Oregon Graduate Institute1 Sensor and energy-efficient networking CSE 525: Advanced Networking Computer Science and Engineering Department Winter 2004

Oregon Graduate Institute2 Energy efficient MAC An energy-efficient MAC protocol for wireless sensor networks by W. Ye, J. Heidemann, and D. Estrin An Energy Efficient MAC Protocol for Wireless LANs by E. Jung and N. Vaidya Energy efficient communications in ad hoc networks using directional antennas by A. Spyropoulos and C. Raghavendra

Oregon Graduate Institute3 Motivation Challenged Networks are normally battery operated, hence power limited Other goals: Self-configuration good scalability collision avoidance Fairness and latency are NOT important

Oregon Graduate Institute4 Conventional MAC Layer An access mechanism for nodes that ensure that NO two nodes have access to the communication channel concurrently If air is clear, send; for receiving, wait How do we know when to receive? Listen always! Usually designed to allow for maximum throughput Hence not energy-efficient

Oregon Graduate Institute5 Sources of energy inefficiency Collision Overhearing Control packet overhead Idle listening

Oregon Graduate Institute6 #1: S (sensor) -MAC It is contention based Tries to reduce wastage of energy from all four sources of energy inefficiency Collision – by using RTS and CTS Overhearing – by switching the radio off when the transmission is not meant for that node Control overhead – by Message Passing Idle listening – by Periodic Sleep

Oregon Graduate Institute7 There is no FREE dinner! In exchange there is some reduction in both per-hop fairness and latency But this does not necessarily result in lower end-to-end application fairness and latency – Which acceptable in Challenged Network

Oregon Graduate Institute8 Periodic Listen and Sleep Each node go into periodic sleep mode during which it switches the radio off and sets a timer to awake later To reduce control overhead, neighboring nodes are synchronized (i.e. Listen and sleep together)

Oregon Graduate Institute9 Periodic Listen and Sleep Broadcasting schedule to all its immediate neighbor neighbor can have different schedules. If multiple neighbor want to talk to a node, use (RTS/CTS) like contention scheme After they start data transmission, they do not follow their sleep schedules until they finish transmission.

Oregon Graduate Institute10 Choosing and Maintaining Schedules Each node maintains a schedule table that stores schedules of all its known neighbors. To establish the initial schedule (at the startup) following steps are followed: A node first listens for a certain amount of time. If it does not hear a schedule from another node, it randomly chooses a schedule and broadcast its schedule immediately. This node is called a SYNCHRONIZER.

Oregon Graduate Institute11 If a node receives a schedule from a neighbor before choosing its own schedule, it just follows this neighbor’s schedule. This node is called a FOLLOWER and it waits for a random delay and broadcasts its schedule. Choosing and Maintaining Schedules

Oregon Graduate Institute12 Border Nodes If a node receives a different schedule after it selects and broadcasts its own schedule, it adopts both schedules Border nodes consume more energy

Oregon Graduate Institute13 Maintaining Synchronization Timer synchronization among neighbors are needed to prevent the clock drift. Synchronizer needs to periodically send SYNC to its followers. If a follower has a neighbor that has a different schedule with it, it also needs update that neighbor.

Oregon Graduate Institute14 Maintaining Synchronization Time of next sleep is relative to the moment that the sender finishes transmitting the SYNC packet Receivers will adjust their timer counters immediately after they receive the SYNC packet Listen interval is divided into two parts: one for receiving SYNC and other for receiving RTS

Oregon Graduate Institute15 Timing Relationship of Possible Situations

Oregon Graduate Institute16 Collision/Overhearing Avoidance Collision Avoidance: RTS/CTS NAV: virtual carrier sense Physical carrier sense Overhearing Avoidance Go to sleep if overhear RTS/CTS packet NAV Avoid overhearing “long” data packet All immediate neighbors of both sender and receiver go to sleep

Oregon Graduate Institute17 Message Passing Short packets are used in wireless networks More robust Overhead of control packets (RTS/CTS) In-network processing requires a complete msg Divide the long message into small fragments and transmit them in a burst. RTS/CTS/Data1/Ack1/Data2/Ack2/…/DataN/AckN If a packet is lost, extend the duration and immediately retransmit it Data and Acknowledge include a field of duration

Oregon Graduate Institute18 Energy Savings vs. Increased Latency Delay includes: Carrier sense Back off Transmission Propagation Processing Queuing Sleeping

Oregon Graduate Institute19 Conclusions and Future work S-MAC has good energy conserving properties comparing to IEEE Future work Analytical study on the energy consumption and latency Analyze the effect of topology changes

Oregon Graduate Institute20 #2: EEM for WLAN Optimize PSM in the DCF in IEEE Standard Dynamic PSM (DPSM) What is PSM? PSM for DCF, divide time into intervals called beacon intervals, each node in power save mode periodically wakes up at the beginning of beacon interval for a duration called ATIM [Ad-hoc Traffic Indication Message] window to exchange control information.

Oregon Graduate Institute21 What’s wrong? Fixed ATIM window does not perform good in all situation Adaptive mechanism to dynamically adjust ATIM window Synchronization of beacon interval of initially partitioned network Not addressed; it assumes, there is a way to synchronize

Oregon Graduate Institute22

Oregon Graduate Institute23 Dynamic - PSM Each node chose its own ATIM window size based on network traffic condition Allow to increase and decrease the ATIM window dynamically; ATIMmin is defined as minimum level Move into doze state, after completing packet transmission, if remaining time is not “too-small” – Save Energy

Oregon Graduate Institute24 Dynamic - PSM Piggyback own window size on all transmitted packets Packet marking use to adjust ATIM window

Oregon Graduate Institute25 Dynamic - PSM Rules to increase ATIM Pending packet can’t be announce in current window time Based on piggyback information Receiving an ATIM frame after ATIM window Received a marked packet Rules to decrease ATIM If node successfully announce ATIM frame and none of the above rule satisfied

Oregon Graduate Institute26 Result Simulation result shows that DPSM improves energy consumption without degrading performance Only when energy gain from doze state is more then energy loss [overhead of beacon, ATIM and ATIM-ACK frame] Energy save in doze mode is 96% compare to idle mode.

Oregon Graduate Institute27 #3: Directional Antenna Energy efficient routing and scheduling algorithm in ad hoc network where each node has single directional antenna. Using topology consisting of all the possible link in the network, find shortest cost path to be energy efficient. Calculate the amount of traffic that has to go over each link and find the maximum amount of time each link can be up. Schedule node’s transmissions, trying to minimize the total time it takes for all possible Tx-Rx pairs to communicate with each other.

Oregon Graduate Institute28 Conclusion and future work Benefits of using directional antennas in ad hoc networks. Energy efficient algorithm for routing. Scheduling 45% improvement in network life time that is achieved by using energy-aware routing. Future work Multicasting and broadcasting in ad hoc networks with directional antennas Scheduling algorithm in this context

Oregon Graduate Institute29 Related work TDMA Based Naturally have a duty cycle It is not easy to change the slot assignment dynamically, hence scalability is not as good as contention based Requires nodes to form real communication clusters and managing inter-cluster communication is difficult Out-of band solutions [PAMAS]: Requires extra band for signaling

Oregon Graduate Institute30 Questions?