Medium Access Control in Sensor Networks

1. Medium Access Control in Sensor Networks Huaming Li Electrical and Computer Engineering Michigan Technological University

2. Outline • Overview • S-MAC: an energy-efficient MAC protocol for wireless sensor networks • Other MAC Techniques • References Computer Engineering Seminar

3. Medium Access Control in Sensor Networks • Sensor networks • Consist of a set of sensor nodes; • Each node is equipped with one or more sensors and is normally battery operated; • Nodes communicate with each other via wireless connection. • Medium Access Control (MAC) • Fundamental task is to avoid collisions so that two interfering nodes do not transmit at the same time Computer Engineering Seminar

4. Energy efficiency Scalability & Self-configuration Adaptivity Adaptivity Fairness not important Trade for energy Characteristics of Sensor Network • A special wireless ad hoc network • Large number of nodes • Battery powered • Topology and density change • Nodes for a common task • In-network data processing • Sensor-net applications • Sensor-triggered bursty traffic • Can often tolerate some delay • Speed of a moving object places a bound on network reaction time Msg-level Latency Computer Engineering Seminar

5. MAC Protocols Classification • Scheduling-Based MAC Protocols • Contention-Based MAC • Collision Free Real Time MAC • Hybrid MAC Computer Engineering Seminar

6. Scheduling Based MAC • Time is divided into slots • Each node knows when to transmit • Schedule is predetermined • TDMA • Synchronization problems • Adaptability problems Computer Engineering Seminar

7. Contention Based MAC • Carrier sensing & collision avoidance • In-band, out-band handshaking • Busy-tone multiple access (BTMA) • Multiple access with collision avoidance (MACA) • High priority packets Computer Engineering Seminar

8. Common MAC Protocol Requirements • Quality of service (QoS) • Tolerate message loss • Support real time guarantees • Decentralized • Global information may not be available • Flexibility • Diversity of applications Computer Engineering Seminar

9. Primary Secondary MAC Requirements in Sensor Networks • Important requirements of MAC protocols • Collision avoidance • Energy efficiency • Scalability & Adaptivity • Latency • Fairness • Throughput • Bandwidth utilization Computer Engineering Seminar

10. Dominant in sensornetworks Energy Efficiency in MAC Design • Energy is primary concern • What causes energy waste on radio? • Long idle time • Control packet overhead • Overhearing unnecessary traffic • Collisions • bursty traffic in sensor-net apps • Idle listening consumes 50—100% of the power for receiving (Stemm97, Kasten) Computer Engineering Seminar

11. Latency Fairness Energy S-MAC Design Overview • Tradeoffs • Major components in S-MAC • Periodic listen and sleep • Collision avoidance • Overhearing avoidance • Massage passing Computer Engineering Seminar

12. sleep sleep listen listen sleep sleep listen listen Node 1 Node 2 Energy Latency Periodic Listen and Sleep • Reduce long idle time • Reduce duty cycle to ~ 10% (120ms on/1.2s off) • Schedules can differ • Prefer neighboring nodes have same schedule • — easy broadcast & low control overhead Computer Engineering Seminar

13. Periodic Listen & Sleep • Nodes are in idle for a long time if no sensing event happens • Put nodes into periodic sleep mode • i.e. in each second, sleep for half second and listen for other half second Computer Engineering Seminar

14. Coordinated Sleeping • Nodes coordinate on sleep schedules • Nodes periodically broadcast schedules • New node tries to follow an existing schedule Schedule 1 Schedule 2 1 2 • Nodes on border of two schedules follow both • Periodic neighbor discovery • Keep awake in a full sync interval over long time Computer Engineering Seminar

15. Choose & Maintain Schedule • Each node maintains a schedule table that stores schedules of all its neighbors • Nodes exchange schedules by broadcasting them to its neighbors • Try to synchronize neighboring nodes together Computer Engineering Seminar

