2. 1 Medium Access Control in Sensor Networks Huaming Li Electrical and Computer Engineering Michigan Technological University

3. Department of ECE Computer Engineering Seminar Outline Overview S-MAC: an energy-efficient MAC protocol for wireless sensor networks Other MAC Techniques References

4. Department of ECE Computer Engineering Seminar 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

5. Department of ECE Computer Engineering Seminar 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 Energy efficiency Scalability & Self-configuration Fairness not important Msg-level Latency Trade for energy Adaptivity

6. Department of ECE Computer Engineering Seminar MAC Protocols Classification Scheduling-Based MAC Protocols Contention-Based MAC Collision Free Real Time MAC Hybrid MAC

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

8. Department of ECE Computer Engineering Seminar 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

9. Department of ECE Computer Engineering Seminar 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

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

11. Department of ECE Computer Engineering Seminar 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) Dominant in sensor networks

12. Department of ECE Computer Engineering Seminar S-MAC Design Overview Tradeoffs  Major components in S-MAC Periodic listen and sleep Collision avoidance Overhearing avoidance Massage passing Latency Fairness Energy

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

14. Department of ECE Computer Engineering Seminar 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

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

16. Department of ECE Computer Engineering Seminar 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

17. Department of ECE Computer Engineering Seminar 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

18. Department of ECE Computer Engineering Seminar 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

19. Department of ECE Computer Engineering Seminar 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

20. Department of ECE Computer Engineering Seminar Maintain Sync (contd.) CS: carrier sense

21. Department of ECE Computer Engineering Seminar 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

22. Department of ECE Computer Engineering Seminar 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

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

24. Department of ECE Computer Engineering Seminar 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

25. Department of ECE Computer Engineering Seminar 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 Fairness Energy Msg-level latency

26. Department of ECE Computer Engineering Seminar 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) Platform Mica Motes (UC Berkeley) 8-bit CPU at 4MHz, 128KB flash, 4KB RAM 20Kbps radio at 433MHz TinyOS: event-driven

27. Department of ECE Computer Engineering Seminar 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 Application Transport Routing MAC/Link Physical

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

29. Department of ECE Computer Engineering Seminar 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

30. Department of ECE Computer Engineering Seminar Experiment Result (contd.) Percentage of time source nodes in sleep

31. Department of ECE Computer Engineering Seminar Experiment Result (contd.) Energy consumption in the intermediate node

32. Department of ECE Computer Engineering Seminar 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

33. Department of ECE Computer Engineering Seminar 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

34. Department of ECE Computer Engineering Seminar 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

35. Department of ECE Computer Engineering Seminar 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

36. Department of ECE Computer Engineering Seminar 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

37. Department of ECE Computer Engineering Seminar Evolution CSMA/CDCSMA/CASMACTMAC IEEE 802.11 Carrier Sense Multiple Access with Collision Detection IEEE 802.3 Carrier Sense Multiple Access with Collision Avoidance Fixed duty cycle Adaptive duty cycle ARC Adaptive Rate Control DMAC/ MMAC Directional Antennas

38. Department of ECE Computer Engineering Seminar 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

39. Department of ECE Computer Engineering Seminar 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

40. Department of ECE Computer Engineering Seminar Protocol Options Market Name Standard GPRS/GSM 1xRTT/CDMA Wi-Fi ™ 802.11b Bluetooth ™ 802.15.1 ZigBee ™ 802.15.4 Application Focus Wide Area Voice & Data Web, Email, Video Cable Replacement Monitoring & Control System Resources 16MB+1MB+250KB+4KB - 32KB Battery Life (days) 1-7.5 - 51 - 7100 - 1,000+ Network Size1327255 / 65,000 Bandwidth (KB/s)64 - 128+11,000+72020 - 250 Transmission Range (meters) 1,000+1 - 1001 - 10+1 - 100+ Success Metrics Reach, Quality Speed, Flexibility Cost, Convenience Reliability, Power, Cost Possible options

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

42. Department of ECE Computer Engineering Seminar 802.15.4 (LR-WPAN) MAC Layer (prefer star topology) Why star topology here?

43. Department of ECE Computer Engineering Seminar 802.15.4 (LR-WPAN)  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 Why star topology here?

44. Department of ECE Computer Engineering Seminar IEEE 802.15.4 superframe structure

45. Department of ECE Computer Engineering Seminar 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

46. Department of ECE Computer Engineering Seminar 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

47. Department of ECE Computer Engineering Seminar Network scenarios and power analysis Data Transfer Mechanisms (Beacon)  Data transfer to a coordinator (upload) Is the upload period

48. Department of ECE Computer Engineering Seminar Network scenarios and power analysis Data Transfer Mechanisms (Beacon)  Data transfer from a coordinator (download) Is the download period

49. Department of ECE Computer Engineering SeminarResultsResults 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.

50. Department of ECE Computer Engineering Seminar Node Lifetime in Beacon Networks

51. Department of ECE Computer Engineering Seminar Node Lifetime in Beacon Networks

52. Department of ECE Computer Engineering Seminar Node Lifetime in Beacon Networks  15-year lifetime may only be obtained for very low upload rates.  It is under very limited data rate conditions and a tight tolerance crystal, which typically must be better than 25 ppm.

53. Department of ECE Computer Engineering Seminar GTS Option The main drawback of using GTS is that the receiver in the sensor remains on for the duration of the timeslot regardless of the size of the data packet.

54. Department of ECE Computer Engineering Seminar Non-Beacon Networks

55. Department of ECE Computer Engineering Seminar Non-Beacon Networks

56. Department of ECE Computer Engineering Seminar ConclusionConclusion  As a solution to the challenge of the personal area network, the IEEE 802.15.4 standard would provide a limited answer in its non-beacon form.  Sensors that do not have large amounts of data to transfer could be used, i.e., small packets of data several times per hour.

57. Department of ECE Computer Engineering Seminar Questions and Comments

58. Department of ECE Computer Engineering Seminar