An Adaptive Energy-Efficient MAC Protocol for Wireless Sensor Network

1. An Adaptive Energy-Efficient MAC Protocol for Wireless Sensor Network Tijs van Dam, Koen Langendoen SenSys’03 Ku Dara Network & Security LAB at KAIST 2006.09.14

2. Contents • Introduction • S- MAC drawbacks • T- MAC • Experiments • Conclusions T-MAC

3. Introduction • Traditional MAC Protocols • Design to maximize packet throughput, minimize latency and provide fairness • Protocol design for wireless sensor networks • Focuses on minimizing energy consumption • What was the most wasted energy in traditional MAC protocol? • Idle listening • 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 at all times • Ex) sensor application : 1/sec, messages fairly short, transmit(5ms) , receive(5ms), 990ms on listening while nothing happens(99%) T-MAC

4. S-MAC • Idle listening problem solution • Duty cycle is involved, each node sleep periodically • S-MAC in sensor network • Single-frequency contention-based protocol • Time is divided into –fairly large-frame (frame: 1sec) • Every frame has two parts : active part (200ms) /sleep part (800ms) • duty cycle = listen interval / frame length (20%) • All messages are packed into the active part • Tradeoff • Energy efficiency ↑, throughput ↓ , latency↑ T-MAC

5. Drawbacks of S-MAC • Active (Listen) interval – long enough to handle to the highest expected load • If message rate is less – energy is still wasted in idle-listening • S-MAC’ fixed duty cycle is NOT OPTIMAL T-MAC

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 T-MAC

7. T-MAC Preliminaries(2) • Normal MAC protocols: messages are spread out over the whole time frame • S-MAC: active time is fixed • T-MAC: the active time is dynamically adjusted (i.e., be shorten) by timing out on hearing nothing during some time period (TA) T-MAC

8. T-MAC : RTS Operation (1) Contention Interval : Fixed contention interval • In contention-based protocols, like IEEE 802.11 • a back-off scheme is used: contention interval increases when traffic is higher • Reduce the probability of collision when load is high • In the T-MAC protocol • Every node transmits its queued messages in a burst at the start of the frame • In burst, the traffic is mostly high • Waiting and listening for random time within a Fixed contention interval • Tuned for maximum load. T-MAC

9. T-MAC : RTS Operation (2) RTS Retries • No CTS reply for RTS? • The receiving node has not heard the RTS due to collision • The receiving node is prohibited from replying due to an overheard 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 T-MAC

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) T-MAC

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 • Although overhearing avoidance saves energy, it must not be used when maximum throughput is required T-MAC

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) • Node goes to sleep when a neighbor still has messages for it T-MAC

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(Data Send) packet; T-MAC

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 • Advantage in a node–to-sink communication pattern T-MAC

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

16. Simulation setup and parameters • Simulator: OMNeT++ • Built a network of 100 nodes in a 10 by 10 grid (8 neighbor) • 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 T-MAC

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 T-MAC

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

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

20. Conclusions And Future Work • T-MAC dynamically adapts a listen/sleep duty cycle • Early sleeping problem • Proposed FRTS & full-buffer priority • Trade-off : throughput vs. energy efficiency • Future work • Experiment with mobile network • Apply virtual clustering in the S-MAC T-MAC