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MAC Layer Design for Wireless Sensor Networks

MAC Layer Design for Wireless Sensor Networks. Wei Ye USC Information Sciences Institute. Introduction. Wireless sensor network Large number of densely distributed nodes Battery powered Multi-hop ad hoc wireless network Node positions and topology dynamically change Self-organization

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MAC Layer Design for Wireless Sensor Networks

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  1. MAC Layer Design for Wireless Sensor Networks Wei Ye USC Information Sciences Institute

  2. Introduction • Wireless sensor network • Large number of densely distributed nodes • Battery powered • Multi-hop ad hoc wireless network • Node positions and topology dynamically change • Self-organization • Sensor-net applications • Nodes cooperate for a common task • In-network data processing

  3. Introduction • Important attributes of MAC protocols • Collision avoidance • Basic task — medium access control • Energy efficiency • Scalability and adaptivity • Number of nodes changes overtime • Latency • Fairness • Throughput • Bandwidth utilization

  4. C A B Hidden terminal: A is hidden from C’s CS Overview of MAC protocols • Contention-based protocols • CSMA — Carrier Sense Multiple Access • Ethernet • Not enough for wireless (collision at receiver) • MACA — Multiple Access w/ Collision Avoidance • RTS/CTS for hidden terminal problem • RTS/CTS/DATA

  5. Overview of MAC Protocols • Contention-based protocols (contd.) • MACAW — improved over MACA • RTS/CTS/DATA/ACK • Fast error recovery at link layer • IEEE 802.11 Distributed Coordination Function • Largely based on MACAW • Protocols from voice communication area • TDMA — low duty cycle, energy efficient • FDMA — each channel has different frequency • CDMA — frequency hopping or direct sequence

  6. Example: IEEE 802.11 DCF • Distributed coordinate function: ad hoc mode • 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 (frag + ACK) • First (frag + ACK) reserve time for second… • Give up tx when error happens

  7. Example: IEEE 802.11 DCF • Timing relationship

  8. Dominant in sensornets 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. Energy Efficiency in MAC Design • TDMA vs. contention-based protocols • TDMA can easily avoid or reduce energy waste from all above sources • Contention protocols needs to work hard in all directions • TDMA has limited scalability and adaptivity • Hard to dynamically change frame size or slot assignment when new nodes join • Restrict direct communication within a cluster • Contention protocols easily accommodate node changes and support multi-hop communications

  10. TDMA Protocols • Bluetooth • Clustering (piconet) • FH-CDMA between clusters • TDMA within each cluster • Centralized control by cluster head • Only master-slave communication • Master polling slave and schedule tx • At most 8 active nodes can be in a cluster — scalability problem

  11. TDMA 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

  12. 3 1 4 2 TDMA Protocols • Self-Organaiztion — by Sohrabi and Pottie • Have a pool of independent channels • Frequency band or spreading code • Potential interfering links select different channels • Talk to each neighbor in different time slots • Sleep during unscheduled time • Example • 6 different channels • Each node has its own scheduled time slots — superframe

  13. TDMA Protocols • Self-Organaiztion (Contd.) • Result (if a node has n neighbors) • Uses n different channels to talk to each of them • Schedules n time slots to send to each of them and n time slots to receive from each of them • Looks like TDMA, but actually FDMA or CDMA • Any pair of two nodes can talk at the same time • Low bandwidth utilization

  14. Contention-Based Protocols • PAMAS: Power Aware Multi-Access with Signalling — by Singh and Raghavendra • Improve energy efficiency from MACA • Avoid overhearing by putting node into sleep • Control channel separates from data channel • 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.

  15. Contention-Based Protocols • Piconet — by Bennett, Clarke, et al. • 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 • Carrier sense before sending data • May wait for long time before sending • Tx rate control —by Woo and Culler • Carrier sense + adaptive rate control • Promote fair bandwidth allocation • Helps for congestion avoidance

  16. Contention-Based Protocols • 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

  17. Contention-Based Protocols • Example of PS mode in IEEE 802.11 DCF

  18. Energy Efficiency: Contention • Extend 802.11 PS mode for Multi-hops — By Tseng, et al. at IEEE Infocom 2002 • Nodes do not synchronize with each other • Designed 3 sleep patterns — ensure nodes listen intervals overlap, example: • Periodically fully-awake interval: similar to S-MAC • Problem on broadcast — wake up each neighbor

  19. Latency Fairness Energy S-MAC: Introduction • S-MAC — by Ye, Heidemann and Estrin • Tradeoffs • Major components in S-MAC • Periodic listen and sleep • Collision avoidance • Overhearing avoidance • Massage passing

  20. 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% (150ms on/1.5s off)

  21. 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

  22. Periodic Listen and Sleep • 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

  23. 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

  24. 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

  25. 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

  26. ... ... ... 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

  27. Platform Mica Motes (UC Berkeley) 8-bit CPU at 4MHz, 128KB flash, 4KB RAM 433MHz 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)

  28. Average energy consumption in the source nodes 1800 802.11-like protocol w/o sleep 1600 Overhearing avoidance S-MAC 1400 1200 1000 Energy consumption (mJ) 800 600 400 200 0 2 4 6 8 10 Message inter-arrival period (second) Source 1 Sink 1 Sink 2 Source 2 Experiments: two-hop network • 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

  29. C A B Recent Progress: Adaptive Listen • Reduces latency due to periodic sleep • At end of each transmission, nodes wake up for a short time • Next transmission can start right away • Example: data flow ABC • A sends RTS, B replies CTS, C overhears CTS • C will wake up when AB is done • BC can start right away

  30. Recent Progress: 10-hop Experiments • Compare S-MAC in 3 different modes • 10-hop linear network with 11 nodes

  31. S-MAC Information • URL: http://www.isi.edu/scadds/ • Released S-MAC source code (for TinyOS 0.6.1) • Currently porting to nesC environment (TinyOS 1.0) Thank You!

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