1. 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

2. 7/19/2001ACM SIGMOBILE 20012 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

3. 7/19/2001ACM SIGMOBILE 20013 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?

4. 7/19/2001ACM SIGMOBILE 20014 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

5. 7/19/2001ACM SIGMOBILE 20015 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 =

6. 7/19/2001ACM SIGMOBILE 20016 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. 7/19/2001ACM SIGMOBILE 20017 Outline Introduction Metrics MAC protocol design Transmission control design Conclusion

8. 7/19/2001ACM SIGMOBILE 20018 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

9. 7/19/2001ACM SIGMOBILE 20019 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)

10. 7/19/2001ACM SIGMOBILE 200110 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.

11. 7/19/2001ACM SIGMOBILE 200111 Platform of Study Rene 4MHz, 8KB flash, 512B RAM 916MHz RF transceiver 10kbps 1 - 100 feet range Sensors: temperature, light, magnetic field, acceleration Operating System: TinyOS tiny network stack and other communication support Small packets size (tens of bytes)

12. 7/19/2001ACM SIGMOBILE 200112 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 802.11 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

13. 7/19/2001ACM SIGMOBILE 200113 Aggregate Bandwidth

14. 7/19/2001ACM SIGMOBILE 200114 Energy Spent in CSMA

15. 7/19/2001ACM SIGMOBILE 200115 Fairness in CSMA Backoff and CSMA is sufficient to adapt transmission rate to available bandwidth

16. 7/19/2001ACM SIGMOBILE 200116 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

17. 7/19/2001ACM SIGMOBILE 200117 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

18. 7/19/2001ACM SIGMOBILE 200118 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 802.11 CSMA CSMA with ARC BS 1b 1a1c 2 3c 3a 3b 4b 4a 5b5a

19. 7/19/2001ACM SIGMOBILE 200119 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

20. 7/19/2001ACM SIGMOBILE 200120 Delivered Bandwidth for each Node (Simulation) BS 1b1a1c 2 3c3a3b 4b4a 5b5a

21. 7/19/2001ACM SIGMOBILE 200121 Delivered Bandwidth for each Node (Empirical) BS 1b1a1c 2 3c3a3b 4b4a 5b5a cell boundaries are no longer the same as in simulation

22. 7/19/2001ACM SIGMOBILE 200122 Taming the Transmission Rate (Empirical) BS 1b1a1c 2 3c3a3b 4b4a 5b5a fair allocation of available bandwidth is ~0.6 packet/s/node

23. 7/19/2001ACM SIGMOBILE 200123 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

24. 7/19/2001ACM SIGMOBILE 200124 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

25. 7/19/2001ACM SIGMOBILE 200125 TinyOS and Hardware Platform http://tinyos.millennium.berkeley.edu

26. 7/19/2001ACM SIGMOBILE 200126 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)

27. 7/19/2001ACM SIGMOBILE 200127 Importance of phase shift