Medium Access Control in Wireless Sensor Networks

1. Medium Access Control in Wireless Sensor Networks Wei Ye USC Information Sciences Institute Guest lecture for CS113, UCLA

2. Outline • Introduction to MAC • MAC attributes and trade-offs • Scheduled MAC protocols • Contention-based MAC protocols • Case studies • Summary Guest lecture for CS113, UCLA

3. Introduction to MAC • The role of medium access control (MAC) • Controls when and how each node can transmit in the wireless channel • Why do we need MAC? • Wireless channel is a shared medium • Radios transmitting in the same frequency band interfere with each other – collisions • Other shared medium examples: Ethernet Guest lecture for CS113, UCLA

4. Application layer Transport layer End-to-end reliability, congestion control Network layer Routing Link/MAC layer Per-hop reliability, flow control, multiple access Physical layer Packet transmission and reception Where Is the MAC? • Network model from Internet • A sublayer of the Link layer • Directly controls the radio • The MAC on each node only cares about its neighborhood Guest lecture for CS113, UCLA

5. What’s New in Sensor Networks? • 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 Guest lecture for CS113, UCLA

6. Next… • Introduction to MAC • MAC attributes and trade-offs • Scheduled MAC protocols • Contention-based MAC protocols • Case studies • Summary Guest lecture for CS113, UCLA

7. Primary MAC Attributes • Collision avoidance • Basic task of a MAC protocol • Energy efficiency • One of the most important attributes for sensor networks, since most nodes are battery powered • Scalability and adaptivity • Network size, node density and topology change Guest lecture for CS113, UCLA

8. Other MAC Attributes • Channel utilization • How well is the channel used? Also called bandwidth utilization or channel capacity • Latency • Delay from sender to receiver; single hop or multi-hop • Throughput • The amount of data transferred from sender to receiver in unit time • Fairness • Can nodes share the channel equally? Guest lecture for CS113, UCLA

9. Dominant factor Energy Efficiency in MAC Design • Energy is primary concern in sensor networks • What causes energy waste? • Collisions • Control packet overhead • Overhearing unnecessary traffic • Long idle time • bursty traffic in sensor-net apps • Idle listening consumes 50—100% of the power for receiving (Stemm97, Kasten) Guest lecture for CS113, UCLA

10. Classification of MAC Protocols • Schedule-based protocols • Schedule nodes onto different sub-channels • Examples: TDMA, FDMA, CDMA • Contention-based protocols • Nodes compete in probabilistic coordination • Examples: ALOHA (pure & slotted), CSMA Guest lecture for CS113, UCLA

11. Next… • Introduction to MAC • MAC attributes and trade-offs • Scheduled MAC protocols • Contention-based MAC protocols • Case studies • Summary Guest lecture for CS113, UCLA

12. Scheduled Protocols: TDMA • Time division multiple access • Divide time into subchannels • Advantages • No collisions • Energy efficient — easily support low duty cycles • Disadvantages • Difficult to accommodate node changes • Requires strict time synchronization Guest lecture for CS113, UCLA

13. Scheduled Protocols: Polling • Master-slave configuration • The master node decides which slave can send by polling the corresponding slave • Only direct communication between the master and a slave • A special TDMA without pre-assigned slots • Examples • IEEE 802.11 infrastructure mode (CPF) • Bluetooth piconets Guest lecture for CS113, UCLA

14. Scheduled Protocols: Bluetooth • Wireless personal area network (WPAN) • Short range, moderate bandwidth, low latency • IEEE 802.15.1 (MAC + PHY) is based on Bluetooth • Nodes are clustered into piconet • Each piconet has a master and up to 7 active slaves – scalability problem • The master polls each slave for transmission • CDMA among piconets • Multiple connected piconets form a scatternet • Difficult to handle inter-cluster communications Guest lecture for CS113, UCLA

15. Scheduled Protocols: Self-Organization • By Sohrabi and Pottie • Have a pool of independent channels • Frequency band or spreading code • Potential interfering links select different channels • Talk to neighbors in different time slots • Sleep in unscheduled time slots • Looks like TDMA, but actual multiple access is accomplished by FDMA or CDMA • Any pair of two nodes can talk at the same time • Low bandwidth utilization Guest lecture for CS113, UCLA

