Energy-Efficient Medium Access Control

1. Energy-Efficient Medium Access Control Koen Langendoen and Gertjan Halkes Delft University of Technology The Embedded Systems Handbook

2. Roadmap • Introduction • Energy efficiency • Looking into some existing MAC protocols • Contention-based protocols • Slotted protocols • TDMA-based protocols • Simulation and benchmarks • Observation and conclusion

3. Roadmap • Introduction • Energy efficiency • Looking into some existing MAC protocols • Contention-based protocols • Slotted protocols • TDMA-based protocols • Simulation and benchmarks • Observation and conclusion

4. What is the role of MAC in sensor networks? • The unattended operation of sensor networks stresses the importance of energy efficiency and reduce the significance of performance considerations such as low latency, high throughput, and fairness. • The task of MAC layer in sensor networks is to use the radio, with limited resources, as efficient as possible to send and receive data generated by upper layers.

5. Some characteristics in hardware • Sending or receiving data consumes two to three orders of magnitude more than standby state. • Turn the radio off! • The time needed to switch the radio between transmit and receive mode is non-negligible. • Do not switch the radio if you don’t need one!

6. Some characteristics in communication • The data rates are very low. • Typically in the order of 1-200 bytes per second. • Two distinct important communication patterns. • Convergecast and local gossip. • The traffic is not distributed evenly over the network. • Nodes in the vicinity of the sink relay much more traffic than other nodes. • Fluctuations in network load is triggered by events.

7. What are convergecast and local gossip? • Convergecast: • Information needs to be sent to a sink node to be processed. • Small packets and the need to travel across the network. • Do aggregation! • Local gossip: • We expect when a sensor node observes an event, so does its neighbor. • They can collaborate to obtain a better estimate by unicast or broadcast. • By sharing (gossip) their sensor readings (rumors), nodes can reduce false positives and report significant events.

8. Roadmap • Introduction • Energy efficiency • Looking into some existing MAC protocols • Contention-based protocols • Slotted protocols • TDMA-based protocols • Simulation and benchmarks • Observation and conclusion

9. How to achieve energy efficiency? • Individual node failures do not harm the overall functioning of a sensor network. • Optimize for network lifetime, that is, the time until the network gets partitioned. • Optimization is easier at higher layers, since they have global view. • Solution: MAC layer ensures that the energy it spends is directly related to the amount of traffic it handles. • Trade off : • Performance (latency, throughput, fairness) v.s. Cost (energy efficiency, reduced algorithmic complexiy)

10. Sources of overheads • Idle listening • Since a node does not know when it will be the receiver of a message from one of its neighbors, it must keep its radio in receive mode all the time. • Collision • Energy used during transmission and reception is wasted. • RTS/CTS • Overhearing • A node may receive packets that are not destined for it. • Turn off the radio. • Protocol overhead • MAC headers and control packets do not contain application data. • The data is short. • Traffic fluctuations • A sudden peak in activity raises the probability of collision. • Much time and energy are spent on waiting in the random backoff. • The performance collapse when the load approaches the channel capacity.

11. Some existing MAC protocols

12. Some trade offs that people used. • The number of the physical channel used. • The degree of organization between nodes • The way in which a node is notified of an incoming message.

13. Is it good to use multiple channels? • Multiple channels • FDMA or CDMA • It requires a rather complicated radio consming considerable of energy. • Tone or control signal • Use a second, extremely low power radio that can be used for signaling an intended receiver to wake up.

14. Do networks need to be organized? • Contention based protocol (random) • Low implementation complexity • Ad-hoc nature • Flexible to accommodate mobile nodes and traffic fluctuation • Frame-based TDMA protocol • Lack of collision • Lack of overhearing • No idle-listening • Slotted System • E.g. S-MAC, T-MAC • Duty cycle • Nodes are awake in the first part of each slot and go to sleep in the second part.

15. How do the receivers get notified? • Schedule-based protocols • Schedule • Data transfers are scheduled ahead of time, so receiving nodes know exactly when to turn on the radio. • Contention-based protocols • Listening • Idle-listening • Use duty cycle • Individually (e.g. Low power listening) • Collectively per slot (e.g. S-MAC) • Use a cellular topology with access points (e.g. Sift) • Wakeup • Send a wake up tone over a second, very low-power radio.

16. Roadmap • Introduction • Energy efficiency • Looking into some existing MAC protocols • Contention-based protocols • Slotted protocols • TDMA-based protocols • Simulation and benchmarks • Observation and conclusion

17. Characteristics of contention-based protocols • Needs to reduce the energy consumption caused by collisions, overhearing, and idle-listening.

18. Low Power Listening and Preamble Sampling • Used in Aloha and B-MAC • Turn the radio off repeatedly • Header starts off with a preamble that is used to notify receivers of the upcoming transfer. • Shift the cost from the receiver (the frequent case) to the transmitter (the rare case). • No collision avoidance to save protocol overhead. • Slight increase in latency • Minor reduction in throughput

19. WiseMAC • Long preamble are not necessary when the sender knows the sampling schedule of the intended receiver. • Nodes maintain the schedule offsets of their neighbors through piggy backed information. • To account for clock drift, the preamble is extended with a time proportional to the length of the interval since the last message exchange. • Not effective for broadcast. • Preamble must span the sampling points of all neighbors and account for drift.

