A Transmission Control Scheme for Media Access in Sensor Networks Alec Woo and David Culler University of California at Berkeley Intel Research ACM SIGMOBILE 2001

7/19/2001ACM SIGMOBILE Sensor Network Scenario Characteristics: discovered spanning forest like topology base station as root of tree and traffic flows to the base station bi-directional connectivity sensors periodically sample the environment correlated traffic small packet size Operating mode: standalone network propagate sensory data into infrastructure infrastructure collects sensory data and distributes control back to the network

7/19/2001ACM SIGMOBILE Key Questions How should Media Access Control (MAC) protocols be designed for sensor networks? What are the metrics for MAC in a multi-hop scenario? How can we achieve these metrics with local algorithms over limited resources?

7/19/2001ACM SIGMOBILE Aggregate Bandwidth Traditional MAC metric channel capacity is a precious resource Maximize total delivered bandwidth from every node in the network to the base station

7/19/2001ACM SIGMOBILE Energy Efficiency Observation: Energy is the precious resource Goal: Minimize energy per unit of successful communication to base station while sustaining reasonable channel utilization Turn off radio whenever possible Avoid over commit the network Total energy spent in data propagation over a network Total packets received by the base station E =

7/19/2001ACM SIGMOBILE Fairness Challenge Challenge: want roughly equal data coverage Observation: originated traffic competes with route-thru traffic at odd with energy efficiency and aggregate bandwidth Goal: minimize variance in bandwidth delivered to base station

7/19/2001ACM SIGMOBILE Outline Introduction Metrics MAC protocol design Transmission control design Conclusion

7/19/2001ACM SIGMOBILE MAC Design Carrier Sense Multiple Access (CSMA) no extra control packets (energy efficient) Save energy: Shorten listening period as much as possible turn radio off during backoff Trade bandwidth for battery life Provide feedback to applications to desynchronize Backoff should signal application to shift phase of sampling Random delay before each transmission break close synchronization

7/19/2001ACM SIGMOBILE Hidden Nodes in Multi-hop Networks Occurs between every other pair of levels CSMA fails to detect Traditionally addressed with contention-based protocols, but Control packets (e.g. RTS/CTS/ACKs) induce high overhead given data packets are small ACKs can be free in multi-hop networks By hearing your parent forwards your packets Data aggregation is application specific Not adequate to solve hidden node problem in multi- hop case (Related Work: V. Bharghavan et al. MACAW)

7/19/2001ACM SIGMOBILE Avoid Hidden Node Corruption A B CD exploits application characteristics “A” refrains from sending for a packet time after parent transmits Hidden node cases like this may be avoided without use of control packets.

7/19/2001ACM SIGMOBILE Platform of Study Rene 4MHz, 8KB flash, 512B RAM 916MHz RF transceiver 10kbps feet range Sensors: temperature, light, magnetic field, acceleration Operating System: TinyOS tiny network stack and other communication support Small packets size (tens of bytes)

7/19/2001ACM SIGMOBILE Simulation Study No physical layer interference model Single hop scenario with 2 to 10 nodes All nodes hear each other Channel capacity is ~20 packet/s Offered load is 5 packet/s/node Compare CSMA with our proposed CSMA schemes with 3 different backoff mechanism Fixed backoff window Binary exponentially increase backoff window Binary exponentially decrease backoff window Empirical study of CSMA schemes validate simulation

7/19/2001ACM SIGMOBILE Aggregate Bandwidth

7/19/2001ACM SIGMOBILE Energy Spent in CSMA

7/19/2001ACM SIGMOBILE Fairness in CSMA Backoff and CSMA is sufficient to adapt transmission rate to available bandwidth

7/19/2001ACM SIGMOBILE Multi-hop Extensions Rate control module inserted between MAC and application Adapts data sampling rate to available bandwidth Balances demand for upstream bandwidth between local, originating traffic and route-thru traffic by adjusting transmission rate Multihop Merging traffic flow Provides a mechanism for progressive feedback deep down into the network

7/19/2001ACM SIGMOBILE Rate Control Mechanism snoop on route-thru traffic to estimate children (n) Open parameters ,  Apply for forwarding route-thru traffic Progressive feedback deep down into the network Packet loss rate provides natural damping effect x  +  /n R x p if fails if success x  +  /n x  +  /n Estimate n based on route-thru traffic Route-thru Traffic

7/19/2001ACM SIGMOBILE Simulation Settings No physical layer interference model Node offers 4 packet/s Hidden unless linked by an edge Nodes start at the same time Compare: CSMA RTS/CTS with CSMA CSMA CSMA with ARC BS 1b 1a1c 2 3c 3a 3b 4b 4a 5b5a

7/19/2001ACM SIGMOBILE Available Bandwidth Observation: a route-thru packet occupies a cell 3 times channel capacity ~20packet/s in 2’s cell is shared by 3 x all route-thru traffic + 2 x 2’s own traffic + 1b’s traffic fair allocation of available bandwidth is ~0.6packet/s/node BS 1b 1a1c 2 3c 3a 3b 4b 4a 5b5a ARC adapts from offered load of 4 packet/s/node

7/19/2001ACM SIGMOBILE Delivered Bandwidth for each Node (Simulation) BS 1b1a1c 2 3c3a3b 4b4a 5b5a

7/19/2001ACM SIGMOBILE Delivered Bandwidth for each Node (Empirical) BS 1b1a1c 2 3c3a3b 4b4a 5b5a cell boundaries are no longer the same as in simulation

7/19/2001ACM SIGMOBILE Taming the Transmission Rate (Empirical) BS 1b1a1c 2 3c3a3b 4b4a 5b5a fair allocation of available bandwidth is ~0.6 packet/s/node

7/19/2001ACM SIGMOBILE Other ARC Results Proposed CSMA with ARC scheme: Aggregate bandwidth: ~ 60% of proposed CSMA without ARC scheme Energy efficiency: ~ 50% of proposed CSMA at a low  Fairness: 5 – 10 times lower variance Additional details are in the paper

7/19/2001ACM SIGMOBILE Conclusion Sensor networks characteristics differ from traditional settings enough to require revisiting the basic protocols MAC design fairness and energy efficiency goals modified CSMA shown effective Transmission control local adaptive scheme on originating traffic effective Implemented and evaluated on simulation and real networked sensors Each node achieves 20% of multi-hop channel capacity

7/19/2001ACM SIGMOBILE TinyOS and Hardware Platform

7/19/2001ACM SIGMOBILE Radio Packet Adaptive Rate Control Implementation RFM Radio byte i2c Magnetic field Temp Messaging Layer clocks bit byte packet Multi-hop Routing Layer sensing application application HW SW ADC messaging multihop routing 3.) Application Originates Message Thru-Route Message 1.) Clock Event 2.) Acquires Data CSMA LFSR ARC (588b)

7/19/2001ACM SIGMOBILE Importance of phase shift