16. Scheduled Protocols: LEACH • Low-Energy Adaptive Clustering Hierarchy — by Heinzelman, et al. • Similar to Bluetooth • CDMA between clusters • TDMA within each cluster • Static TDMA frame • Cluster head rotation • Node only talks to cluster head • Only cluster head talks to base station (long dist.) • The same scalability problem Guest lecture for CS113, UCLA

17. Next… • Introduction to MAC • MAC attributes and trade-offs • Scheduled MAC protocols • Contention-based MAC protocols • Case studies • Summary Guest lecture for CS113, UCLA

18. Contention Protocols: Classics • ALOHA • Pure ALOHA: send when there is data • Slotted ALOHA: send on next available slot • Both rely on retransmission when there’s collision • CSMA — Carrier Sense Multiple Access • Listening (carrier sense) before transmitting • Send immediately if channel is idle • Backoff if channel is busy • non-persistent, 1-persistent and p-persistent Guest lecture for CS113, UCLA

19. c a b Node a is hidden from c’s carrier sense Contention Protocols: CSMA/CA • Hidden terminal problem • CSMA is not enough for multi-hop networks (collision at receiver) • CSMA/CA (CSMA with Collision Avoidance) • RTS/CTS handshake before send data • Node c will backoff when it hears b’s CTS Guest lecture for CS113, UCLA

20. Contention Protocols: MACA and MACAW • MACA — Multiple Access w/ Collision Avoidance • Based on CSMA/CA • Add duration field in RTS/CTS informing other node about their backoff time • MACAW • Improved over MACA • RTS/CTS/DATA/ACK • Fast error recovery at link layer Guest lecture for CS113, UCLA

21. Contention Protocols: IEEE 802.11 • IEEE 802.11 ad hoc mode (DCF) • Virtual and physical carrier sense (CS) • Network allocation vector (NAV), duration field • Binary exponential backoff • RTS/CTS/DATA/ACK for unicast packets • Broadcast packets are directly sent after CS • Fragmentation support • RTS/CTS reserve time for first (fragment + ACK) • First (fragment + ACK) reserve time for second… • Give up transmission when error happens Guest lecture for CS113, UCLA

22. Contention Protocols: IEEE 802.11 (cont.) • Power save (PS) mode in IEEE 802.11 DCF • Assumption: all nodes are synchronized and can hear each other (single hop) • Nodes in PS mode periodically listen for beacons & ATIMs (ad hoc traffic indication messages) • Beacon: timing and physical layer parameters • All nodes participate in periodic beacon generation • ATIM: tell nodes in PS mode to stay awake for Rx • ATIM follows a beacon sent/received • Unicast ATIM needs acknowledgement • Broadcast ATIM wakes up all nodes — no ACK Guest lecture for CS113, UCLA

23. Contention Protocols: IEEE 802.11 (cont.) • Unicast example of PS mode in 802.11 DCF Guest lecture for CS113, UCLA

24. Contention Protocols: Tx Rate Control • By Woo and Culler • Based on a special network setup • A base station tries to collect data equally from all sensors in the network • CSMA + adaptive rate control • Promote fair bandwidth allocation to all sensors • Nodes close to the base station forward more traffic, and have less chances to send their own data • Helps in congestion avoidance Guest lecture for CS113, UCLA

25. Contention Protocols: Piconet • By Bennett, Clarke, et al. • Not the same piconet in Bluetooth • Low duty-cycle operation — energy efficient • Sleep for 30s, beacon, and listen for a while • Sending node needs to listen for receiver’s beacon first, then • CSMA before sending data • May wait for long time before sending Guest lecture for CS113, UCLA

26. Contention Protocols: PAMAS • PAMAS: Power Aware Multi-Access with Signalling — by Singh and Raghavendra • Improve energy efficiency from MACA • Avoid overhearing by putting node into sleep • Use separate control and data channels • RTS, CTS, busy tone to avoid collision • Probe packets to find neighbors transmission time • Increased hardware complexity • Two channels need to work simultaneously, meaning two radio systems. Guest lecture for CS113, UCLA

