8/1/2005 Hong-Shi Wang

1. An Adaptive Energy-Efficient MAC Protocol for Wireless Sensor NetworksTijs van Dam, Koen LangendoenIn ACM SenSys 2003. 8/1/2005 Hong-Shi Wang

2. Contents • Introduction • Related work • Drawbacks of S-MAC • T-MAC • Experiments • Conclusions and Future Work

3. Introductions • Traditional MAC Protocols • Design to maximize packet throughput, minimize latency and provide fairness • Protocol design for wireless sensor networks • focuses on minimizing energy consumption

4. Related Work • TDMA-based protocol • Have advantage of energy conservation compared to contention protocols, because there is no contention-introduced overhead and collisions • But requires the nodes to form real communication clusters like LEACH • Managing inter-cluster communication and interference is not an easy task. • Contention-based protocol • simplicity • Energy consumption using this MAC is very high when nodes are in idle mode

5. Drawbacks of S-MAC • Active (Listen) interval • If message rate is less – energy is still wasted in idle-listening • S-MAC’ fixed duty cycle is NOT OPTIMAL

6. T-MAC : Preliminaries (1) • Basic idea • To utilize an active and a sleep cycle, similar to S-MAC • To introduce an adaptive duty cycle by dynamically ending the active part • An active period ends when no activation event has occurred for a time TA • Activation event • The reception of any data on the radio (RTS, CTS, DATA, ACK) • The sensing of communication on the radio (overhearing) • Difference in the duty cycle • S-MAC - fixed duty cycle • T-MAC – Dynamic duty cycle

7. Active Active Active Sleep Sleep S-MAC Active Active Active Sleep Sleep T-MAC TA TA TA T-MAC : Preliminaries (2) • With normal MAC protocols, messages are spread out over the whole time frame • With S-MAC, active time is fixed • With T-MAC, the active time is dynamically adjusted (i.e., be shorten) by timing out on hearing nothing during some time period (TA) • TA determines the minimal amount of idle listening per frame

8. T-MAC : RTS Operation (1) Contention Interval • In contention-based protocols, like IEEE 802.11, a back-off scheme is used: • Contention interval increases when traffic is higher. • In the T-MAC protocol, waiting and listening for a random time within a fixed contention interval • Tuned for maximum load.

9. T-MAC : RTS Operation (2) RTS Retries • No CTS reply for RTS? • Collision • The receiving node is prohibited from replying due to an overhead RTS or CTS • Receiving node is asleep • Solutions: • Retransmit RTS if no answer • If there is still no reply after two retries, it should give up and go to sleep

10. contend RTS CTS DATA ACK A B contend C TA T-MAC : Choosing TA Determining TA • The interval TA must be long enough to receive at least the start of the CTS packet • TA > C+R+T • C –contention interval length; R–RTS packet length; • T –turn-around time, time between RTS end & CTS start • Larger TA increases the energy used • In experiments, used TA = 1.5 x (C + R + T)

11. T-MAC : Overhearing Avoidance • ~= S-MAC • But implemented as an option in T-MAC • Node goes to sleep after overhearing RTS/CTS of other nodes communication • miss other RTS/CTS while sleeping • throughput decreases • Although overhearing avoidance saves energy, it must not be used when maximum throughput is required

12. contend RTS CTS DATA ACK A B contend C active sleep D RTS? TA T-MAC: Asymmetric Communication (1) Early-Sleeping Problem–unidirectional (A to D) • If node C looses contention because it overhears a CTS packet from B to A, C must remain silent. • Since D does not know of the communication between A and B, its active time will end, and node D will go to sleep. • Only at the start of the next frame will node C have a new chance to send to node D • Early-Sleeping Problem • Node goes to sleep when a neighbor still has messages for it

13. contend RTS CTS DS DATA ACK A B contend C active active D FRTS RTS TA TA > C+R+T+CTS_length T-MAC: Asymmetric Communication (2) Future request-to-send (FRTS) • Let others know that we still have a message for it, but cannot access the medium; • C sends FRTS to future target of an RTS packet • FRTS has duration field • FRTS might affect data; so, DATA postponed until FRTS is over; To prevent others from taking medium, A send DS packet;

14. contend A contend B contend C RTS active D RTS CTS DATA ACK TA T-MAC: Asymmetric Communication (3) Taking priority on full buffers • When a node’s transmit/routing buffers are almost full, it may prefer sending than receiving • Receive RTS, send its own RTS to others instead of CTS • limited form of flow control

15. Experiments S-MAC Vs. T-MAC

16. Simulation setup and parameters • Simulator: OMNeT++ • Built a network of 100 nodes in a 10 by 10 grid • Energy consumption • S-MAC protocol • A frame length of one second, and with several lengths of the active time, varying from 75 ms to 915 ms. • T-MAC protocol • Always used a frame length of 610ms and an interval TA with a length of 15 ms • Can optionally be deployed with overhearing avoidance, full-buffer priority, and FRTS

17. Homogeneous local unicast • Nodes send packets to one of their neighbors at random • T-MAC: Used overhearing avoidance, but no FRTS or full-buffer priority mechanisms

18. Nodes-to-sink communication • Nodes send messages to a single sink node • Shortest path routing, no data aggregation • T-MAC: Used overhearing avoidance, FRTS or full-buffer priority mechanisms

19. Early-sleeping Problem • Nodes send messages to a single sink node • Shortest path routing, no data aggregation T-MAC: FRTS Vs. Priority Vs. FRTS + Priority Vs. No measures

20. Event-Based Local Unicast • When no events happen, nodes exchange local messages of 10 bytes with each other every 20 seconds. Also report to a sink node every 100 seconds. • When an event happens, nodes start sending local unicast messages of 30 bytes. Then also send messages of 50 bytes to the sink.

21. Conclusions And Future Work Conclusions • T-MAC dynamically adapts a listen/sleep duty cycle • T-MAC Protocol • During a high load, nodes communicate without sleeping • During a very low load, nodes will use their radios for as little as 2.5% of the time, saving as much as 96% of the energy compared to a traditional Future Work • Throughput and Early-Sleeping Problem

22. The End • Thanks for your listening !