An Adaptive Energy-Efficient and Low- Latency MAC for Data Gathering in Wireless Sensor Networks Gang Lu, Bhaskar Krishnamachari, and Cauligi S. Raghavendra Department of Electrical Engineering University of Southern California 18 th Intl. Parallel and Distributed Processing Symposium (IPDPS’04)

Outline Introduction DMAC Protocol Performance Evaluation Conclusions

Main Concern Primary goal: Energy efficiency Limited battery power Secondary goal: Latency, throughput, fairness Source of energy consumption Idle listening Collisions Overhearing Control packet overheads

Issues in S-MAC Main idea Eliminate idle listening Major shortcoming Long delay Fixed duty cycle Unadapted to the traffic variation Improvement Adaptive listening Challenge Data Forwarding Interruption (DFI) problem

DFI Problem Data forwarding process will stop at the node whose next hop is out of the overhearing range because it is in sleep mode Packets are queued until the next active period, which increases latency

DMAC Protocol Application Data gathering Motivation DFI problem Objective Design of a MAC protocol, where nodes can dynamically increase duty cycles and their nearby nodes keep asleep

DMAC Overview RxTxRxTxRxTxRxTxRxTxRxTxRxTxRxTx …… sleep data ack data ack data ack data ack data ack data ack time duty cycle adaptation data prediction more-to-send staggered wakeup schedule

Staggered Wakeup Schedule Nodes wake up sequentially like a chain reaction An interval comprises receiving, sending, and sleep periods RxTxRxTxRxTxRxTxRxTxRxTxRxTxRxTx …… sleep data ack data ack data ack data ack data ack data ack time

Staggered Wakeup Schedule (cont’d) Tx period = Rx period =  Tx: transmit data packet, receive ack packet Rx: receive data packet, transmit ack packet Collision avoidance  = BP + CW + DATA + SP + ACK BP: backoff period (like DIFS in IEEE ) CW: contention window SP: short period (like SIFS in IEEE ) BP > SP

Duty Cycle Adaptation Scenario A node has multiple packets to send at a sending slot Idea Increase own duty cycle Request other nodes on the multihop path to increase their duty cycles Solution slot-by-slot renewal mechanism More data flag (piggybacked in both data and ack packets)

Duty Cycle Adaptation (cont’d) RxTx RxTx time

Duty Cycle Adaptation (cont’d) RxTx RxTx Multiple packets time

Duty Cycle Adaptation (cont’d) RxTx RxTx data (more data) Multiple packets time

Duty Cycle Adaptation (cont’d) RxTx Rx ack (more data) time

Duty Cycle Adaptation (cont’d) RxTx Rx Tx data (more data) time

Duty Cycle Adaptation (cont’d) RxTx Rx Tx Rx ack (more data) time

Duty Cycle Adaptation (cont’d) RxTx Rx time data Tx

Duty Cycle Adaptation (cont’d) RxTx Rx time ack Tx

Duty Cycle Adaptation (cont’d) RxTx Rx Tx sleep time Rx Tx data

Data Prediction A BC SSSRxTx …… Rx SSSSS …… RxTx SSS S …… RxTx A B C 1 packet additional sending slot additional receiving slot data ack 33 33

A More-to-Send Packet MTS packet Explicit control packet Dest. local ID and a flag Request MTS (flag=1) /clear MTS (flag=0) The MTS packet incurs small increase in slot length, and energy consumption B A D C

More-to-Send Packet (cont’d) A node sends a request MTS when Channel is busy Receives a request MTS from its children A node sends a clear MTS when Buffer is empty All request MTSs received from children are cleared It sends a request MTS to its parent before and has not sent a clear MTS

Performance Evaluation --- Model Energy cost The total energy cost of deliver a certain number of packets from sources to the sink Latency End-to-end delay of a packet Delivery ratio The ratio of the successfully delivered packets to the total packets originating from all sources Radio bandwidth100 Kbps Radio transmission range250 m Radio interference range550 m Data packet length100 bytes Transmit power0.66 W Receive power0.395 W Idle power0.35 W MTS packet length3 bytes Receiving/sending slot (w/ MTS)10 ms (11 ms) Active period in SMAC20 ms Duty cycle10%

Multihop Chain Evaluation source Scenario (11 nodes) sink 200 m Increase linearly Non-next hop nodes have additional active periods

Traffic Load Evaluation --- Random Data Gathering Tree Sensor field: 1000m  500m area 50 nodes randomly deployed Sink is at the right bottom corner 5 sources are at the margin

Traffic Load Evaluation --- Latency full active CSMA/CA has the smallest delay for all traffic loads DMAC/MTS handle the highest traffic load with the smallest delay

Traffic Load Evaluation --- Energy DMAC and DMAC/MTS are two most energy efficient MAC protocols DMAC outperforms DMAC/MTS because of less overheads requested by MTS packets

Traffic Load Evaluation --- Delivery Ratio DMAC/MTS is better than other protocols SMAC and DMAC have lower delivery ratio when the traffic load is heavy

Scalability Evaluation 100 nodes randomly deployed in a 100m  500m area Traffic generation: 1 message per 3 seconds Sink: at the right bottom corner Sources: at the margin

Scalability Evaluation --- Latency and Energy

Energy  Latency SMAC: a general-purpose energy-efficient MAC protocol DMAC: not suitable for applications that require data exchange between arbitrary sensor nodes

Conclusions Techniques in DMAC protocol Staggered active/sleep schedule Duty cycle adaptation Data prediction More-to-send packet Advantages in DMAC protocol Tree-based data gathering Energy-efficient Low latency