27. Contention Protocols: ZigBee • Based on IEEE 802.15.4 MAC and PHY • Three types devices • Network Coordinator • Full Function Device (FFD) • Can talk to any device, more computing power • Reduced Function Device (RFD) • Can only talk to a FFD, simple for energy conservation • CSMA/CA with optional ACKs on data packets • Optional beacons with superframes • Optional guaranteed time slots (GTS), which supports contention-free access Guest lecture for CS113, UCLA

28. Contention Protocols: ZigBee (cont.) • Low power, low rate (250kbps) radio • MAC layer supports low duty cycle operation • Target node life time > 1 year Guest lecture for CS113, UCLA

29. Next… • Introduction to MAC • MAC attributes and trade-offs • Scheduled MAC protocols • Contention-based MAC protocols • Case studies • Summary Guest lecture for CS113, UCLA

30. Latency Fairness Energy Case Study 1: S-MAC • By Ye, Heidemann and Estrin • Tradeoffs • Major components in S-MAC • Periodic listen and sleep • Collision avoidance • Overhearing avoidance • Massage passing Guest lecture for CS113, UCLA

31. sleep listen listen sleep Energy Latency Coordinated Sleeping • Problem: Idle listening consumes significant energy • Solution: Periodic listen and sleep • Turn off radio when sleeping • Reduce duty cycle to ~ 10% (120ms on/1.2s off) Guest lecture for CS113, UCLA

32. Node 1 sleep sleep listen listen Node 2 sleep sleep listen listen Schedule 1 Schedule 2 Coordinated Sleeping • Schedules can differ • Prefer neighboring nodes have same schedule • — easy broadcast & low control overhead Border nodes: two schedules or broadcast twice Guest lecture for CS113, UCLA

33. Coordinated Sleeping • Schedule Synchronization • New node tries to follow an existing schedule • 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 • Periodic neighbor discovery • Keep awake in a full sync interval over long periods Guest lecture for CS113, UCLA

34. RTS CTS CTS listen listen t1 t2 Coordinated Sleeping • Adaptive listening • Reduce multi-hop latency due to periodic sleep • Wake up for a short period of time at end of each transmission 2 4 1 3 listen • Reduce latency by at least half Guest lecture for CS113, UCLA

35. Collision Avoidance • S-MAC is based on contention • 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 Guest lecture for CS113, UCLA

36. 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 Guest lecture for CS113, UCLA

37. 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 Guest lecture for CS113, UCLA

38. Platform: Mica Motes Topology: 10-hop linear network S-MAC saved a lot of energy compared with a MAC without sleep Energy consumption at different traffic load 30 25 No sleep cycles 20 Energy consumption (J) 15 10 10% duty cycle without adaptive listen 5 10% duty cycle with adaptive listen 0 0 2 4 6 8 10 Message inter-arrival period (S) Implementation and Experiments Guest lecture for CS113, UCLA

39. Case Study 2: B-MAC • Another low-power MAC for sensor networks • B-MAC design considerations • Simplicity: based on simple CSMA • Configurable options • Minimize idle listening • Based on model of periodic sensor data transfer • B-MAC components • CSMA without RTS/CTS • Optional Low-power listening (LPL) • Optional ACK Guest lecture for CS113, UCLA

40. Low-Power Listening • Determine channel status by quick sampling • Very low overhead on wake-up Joe Polastre, et al., SenSys’04 Guest lecture for CS113, UCLA

41. Low Duty Cycle with LPL • Nodes periodically sleep and perform LPL • Nodes do not synchronized on listen time • Sender uses a long preamble before each packet to wake up the receiver • Shift most burden to the sender Guest lecture for CS113, UCLA

42. Comparison of S-MAC and B-MAC Guest lecture for CS113, UCLA

43. Next… • Introduction to MAC • MAC attributes and trade-offs • Scheduled MAC protocols • Contention-based MAC protocols • Case studies • Summary Guest lecture for CS113, UCLA

44. MAC Design for Sensor Networks • MAC protocols can be classified as scheduled and contention-based • Major considerations • Energy efficiency • Scalability and adaptivity to number of nodes • Major ways to conserve energy • Low duty cycle to reduce idle listening • Effective collision avoidance • Overhearing avoidance • Reducing control overhead Guest lecture for CS113, UCLA

45. Scheduled vs. Contention Protocols Guest lecture for CS113, UCLA

46. Questions? Guest lecture for CS113, UCLA