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

Contents Introduction S- MAC drawbacks T- MAC Experiments Conclusions

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

S-MAC Idle listening problem solution S-MAC in sensor network 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

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

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

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

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

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

T-MAC : Choosing TA Determining TA TA > C+R+T 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 RTS CTS DATA ACK TA T-MAC

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

T-MAC: Asymmetric Communication (1) Early-Sleeping Problem – unidirectional (A to D) Node goes to sleep when a neighbor still has messages for it A B C D contend RTS CTS DATA ACK RTS? active sleep TA T-MAC

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; A B C D contend RTS CTS DATA ACK active TA DS FRTS TA > C+R+T+CTS_length T-MAC

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 A B C D contend RTS CTS DATA ACK active TA T-MAC

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

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

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

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

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

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