MAC Layer Protocols for Sensor Networks

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

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. 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. Talk Outline • SMAC: http://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.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.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. Primary Secondary Medium Access Control in Sensor Nets • Important attributes of MAC protocols • Collision avoidance • Energy efficiency • Scalability in node density • Latency • Fairness • Throughput • Bandwidth utilization

6. Dominant in sensornets Common to all wireless networks Energy Efficiency in MAC • 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

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

8. sleep listen listen sleep Energy Latency 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)

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

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. 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. 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. Energy Msg-level latency Fairness 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

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

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

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

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

18. Talk Outline • SMAC: http://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.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.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. BMAC Objectives • Information sharing with higher layers • Control and reconfiguration of link protocol • Tradeoffs in link protocols

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. 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. wakeup wakeup wakeup wakeup wakeup wakeup wakeup wakeup wakeup 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 TX sleep sleep sleep Node 1 time RX sleep sleep sleep Node 2 time

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. 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) topology

25. A E C B D 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 relieson controlling link layer primitives 10 packets every 10 seconds 10 packets every 100 seconds

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

27. Talk Outline • SMAC: http://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.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.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. Existing MAC Layer Approaches • Synchronized Solutions • SMAC, TMAC, DMAC • Unsynchronized Solutions • BMAC, GeRaF

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. Unsynchronized Approaches - BMAC • Long Preamble Approach • Wasteful if the receiver wakes up early Sleep Long Preamble Packet Sender Sleep Receiving Preamble Packet Receiver

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

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

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 RTS RTS RTS (a) (b) Channel check Channel check RTS RTS (c) (shouldn’t happen) Channel check

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. 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. mini-slot CTS slot RTS Canceled RTS Sender CTS Node in R1 Node in R1 Canceled CTS Canceled CTS Node in R2 Canceled CTS Node in R3 Contention Among Anycast Receivers • Anycast candidate prioritization

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

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