16. Choose Schedule • If not hear a schedule from others, the node randomly chooses a schedule and broadcast the schedule • If receive a schedule, the node follows that schedule, wait for a random delay then rebroadcast this schedule • If receive a different schedule, the node adopt both, broadcast its own schedule Computer Engineering Seminar

17. Maintain Synchronization • Listen/sleep scheme requires synchronization among neighboring nodes • Looser synchronization (compared to TDMA) • Listen period is significantly longer than clock error or drift • Use relative time rather than absolute • Update schedule by sending SYNC packets Computer Engineering Seminar

18. Maintain Sync (contd.) • Divide listen time into two parts: • For receiving SYNC packets • For receiving data packets • Each part is further divided into many time slots for senders to perform carrier sense Computer Engineering Seminar

19. Maintain Sync (contd.) CS: carrier sense Computer Engineering Seminar

20. Collision Avoidance • Adopt IEEE 802.11 collision avoidance • Virtual carrier sense • During field • Network allocation vector (NAV) • Physical carrier sense • RTS/CTS exchange (for hidden terminal problem) • Broadcast packets (SYNC) are sent without RTS/CTS • Unicast packets (DATA) are sent with RTS/CTS Computer Engineering Seminar

21. 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 Computer Engineering Seminar

22. Example • Who should sleep when node A is transmitting to B? • All immediate neighbors of both sender & receiver should go to sleep Computer Engineering Seminar

23. Message Passing • How to efficiently transmit a long message? • Single packet vs. fragmentations • Single packet: high cost of retransmission if only a few bits have been corrupted • Fragmentations: large control overhead (RTS & CTS for each fragment), longer delay • Problem: Sensor network in-network processing requires entire message Computer Engineering Seminar

24. Energy Msg-level latency Fairness Message Passing • 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 Computer Engineering Seminar

25. Platform Mica Motes (UC Berkeley) 8-bit CPU at 4MHz, 128KB flash, 4KB RAM 20Kbps radio at 433MHz TinyOS:event-driven Implementation on Testbed Nodes • Configurable S-MAC options Low duty cycle with adaptive listen Low duty cycle without adaptive listen Fully active mode (no periodic sleeping) Computer Engineering Seminar

26. Application Transport Routing MAC/Link Physical Implementation on Testbed Nodes • Layered model on Motes • MAC layer: S-MAC • Physical layer • Radio state control, Carrier sense • CRC checking, Channel coding, Byte buffering • Nested headers • Avoid memory • copy across • layers Computer Engineering Seminar

27. Source 1 Sink 1 Sink 2 Source 2 Test Bed • Three test MAC modules • Simplified IEEE 802.11 DCF • Message passing with overhearing avoidance • Complete S-MAC • Topology in experiments Computer Engineering Seminar

28. Average energy consumption in the source nodes 1800 1600 1400 802.11-like protocol without sleep 1200 1000 Energy consumption (mJ) Overhearing avoidance 800 600 400 S-MAC w/o adaptive listen 200 0 2 4 6 8 10 Message inter-arrival period (second) Experiment Result • Average source nodes energy consumption • S-MAC consumes much less energy than 802.11-like protocol w/o sleeping • At heavy load, overhearing avoidance is the major factor in energy savings • At light load, periodic sleeping plays the key role Computer Engineering Seminar

29. Experiment Result (contd.) • Percentage of time source nodes in sleep Computer Engineering Seminar

30. Experiment Result (contd.) • Energy consumption in the intermediate node Computer Engineering Seminar

31. S-MAC Conclusions • Advantages: • Periodically sleep reduces energy consumption in idle listening • Sleep during transmissions of other nodes • Message passing reduces contention latency and control packet overhead • Disadvantages: • Reduction in both per-node fairness & latency Computer Engineering Seminar

32. Other MAC Techniques • Timeout-MAC (T-MAC) • S-MAC has fixed duty cycle and not optimal • Reduce idle listening by transmitting data in bursts • Sleep in between bursts to save power • End the active time in an intuitive way • Timeout on hearing nothing Computer Engineering Seminar

