An EnergyEfficient MAC Protocol for Wireless Sensor Networks

An Energy-Efficient MAC Protocol for Wireless Sensor Networks Wei Ye, John Heidemann, Deborah Estrin -- Adapted the authors’ Infocom 2002 talk

Introduction v. Wireless sensor network • Special ad hoc wireless network • Large number of nodes w/ sensors & actuators • Battery-powered nodes energy efficiency • Unplanned deployment self-organization • Node density & topology change robustness v. Sensor-net applications • Nodes cooperate for a common task • In-network data processing

Medium Access Control in Sensor Nets v Important attributes of MAC protocols 1. 2. 3. 4. 5. 6. 7. Collision avoidance Energy efficiency Scalability in node density Latency Fairness Throughput Bandwidth utilization Primary Secondary

Energy Efficiency in MAC v. Major sources of energy waste Idle listening ü Energy consumption of typical 802. 11 WLAN cards idle: receive: send — 1: 1. 05: 1. 4, 1: 2: 2. 5 (Stemm 1997) ü Example: directed diffusion (Intanagonwiwat 2000) 0. 14 0. 12 0. 1 Diffusion Flooding Omniscient Multicast 0. 08 0. 06 0. 04 0. 02 00 50 100 150 200 250 300 Network Size Over always-listening MAC Average Dissipated Energy (Joules/Node/Received Event) Average Dissipated Energy • 0. 018 0. 016 0. 014 0. 012 0. 01 0. 008 0. 006 0. 004 0. 002 00 Flooding Omniscient Multicast Diffusion 50 100 150 200 250 Network Size Over energy-aware MAC 300

Reminder: IEEE 802. 11 MAC v Very popular wireless MAC protocol v Two modes: DCF (distributed coordination function) & PCF (point coordination function) v DCF is based on CSMA/CA ≈ CSMA + MACA • RTS-CTS-DATA-ACK • Physical carrier sensing + NAV (network allocation vector) containing time value that indicates the duration up to which the medium is expected to be busy due to transmissions by other nodes • Every packet contains the duration info for the remainder of the message • Every node overhearing a packet continuously updates its own NAV for virtual carrier sensing v IFS (inter frame spacing) • Short IFS (SIFS), PCF IFS (PIFS), DCF IFS (DIFS), Extended IFS (EIFS)

Big problem with existing wireless MACs v. Idle listening • Does anybody send me a RTS? • Does anyone send CTS to my neighbor which I want to communicate with? • Huge energy consumption! v. PAMAS (Power Aware Medium Access with Signaling) • Separate radio channel for RTS/CTS • Sleep for the duration of the transmission indicated in the control packets S-MAC aims to achieve this without requiring a separate channel for RTS & CTS

Idle listening in 802. 11 v RTS & CTS only reserves the medium for the first data fragment & the first ACK v The 1 st fragment & ACk reserves the medium for the 2 nd fragment and so on v After overhearing a fragment or an ACK, a neighboring node knows that there is one more fragment to be sent • It has to keep listening until all the fragments are sent • Promote fairness; If the sender fails to get ACK after • sending a fragment, it must give up the transmission and recondtend for the medium For in-network processing, an entire message is needed in sensor networks 802. 11 may cause a largee delay

Energy Efficiency in MAC v. Major sources of energy waste (cont. ) • Idle listening Dominant in sensor nets üLong idle time when no sensing event happens • Collisions • Control overhead • Overhearing Common to all wireless networks v. Reduce energy consumption from all above sources v. Combine benefits of TDMA + contention protocols

Sensor-MAC (S-MAC) Design v. Tradeoffs Latency Fairness Energy v. Major components in S-MAC • • Periodic listen and sleep Collision avoidance Overhearing avoidance Massage passing

Periodic Listen and Sleep v. Problem: Idle listening consumes significant energy v. Solution: Periodic listen and sleep listen sleep • Turn off radio when sleeping • Reduce duty cycle to ~ 10% (200 ms on/2 s off) Latency Energy

Periodic Listen and Sleep v. Schedules can differ Node 1 Node 2 listen sleep listen sleep • Prefer neighboring nodes have same schedule — easy broadcast & low control overhead Schedule 1 Schedule 2 Border nodes: two schedules broadcast twice

