Introduction Energy Efficient Channel Access Protocols l l l Sensor networks are a special class of multi hop wireless networks Energy conservation is very critical in improving the life time of the networks Some of the interesting features are Ad hoc deployment Very low or no mobility Motivation l l l l Major source of energy consumption is the radio Energy wastage due to idle listening and collisions Today s radios have a special low power standby mode or sleep mode to save power Switching the radio to sleep mode whenever possible could achieve potential savings Measurement on WLAN Cards Measurement on WLAN Cards Sensor Radios RFM TR1000 S MAC l l S MAC Sensor MAC l l Contention based MAC Energy savings by periodic sleep overhearing avoidance and message passing SYNC packets are used to maintain sleep schedules Less throughput and per node fairness S MAC Overview l l l Time is divided in to cycles of listen and sleep intervals Schedules are established such that neighboring nodes have synchronous sleep and listen periods SYNC packets are exchanged periodically to maintain schedule synchronization SMAC Operation l l S MAC Operation SYNC packets are transmitted only during the SYNC period long enough to send 1 SYNC packet of the listen interval Data transmissions are initiated by sending RTS during the DATA period long enough to send an RTS and overhear a CTS of the listen interval Schedule Establishment l l l l l Node listens for certain amount of time If it does not hear a schedule it chooses a time to sleep and broadcast this information immediately This node is called the Synchornizer If a node receives a schedule before establishing its schedule it just follows the received schedule If a node receives a different schedule after it has established its schedule it listens for both the schedules S MAC Features l Collision Avoidance l Overhearing Avoidance l Message Passing Similar to 802 11 RTS CTS handshake All the immediate neighbors of the sender and receiver goes to sleep Long messages are broken down in to smaller packets and sent continuously once the channel is acquired by RTS CTS handshake Increases the sleep time but leads to fairness problems Features l TRAMA Traffic Adaptive Medium Access Control Protocol l l Collision freedom by distributed election based on Neighborhood Aware Contention Resolution NCR Traffic adaptive scheduling to increase the channel utilization Radio mode control for energy efficiency Overview l l l Time slot organization Single time slotted channel access for both data and signaling Organized as sections of random and scheduled access periods Random access period used for signaling and scheduled access period used for data transmission Neighborhood aware Contention Resolution NCR NCR l l l Each node maintains two hop neighbor information For every contention slot all the nodes compute priorities based on the unique node id n and current time slot id t i as p n randomhash n t i l The node with the highest priority among the two hop neighborhood is elected as the transmitter l l NCR prevents collisions due to hidden terminals A set of protocols based on NCR is developed by Lichun and JJ for schedulingbased channel access in ad hoc networks TRAMA uses NCR to elect the transmitter To elect the receiver TRAMA uses the schedule information announced by the transmitter Components of TRAMA l l l Neighbor Protocol NP Schedule Exchange Protocol SEP Adaptive Election Algorithm AEA Neighbor Protocol l l l Packet Formats Main Function Gather two hop neighborhood information by using signaling packets Incremental neighbor updates to keep the size of the signaling packet small Periodically operates during random access period Schedule Exchange Protocol SEP l l l l Schedule consists of list of intended receivers for future transmission slots Schedules are established based on the current traffic information at the node Propagated to the neighbors periodically SEP maintains consistent schedules for the one hop neighbors Schedule Packet Format Adaptive Election Algorithm AEA l l l Experimental Setup l l Performance analysis by extensive simulation using Qualnet Compared the performance with both contention based protocols IEEE802 11 CSMA and S MAC and schedulingbased protocols NAMA Decides the node state as either Transmit Receive or Sleep Uses the schedule information obtained by SEP and a modified NCR to do the election Nodes without any data to send are removed from the election process thereby improving the channel utilization Simulation Setup l l l l Randomly placed 50 nodes in 500x500m area Typical sensor radio TR1000 with 100m range and 115 2kbps Data size 512 bytes Two different scenarios Scenarios l Sensor Scenario Synthetic scenario Synthetic traffic generated at the MAC level based on Poisson arrivals l Data gathering application Sink collects information from sensors by sending out a query Different placements for sink Reverse path routing for sending data back to sink Performance Metrics l Average Packet Delivery Ratio l l Ratio of number of packets delivered to the number of packets sent Average Queuing Delay Percentage Sleep Time l Delivery Ratio Synthetic Percentage of time nodes can to be put to sleep mode Average Length of Sleep Time A measure of energy savings as longer sleeps involves less switching and hence less transient power consumption due to switching Unicast Traffic Broadcast Traffic Energy Savings synthetic Percentage Sleep Delivery Ratio Sensor Sleep Interval Energy Savings sensor Corner Sink Conclusion l l l Percentage Sleep Sleep Time Center Sink Significant improvement in delivery ratio in all scenarios when compared to contention based protocols Significant energy savings compared to S MAC which incurs more switching Acceptable latency and traffic adaptive Future Works l l Traffic predictions to improve the delay performance Predicting the transmitter without transmitting entire bitmaps based on history to reduce scheduling 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