20 B-MAC Design Principles  Reconfigurable MAC protocol  Flexible control  Hooks for sub-primitives Backoff/Timeouts Duty Cycle Acknowledgements  Feedback to higher protocols  Minimal implementation  Minimal state Primary Goals  Low Power Operation  Effective Collision Avoidance  Simple/Predicable Operation  Small Code Size  Tolerant to Changing RF/Networking Conditions  Scalable to Large Number of Nodes Implementation is on Mica2 motes with CC1000

21. 21 B-MAC Link Protocol Interaction Reconfiguration and control of link layer protocol parameters  Acknowledgements, Backoff/Timeouts, Power Management, Ability to choose tradeoffs – “knobs”  Fairness, Latency, Energy Consumption, Reliability Power consumption estimation through analytical and empirical models  Feedback to network protocols  Lifetime estimation Mechanisms to achieve network protocols’ goals

22. 22 Low Power Listening (LPL) Higher level communication scheduling  Energy Cost = RX + TX + Listen  Start by minimizing the listen cost Example of a typical low level protocol mechanism Periodically  wake up, sample channel, sleep Properties  Wakeup time fixed  “Check Time” between wakeups variable  Preamble length matches wakeup interval Overhear all data packets in cell  Duty cycle depends on number of neighbors and cell traffic RX wakeup TX sleep Node 2 Node 1 time

23. 23 Effect of Neighborhood Size Neighborhood Size affects amount of traffic in a cell  Network protocols typically keep track of neighborhood size  Bigger Neighborhood  More traffic

24. 24 B-MAC Performance Experimental Setup:  n nodes send as quickly as possible to saturate the channel B-MAC never worse than traditional approach  Often much better Flexible configuration yields efficient:  Reliable transport (Acks)  Hidden Terminal support (RTS-CTS) ProtocolROMRAM B-MAC3046166 B-MAC w/ ACK3340168 B-MAC w/ Duty Cycling4092170 B-MAC w/ DC & ACK4386172 S-MAC6274516 topology

25. 25 Fragmentation Support S-MAC  RTS-CTS Fragmentation Support B-MAC w/app control  Network protocol sends initial data packet with number of fragments pending  Disable backoff & LPL for rest of fragments Measure energy consumption at C (bottleneck node) Minimizing power relies on controlling link layer primitives A B C E D 10 packets every 10 seconds 10 packets every 100 seconds

26. 26 BMAC Conclusions Coordination with higher protocols is essential for long lived operation Feedback allows protocols to make informed decisions

27. 27 Talk Outline SMAC: http://www.isi.edu/~weiye/pub/smac_ton.pdfhttp://www.isi.edu/~weiye/pub/smac_ton.pdf  “Medium Access Control With Coordinated Adaptive Sleeping for Wireless Sensor Networks”, Wei Ye, John Heidemann, and Deborah Estrin, Transactions on Networking, 2004, (also Infocom 2002) BMAC: http://www.polastre.com/papers/sensys04-bmac.pdfhttp://www.polastre.com/papers/sensys04-bmac.pdf  “Versatile Low Power Media Access for Wireless Sensor Networks”, Joseph Polastre, Jason Hill and David Culler, ACM SENSYS 2004 CMAC: http://www.cse.ohio-state.edu/~prasun/publications/conf/secon07- cmac.pdfhttp://www.cse.ohio-state.edu/~prasun/publications/conf/secon07- cmac.pdf  “CMAC: An Energy Efficient MAC Layer Protocol Using Convergent Packet Forwarding for Wireless Sensor Networks”, Sha Liu, Kai-Wei Fan and Prasun Sinha, IEEE SECON 2007

28. 28 Existing MAC Layer Approaches Synchronized Solutions  SMAC, TMAC, DMAC Unsynchronized Solutions  BMAC, GeRaF

29. 29 Synchronized Approaches Unnecessary power consumption on synchronization message exchanges  Can be improved if synchronization is infrequent Can not achieve very low duty cycles  10% level

30. 30 Unsynchronized Approaches - BMAC Long Preamble Approach Wasteful if the receiver wakes up early Sender Receiver SleepLong Preamble SleepReceiving Preamble Packet

31. 31 Our Approach - CMAC Unsynchronized Duty Cycling Flow Initialization  Aggressive RTS  Anycasting for Packet Forwarding Flow Stabilization  Convergent Packet Forwarding

32. 32 CMAC: Aggressive RTS Aggressive RTS Sender Receiver SleepRTS SleepRX Packet Sleep RTS RX CTS

33. 33 CMAC: Aggressive RTS (Double Channel Check) The receiver only needs to check if the channel is busy after waking up Check the channel twice to avoid missing activities Time between the two checks  Larger than inter-RTS separation  Smaller than RTS duration RTS Channel check RTS Channel check RTS Channel check (a)(b) (c) (shouldn’t happen)

34. 34 CMAC: Anycasting Anycast Packet Forwarding  Exploits network density Nodes other than the target receiver may  wake up earlier  can make some progress toward the sink

35. 35 Contention Among Anycast Receivers Anycast to nodes which are  awake  closer to the destination More than one potential participants  Nodes closer to the sink send CTS’s earlier

36. 36 Contention Among Anycast Receivers Anycast candidate prioritization Canceled RTS CTS RTS Sender CTS slot Canceled CTS mini-slot Node in R 1 Node in R 2 Node in R 3 Canceled CTS

37. 37 CMAC: Convergent Forwarding Anycast has higher overhead than unicast Nodes stay awake for a short duration after receiving a packet  For how long? Switch from anycast to unicast if  Node is able to communicate with a node in R1  Cannot find a better next hop than current one

38. 38 Active nodes Sleeping nodes Unicast links Anycast links Time 1Time 2Time 3 CMAC: Convergent Forwarding Illustration

39. 39 Experiments Testbed: Kansei Testbed  7 x 15 XSM nodes Metrics  Normalized Energy Consumption Average energy consumption to deliver one packet  Throughput: Number of packets received by sink  Latency Scenarios:  Static Event  Moving Event

40. 40 Experimental Results: Static Scenario Sink is at one corner of the network The node that is diagonally opposite to sink sends data to the sink Vary data rates

41. 41 Experimental Results: Moving Event One node generates data at any point for the sink The node generating data (event) moves along one side of the network that does not include the sink. Vary moving speeds

42. 42 CMAC Conclusion CMAC supports high throughput, low latency and consumes less energy than existing solutions. CMAC’s performance difference from existing approaches increases with speed of the moving event.

43. 43 Thanks for your attention! For more information on my research please check my webpage at http://www.cse.ohio-state.edu/~prasun