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An Adaptive EnergyEfficient MAC Protocol for Wireless Sensor Networks Tijs van Dam Koen Langendoen Presenter: Michael Curcio ACM SenSys 2003 Sensor Networks • • • • • Low message rate Insensitive to latency Low processing power and memory capacity Lots of redundancy Often battery operated! Goals • • • Focus has moved away from maximizing throughput and fairness; minimizing latency Power consumption kept to a minimum Memory/Network processing kept low Energy Sinks • • • • • Processor Radio • • Receiving/Transmitting Idle Listening Collisions Protocol Overhead Overhearing What else addresses this? • • • • • • TDMA 802.11 (CSMA) Extra wake-up radio TinyOS S-MAC Radio-triggered wake-up hardware How do they do that? • • TDMA • • Built-in duty cycle; eliminates collisions But-- Hard to do for ad-hoc network • • Scheduling, slot allocation, coordination Clock drift (especially for small slots) Extra radio • • Solve problems with more hardware Add a radio on a different frequency that can wake up other nodes How do they do that? • 802.11 • • • Has power-saving features, but not good enough for sensor networks Was designed for nodes all existing in one cell (no multi-hop) TinyOS • • Instead of listening really long for a short transmission, listen realy short for a long transmission Makes the transmitter, not the receiver pay the energy bill How do they do that? • S-MAC • • • • Fixed duty cycle Compress spread out transmits and receives into a shorter amount of time so we can sleep the rest Event, issue, request-based transfer of information hop-to-hop Radio-triggered wake-up • • Stay awake for Soji’s presentation Teaser: On-demand node wake-up T-MAC Approach • • “Timeout”-MAC Adaptive duty cycle • • • Period of radio activity can be ended dynamically Reduce idle listening to a minimum Better handle variable network load Sensor Network Communication Patterns • • Local uni-/broadcast • Event processed in network among nodes Nodes to sink(s) reporting • • Messages move through the network in a generally unified direction (to the sink(s)) May or may not be aggregated/processed en-route Sensor Network Communication Patterns • • • Traditional, multi-hop routing not used • Might there be a case where it would be useful? Time dependence • Nothing to do if no events occur Location dependence • Network in proximity to sink nodes experiences heavier traffic than at remote edge of network EYES Nodes • • • • • • 16-bit, 5MHz, variable clock rate processor 2KB RAM 60KB FLASH 2MB EEPROM JTAG, RS232, 2 LEDs, 16 GPIO (8 ADC) pins Runs on 2 AA batteries Photo Courtesy: Eyes - Energy Efficient Sensor Networks, http://www.eyes.eu.org/ T-MAC Duty Cycle • • • • Variable length duty cycle Transmit in bursts Maintain optimal active time under variable load Sleep after time of hearing nothing S-MAC Duty Cycle • • • Fixed duty cycle Frame time - limited by latency requirements and buffer space Active time configured to be long enough to handle highest expected load T-MAC Protocol Basics • • • • Burst communication schedule Messages are queued while node is asleep • Buffer capacity determines Frame Time RTS/CTS/<Data>/ACK • • Collision avoidance Reliability Active Period (Active Time) T-MAC Active Period • • Starts at scheduled intervals Ends when no Activation Event is heard for a time = TA • • • • • firing of a periodic frame timer reception of any data sensing of any communication activity end-of-transmission of own data or acknowledgment overhearding end of neighbor’s data exchange T-MAC Considerations • • • • Clustering and Synchronization RTS Operation and Choosing TA Overhearing Avoidance Asymmetric Communication Clustering and Synchronization • • From S-MAC protocol Virtual Clustering • • Frame schedules and SYNC packets define a node’s active time Shared with neighbors to ensure transmissions go to nodes that are awake RTS and TA • • • Fixed contention window at beginning of active time for RTS signaling RTS retry after loss (max of two times) TA > C + R + T Overhearing Avoidance • • Results in energy savings, but decreases max throughput (do not use if speed is required) Experiments show going to sleep to avoid overhearing makes nodes miss other RTS/CTS transmissions. When they wake-up, they cause interference collisions. Asymmetric Communication • • Most communication is unidirectional (node-to-sink) Early Sleeping Problem • • Future-Requestto-Send (FTRS) Full-buffer Priority • • • • Future RTS Nodes that lose RTS/CTS contention have opportunity to send FRTS Collides with empty DS packet of contention winner Requires increase in TA (increased energy usage) 75% throughput gain Full-buffer Priority • • When a node’s sender buffer is or is almost full, it can decide to ignore an incoming RTS, i.e., refuse to send a CTS reply, and sends its own RTS to a different node But... heavy load increases collisions rapidly Experimental Setup • • • • • OMNeT++ discrete event simulation package EYES nodes modeling 100 nodes on 10 x 10 grid; non-edge nodes have 8 neighbors each Local Unicasts Nodes-to-sink communication; shortest-path routing Results • • Simulations • • • • Homogenous local unicast Nodes-to-sink communication The effects of early sleep Event-based local unicast and node-to-sink reporting Real Implementation • Energy use Homogenous Local Unicast Nodes-to-sink Communication Early Sleeping Event-based Unicast and Node-to-sink Energy Consumption Future Work • • • Experimentation with FRTS and full-buffer priority to solve early sleeping problem Node mobility Virtual clustering and multi-hop synchronization Conclusions • • • Power consumption reductions achieved As much as 96% with low loads as compared to traditional protocols Improves upon S-MAC performance in volatile environments where message rates change with both time and place