20. Roadmap • Introduction • Energy efficiency • Looking into some existing MAC protocols • Contention-based protocols • Slotted protocols • TDMA-based protocols • Simulation and benchmarks • Observation and conclusion

21. Characteristics of slotted protocols • They address the inherent idle listening overhead by synchronizing the nodes, and implementing a duty cycle within each slot.

22. S-MAC • Broadcast SYNC packets at the beginning of a slot, so other nodes receiving these packets can adjust their clock. • Collision avoidance and overhearing avoidance. • RTS/CTS handshake

23. T-MAC • Improvement of S-MAC • Use adaptive duty cycle to account for traffic fluctuation. • Fixed slot time and variable active time. • Use time-out to determine the end of active time. • Time out value is 15 ms • Early sleep problem • S wants to send data to R Go to sleep Keep silence Sending msg N S R

24. DMAC • Convergecast only • Each node repeatedly executes a basic sequence of 1 receive, 1 send, n sleep slots. • Ensures that the sequence are staggered to match the structure of the convergecast tree rooting at the sink node.

25. Roadmap • Introduction • Energy efficiency • Looking into some existing MAC protocols • Contention-based protocols • Slotted protocols • TDMA-based protocols • Simulation and benchmarks • Observation and conclusion

26. Characteristics of TDMA-based protocols • Collision free • No idle listening.

27. Scheduling of TDMA-based protocols • Sink-based scheduling • The traffic within each cluster is scheduled by a sink node who is connected to the wired backbone network. • Sink node will decide which node will sense, relay or sleep. • All nodes need to directly communicate with the sink node. • Static scheduling • Fixed schedule. • Acceptable performance for typical communication patterns. • Rotation duties • The cost of serving as an access point may be amortized over multiple nodes by rotating duties. • Partitioned scheduling • Each slot serves as a mini TDMA frame and consists of a contention phase, a traffic control section and a data section. • Active nodes owns a slot and always transmit in its slot. • Passive nodes must content for a slot. • All nodes listen to their neighbors’ control section. • Replicated scheduling • Node broadcasts its traffic and the identity of their one-hop neighbor • Use a distributed hash function to compute the winner of each slot. • Collision-free assignment. • High channel utilization but at cost of high algorithmic complexity.

28. LMAC • Each active node is in control of a slot. • Does not acknowledge on the reception of the data. • Ensure collision-free transmission by having nodes select a slot number that is not in use within a two-hop neighbor • Broadcast a bit-set detailing which slots are occupied by the one-hop neighbors. • Nodes must always listen to the control section of all slots in a frame. • Sample the unused slots first to determine whether there is any activity

29. Roadmap • Introduction • Energy efficiency • Looking into some existing MAC protocols • Contention-based protocols • Slotted protocols • TDMA-based protocols • Simulation and benchmarks • Observation and conclusion

30. Performance in empty network • Latency • LMAC • A node want to send pkts must wait until its slot • S-MAC/T-MAC • Source node must wait for the next active period • Early sleep in T-MAC • Throughput • LPL • Collision generated by hidden nodes • S-MAC • Loss is caused at contention period • RTS/CTS overhead • T-MAC • Adaptive duty cycle • Fixed contention window

31. Homogeneous unicast • Delivery ratio collapsed at some point for LPL, S-MAC and T-MAC • The energy consumption jump in T-MAC and LPL is due to breakdown. Most of the energy is used in retransmission • The energy consumption of the 802.11 protocol decreases for higher load. • Overhearing avoidance mechanism

32. Homogeneous broadcast • LMAC guarantees collision-free communications • Delivery rates degrade at light load, but improve for high load • No retransmission • Slotted protocol perform worse than contention-based protocols • In a rather short active period, the probability of a collision is increased. • Overhearing is no longer applicable, forcing the radio to be on all the time • For LPL • Longer preamble cause more collision

33. Local gossip • In essence local gossip is a mixture of empty workload and homogeneous workload • The delivery rate associated with local gossip are completely determined by homogeneous unicast component. • Collision occur less frequently due to a relatively large number of edge nodes with inactive neighbors.

34. Convergecast • The generated load that can be handled is much lower than with homogeneous unicast, since the average hop length is large. • The performace do not simply scale with the path-length vector. • LMAC cannot handle heavy load around the sink, since each node can use only one slot.

35. Roadmap • Introduction • Energy efficiency • Looking into some existing MAC protocols • Contention-based protocols • Slotted protocols • TDMA-based protocols • Simulation and benchmarks • Observation and conclusion

36. Observation • Communication grouping considered harmful • Slotted protocols • Very low duty cycles can be obtained, but at the expense of high latency and a collapse under high load • TDMA-based protocols • High latency and reduced throughput under high load • Energy consumption does not deteriorate • Contention-based protocols • LPL is most flexible • Sampling approach depends on the radio’s ability to switch quickly • Collision avoidance considered prohibitive • RTS/CTS prevents collision due to hidden nodes • Reduce the effective channel capacity • Small messages in sensor networks. (A high overhead) • Adapteivity considered essential • Static protocol either consume too much energy under low load, or throttle throughput too much under high loads.

37. Conclusion • No MAC protocol outperform the others in all ways. • Adaptivity is mandatory to handle the generic local gossip and convergecast communication, since these patterns showing traffic fluctuation both in time and space.