Sensor MAC Design

1. Sensor MAC Design • Requirements: • Energy efficiency • Simple operations • Working with a large number of sensors • Fair share of the channel among competing sensor nodes

2. 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

3. 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

4. Key Questions • How should Media Access Control (MAC) protocols be designed for sensor networks? • What are the metrics for MAC in a multi-hop scenario? • How can we achieve these metrics with local algorithms over limited resources?

5. What’s New in Sensor Networks? • A special multi-hop wireless network • Large number of nodes • Battery powered • Topology and density change due to node’s sleeping or failure • 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

6. 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

7. 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?

8. 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)

9. Classification of MAC Protocols • Schedule-based protocols • Schedule nodes onto different sub-channels • Examples: TDMA, FDMA, CDMA • Contention-based protocols (our focus) • Nodes compete in probabilistic coordination • Example: 802.11 DCF

10. Contention Protocols: CSMA • 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

11. 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 sending data • Node c will backoff when it hears b’s CTS

12. 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

13. Latency Fairness Energy Case Study 1: S-MAC • By Ye, Heidemann and Estrin • Tradeoffs

14. S-MAC Overview • Design goals • Energy efficiency • Self-configuration and flexibility to node changes • Approaches • Contention-based MAC with various energy-conserving features • Major components in S-MAC • Periodic listen and sleep • Collision avoidance • Overhearing avoidance • Massage passing

15. 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)

16. 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

17. 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

18. 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

19. 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

20. 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

21. 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

22. 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 • Platform: Mica Motes • Topology: 10-hop linear network • S-MAC saved a lot of energy compared with a MAC without sleep

23. Evaluation Metric: Aggregate Bandwidth • Traditional MAC metric • channel capacity is a precious resource • Maximize total delivered bandwidth from every node in the network to the base station

24. Evaluation Metric: Energy Efficiency • Observation: • Energy is the precious resource • Goal: • Minimize energy per unit of successful communication to base station while sustaining reasonable channel utilization • Turn off radio whenever possible • Avoid over commit the network Total energy spent in data propagation over a network Total packets received by the base station E =

25. Limitations of SMAC • Look at the assumptions made • Broadcast assumes every one can hear the message • Bursty data applications versus periodic applications • Power consumption model • In-band signaling? • Let us revisit the design using the link measurement findings

26. Fairness Challenge • Challenge: • want roughly equal data • coverage • Observation: • originated traffic competes • with route-thru traffic • at odd with energy • efficiency and aggregate • bandwidth • Goal: • minimizevariance in • bandwidth delivered • to base station

27. Case Study 2: Berkeley MAC Design • Carrier Sense Multiple Access (CSMA) • no extra control packets (energy efficient) • Save energy: • Shorten listening period as much as possible • turn radio off during backoff • Trade bandwidth for battery life • Provide feedback to applications to desynchronize • Backoff should signal application to shift phase of sampling • Random delay before each transmission • break close synchronization

28. Hidden Nodes inMulti-hop Networks • Occurs between every other pair of levels • CSMA fails to detect • Traditionally addressed with contention-based protocols, but • Control packets (e.g. RTS/CTS/ACKs) induce high overhead given data packets are small • ACKs can be free in multi-hop networks • By hearing your parent forwards your packets • Data aggregation is application specific • Not adequate to solve hidden node problem in multi-hop case (Related Work: V. Bharghavan et al. MACAW)

29. Avoid Hidden Node Corruption Hidden node cases like this may be avoided without use of control packets. A C D B • exploits application characteristics • “A” refrains from sending for a packet time after parent transmits

30. Platform of Study • Rene • 4MHz, 8KB flash, 512B RAM • 916MHz RF transceiver • 10kbps • 1 - 100 feet range • Sensors: temperature, light, magnetic field, acceleration • Operating System: TinyOS • tiny network stack and other communication support • Small packets size (tens of bytes)

31. Multi-hop Extensions • Rate control module inserted between MAC and application • Adapts data sampling rate to available bandwidth • Balances demand for upstream bandwidth between local, originating traffic and route-thru traffic by adjusting transmission rate • Multihop • Merging traffic flow • Provides a mechanism for progressive feedback deep down into the network

32. Route-thru Traffic x  x  x  + /n + /n + /n Rate Control Mechanism if fails if success R x p Estimate n based on route-thru traffic • snoop on route-thru traffic to estimate children (n) • Open parameters •  , • Apply for forwarding route-thru traffic • Progressive feedback deep down into the network • Packet loss rate provides natural damping effect

33. Other ARC Results • Proposed CSMA with ARC scheme: • Aggregate bandwidth: • ~ 60% of proposed CSMA without ARC scheme • Energy efficiency: • ~ 50% of proposed CSMA at a low  • Fairness: • 5 – 10 times lower variance

34. Summary of B-MAC • Sensor networks characteristics differ from traditional settings enough to require revisiting the basic protocols • MAC design • fairness and energy efficiency goals • modified CSMA shown effective • Transmission control • local adaptive scheme on originating traffic effective • Implemented and evaluated on simulation and real networked sensors • Each node achieves 20% of multi-hop channel capacity

35. Summary for MAC Design in 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

36. What is next: New Sensor Link Protocols • Look at the experimental results ! • These new findings are not addressed yet • Learn from the topology • With static sensor settings, link protocols can be more efficient • Application-aware link protocol • Get hints from applications about traffic patterns, rate, etc. • New forms of cross-layer design