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

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

3. 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. 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. 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. 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. 7 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 Active Sleep S-MAC Active Sleep TA T-MAC

8. 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. 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. 10 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) A B C contend RTSCTSDATAACK TA

11. 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. 12 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 A B C D contend RTSCTSDATAACK RTS? activesleep TA

13. 13 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; A B C D contend RTSCTSDATAACK RTS active TA DS FRTS active TA > C+R+T+CTS_length

14. 14 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 A B C D contend RTS CTSDATAACK active RTS contend TA

15. 15 Experiments S-MAC Vs. T-MAC

16. 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. 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. 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. 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. 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. 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. 22 The End  Thanks for your listening !