A Transmission Control Scheme for Media Access in Sensor Networks

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. 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 ACM SIGMOBILE 2001

3. 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? ACM SIGMOBILE 2001

4. 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 ACM SIGMOBILE 2001

5. 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 = ACM SIGMOBILE 2001

6. 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: • minimizevariance in • bandwidth delivered • to base station ACM SIGMOBILE 2001

7. Outline • Introduction • Metrics • MAC protocol design • Transmission control design • Conclusion ACM SIGMOBILE 2001

8. 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 ACM SIGMOBILE 2001

9. Hidden Nodes inMulti-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) ACM SIGMOBILE 2001

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

11. 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) ACM SIGMOBILE 2001

12. 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 ACM SIGMOBILE 2001

13. Aggregate Bandwidth ACM SIGMOBILE 2001

14. Energy Spent in CSMA ACM SIGMOBILE 2001

15. Fairness in CSMA • Backoff and CSMA is sufficient to adapt transmission rate to • available bandwidth ACM SIGMOBILE 2001

16. 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 ACM SIGMOBILE 2001

17. Route-thru Traffic x  x  x  + /n + /n + /n 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 if fails if success R x p Estimate n based on route-thru traffic ACM SIGMOBILE 2001

18. Simulation Settings BS • 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 1a 1c 1b 2 3c 3a 3b 4a 4b 5a 5b ACM SIGMOBILE 2001

19. BS 1a 1c 1b 2 3c 3a 3b 4a 4b 5a 5b 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 ARC adapts from offered load of 4 packet/s/node ACM SIGMOBILE 2001

20. BS 1a 1b 1c 2 3a 3b 3c 4a 4b 5a 5b Delivered Bandwidth for each Node (Simulation) ACM SIGMOBILE 2001

21. BS 1a 1b 1c 2 3a 3b 3c 4a 4b 5a 5b Delivered Bandwidth for each Node (Empirical) • cell boundaries are no longer the same as in simulation ACM SIGMOBILE 2001

22. BS 1a 1b 1c 2 3a 3b 3c 4a 4b 5a 5b Taming the Transmission Rate(Empirical) • fair allocation of available bandwidth is ~0.6 packet/s/node ACM SIGMOBILE 2001

23. 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 ACM SIGMOBILE 2001

24. 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 ACM SIGMOBILE 2001

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

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

27. Importance of phase shift ACM SIGMOBILE 2001