MAC Layer Design for Wireless Sensor Networks

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!