Periodic Listen and Sleep - Choosing and Maintaining Schedules v Each nodes has a schedule table - Stores the schedules of all its known neighbors 1. Listens for a certain amount of time - If it doesn’t hear a schedule, it becomes a synchronizer - It randomly chooses a schedule and broadcast it in SYNC message - SYNC message indicates that it will go to sleep after t seconds

Periodic Listen and Sleep - Choosing and Maintaining Schedules v 2. If the node receives a schedule before choosing its own schedule, it follows that schedule - follower - waits for a random delay td and re-broardcasts this schedule, indicating that it will sleep in t – td v 3. If a node receives a different schedule after it selects and broadcast (border node) - adopts both schedules - less time to sleep - consume more energy

Periodic Listen and Sleep - Schedule Synchronization v. Remember neighbors’ schedules — to know when to send to them v. Each node broadcasts its schedule for multiple periods of sleeping and listening • Update period can be long, e. g. , tens of seconds v. Re-sync when receiving a schedule update v. Sync packets also serve as beacons for new nodes to join a neighborhood

Periodic Listen and Sleep Listen Receiver For SYNC For RTS Sleep SYNC Sender 1 Sleep CS RTS Sender 2 CS Send data if CTS received RTS SYNC Sender 3 CS CS Send data if CTS received Figure 2. Timing relationship between a receiver and different senders

Collision Avoidance v. Problem: Multiple senders want to talk v. Options: Contention vs. TDMA v. Solution: Similar to IEEE 802. 11 ad hoc mode (DCF) • • Physical and virtual carrier sense Randomized backoff time RTS/CTS for hidden terminal problem RTS/CTS/DATA/ACK sequence

Overhearing Avoidance v Problem: Receive packets destined to others • • In 802. 11, each node keeps listening to all transmissions from its neighbors for virtual carrier sensing Each node should overhear a lot of packets not destined to itself • • Basic idea from PAMAS (Singh, Raghavendra 1998) But S-MAC only uses in-channel signaling • • All immediate neighbors of sender and receiver S-MAC lets interfering nodes go to sleep after they hear an RTS or CTS ü DATA packets are normally much longer than control packets v Solution: Sleep when neighbors talk v Who should sleep? v How long to sleep? • • The duration field in each packet informs other nodes the sleep interval After hearing the RTS/CTS packet destined to a node, all the other immediate neighbors of both the sender and receiver should sleep until the NAV becomes zero

Message Passing v Problem: Sensor net in-network processing requires entire message v Solution: Don’t interleave different messages • • • Long message is fragmented & sent in burst RTS/CTS reserve medium for entire message Fragment-level error recovery — ACK — Extend Tx time and re-transmit immediately if no ACK is received v Other nodes sleep for whole message time v Fragmentation in IEEE 802. 11 • • No indication of entire time — other nodes keep listening If ACK is not received, give up Tx & recontend for medium — fairness Fairness Energy Msg-level latency

Energy Savings vs. Increased Latency v. General delay factors • • • Carrier sensor delay Backoff delay Transmission delay Propagation delay Processing delay Queuing delay v. Sleep delay: S-MAC specific • A sender has to wait until the receiver wakes up

Sleep delay and Relative energy savings v. Average sleep delay on the sender is 0. 5 Tframe = 0. 5(Tlisten + Tsleep) Duty Cycle v. Relative energy saving • Es = Tsleep/Tframe = 1 – Tlisten/Tframe

Implementation on Testbed Nodes v. Platform Motes (UC Berkeley) 8 -bit CPU at 4 MHz, 8 KB flash, 512 B RAM 916 MHz radio Tiny. OS v. Compared MAC modules 1. IEEE 802. 11 -like protocol w/o sleeping 2. Message passing with overhearing avoidance 3. S-MAC (2 + periodic listen/sleep)

Experiments v. Topology and measured energy consumption on source nodes Source 1 Sink 1 Source 2 Sink 2 • Each source node sends 10 messages — Each message has 400 B in 10 fragments • Measure total energy over time to send all messages

Conclusions v S-MAC offers significant energy efficiency over always-listening MAC protocols v Sleep delay can be accumulated in intermediate hops Potential problem for real-time sensing in multiple environments v Future Plans • Measurement of throughput and latency üThroughput reduces due to latency, contention, control overhead and channel noise • Experiments on large testbeds ü~100 Motes, ~30 embedded PCs w/ Mote. NIC v. URL: http: //www. isi. edu/scadds/

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