33. T-MAC • Every node periodically wakes up and communicates with its neighbors • A node will keep listening and potentially transmitting, as long as it is in active period • An active period ends when no activation event has occurred for time TA Computer Engineering Seminar

34. Activation event • The firing of periodic timer • The reception of any data on radio • The sensing of communication on the radio • The end of transmission of a node’s own data packet • The knowledge through prior RTS and CTS packets Computer Engineering Seminar

35. T-MAC • A node will sleep if it is not in an active period • TA determines the minimum amount of idle listening per frame • All communication occurs as a burst in the beginning of the frame • Buffer capacity determines the upper bound on the maximum frame time Computer Engineering Seminar

36. Evolution Adaptive Rate Control ARC IEEE 802.3 IEEE 802.11 CSMA/CD CSMA/CA SMAC TMAC Carrier Sense Multiple Access with Collision Detection Carrier Sense Multiple Access with Collision Avoidance Fixed duty cycle Adaptive duty cycle DMAC/ MMAC Directional Antennas Computer Engineering Seminar

37. Performance Analysis of 802.15.4 in WPAN One promising kind of sensor network: Wireless Personal Area Network (WPAN) • Medical sensing and control • Wearable computing • Location awareness and identification • Implanted medical sensors (Focus) • Coronary care • Diabetes • Optical aids • Drug delivery Computer Engineering Seminar

38. Critical Metric : Battery Life Implanted medical sensors (Main concern) • Objective Make Batteries work 10-15 years • Method Ensure that all sensors are powered down or in sleep mode when not in active use • Tradeoff Battery life VS. latency Computer Engineering Seminar

39. Protocol Options Possible options Computer Engineering Seminar

40. 802.15.4 (LR-WPAN) Upper Layers Other LLC IEEE 802.2 LLC IEEE 802.15.4 MAC IEEE 802.15.4 IEEE 802.15.4 868/915 MHz 2400 MHz PHY PHY Physical Medium Computer Engineering Seminar

41. 802.15.4 (LR-WPAN) MAC Layer (prefer star topology) Why star topology here? Computer Engineering Seminar

42. 802.15.4 (LR-WPAN) Why star topology here? • Coordinator is external to the body • PDA, mobile phone or bedside monitor station • Easy to replace of charge batteries • Easy to communicate with other networks • Coordinator defines the start and end of a superframe and is charge of the association and disassociation of the other nodes Computer Engineering Seminar

43. IEEE 802.15.4 superframe structure Computer Engineering Seminar

44. Two Communication methods • Beacon mode • Pros: Coordinator can communicate at will • Cons: Listeners have to keep awake • Non-beacon mode • Pros: Nodes can sleep more • Cons: Communication latency Computer Engineering Seminar

45. Network scenarios and power analysis Sensor power consumption with beacon reception • Problem: The sensor devices within a beacon network have to wake up to receive the beacon from the coordinator (Power consuming) Timebase Tolerances Warm-up time Computer Engineering Seminar

46. Network scenarios and power analysis Data Transfer Mechanisms (Beacon) • Data transfer to a coordinator (upload) Is the upload period Computer Engineering Seminar

47. Network scenarios and power analysis Data Transfer Mechanisms (Beacon) • Data transfer from a coordinator (download) Is the download period Computer Engineering Seminar

48. Results Average Back-off With a small number of sensors that are effectively off most of the time, the probability of a channel being free is greater than 99 %. Therefore, for the relatively small number of sensors used in the WBAN networks explored here, it would be more economical to keep the CSMA/CA switched off. This is to ensure that the automatic initial back-off is avoided. Computer Engineering Seminar

49. Node Lifetime in Beacon Networks Computer Engineering Seminar

50. Node Lifetime in Beacon Networks Computer Engineering Seminar