1. Mitigating Congestion in Wireless Sensor Networks Bret Hull, Kyle Jamieson, Hari Balakrishnan MIT Computer Science and Artificial Intelligence Laborartory ACM SenSys 2004 Speaker: Hao-Chun Sun

2. Outline Introduction Introduction Mitigating Congestion Mitigating Congestion Hop-by-hop flow control Hop-by-hop flow control Source rate limiting Source rate limiting Prioritized MAC layer Prioritized MAC layer Experimental Evaluation Experimental Evaluation Conclusion Conclusion

3. Introduction Myriad types of traffic in WSNs Myriad types of traffic in WSNs Simple periodic reports Simple periodic reports Unpredictable bursts of messages Unpredictable bursts of messages Channel quality Channel quality Noise Noise Traffic density Traffic density Data transmissions over different radio links interact with each other. Data transmissions over different radio links interact with each other.

4. Introduction The symptoms of congestion in wired networks The symptoms of congestion in wired networks Buffer drops Buffer drops Increased delays Increased delays The solution of congestion in wired networks The solution of congestion in wired networks End-to-end rate (window) adaptation End-to-end rate (window) adaptation Signaling techniques Signaling techniques

5. Introduction A key symptom of congestion in WSNs A key symptom of congestion in WSNs Degradation in the quality of the radio channel caused by an increase in the amount of traffic being sent in other parts of the network. Degradation in the quality of the radio channel caused by an increase in the amount of traffic being sent in other parts of the network. Traffic traversing any given part of the network has a deleterious impact on channel quality and loss rates in other parts of the network. Traffic traversing any given part of the network has a deleterious impact on channel quality and loss rates in other parts of the network. Poor, time-varying channel quality Poor, time-varying channel quality Asymmetric communication channels Asymmetric communication channels Hidden terminals Hidden terminals

6. Introduction Symptoms of congestion — Loss rate Symptoms of congestion — Loss rate

7. Introduction Symptoms of congestion — Starvation Symptoms of congestion — Starvation

8. Introduction Congestion collapse consequence Congestion collapse consequence

9. Introduction Motivation Motivation Tree congestion control techniques Tree congestion control techniques Fusion Fusion Hop-by hop flow control Hop-by hop flow control Source rate limiting scheme Source rate limiting scheme Prioritized MAC layer Prioritized MAC layer Experimental evaluation Experimental evaluation In-door environment In-door environment

10. Introduction Metrics Metrics Pi i ζ>1

11. Mitigating Congestion Fusion integrates three techniques Fusion integrates three techniques Hop-by hop flow control Hop-by hop flow control Prevent nodes from transmitting. Prevent nodes from transmitting. Source rate limiting Source rate limiting Prevent unfairness. Prevent unfairness. Prioritized MAC layer Prioritized MAC layer Prevent buffer overflow Prevent buffer overflow Focus topology Focus topology Single-sink Single-sink Spanning – tree topology Spanning – tree topology

12. Mitigating Congestion Hop-by-hop flow control Hop-by-hop flow control Congestion detection Congestion detection Each sensor sets a congestion bit in the header of every outgoing packet Each sensor sets a congestion bit in the header of every outgoing packet Queue occupancy Queue occupancy Queue space falls below a water mark α=0.25 Queue space falls below a water mark α=0.25 Channel sampling Channel sampling Sample the state of the channel at a fixed interval. Sample the state of the channel at a fixed interval. Channel utilization rises above a certain level. Channel utilization rises above a certain level.

13. Mitigating Congestion Hop-by-hop flow control Hop-by-hop flow control Congestion mitigation Congestion mitigation Nodes in a given radio neighborhood Nodes in a given radio neighborhood throttle their transmissions to prevent queues at their next-hop node from overflowing. Feedback mechanism Feedback mechanism i Sink Pi Ci Congestion bit=1 Source Congestion bit=0 Application adaptation

14. Mitigating Congestion Source rate limiting Source rate limiting Unfairness problem Unfairness problem There is a natural tendency for the network to deliver traffic originating close to a sink at the expense of traffic sourced deeper inside the network. There is a natural tendency for the network to deliver traffic originating close to a sink at the expense of traffic sourced deeper inside the network. S2 S3 S4 S1 sink source

15. Mitigating Congestion Source rate limiting Source rate limiting Token bucket scheme Token bucket scheme Completely passive approach Completely passive approach Each node listens to its parent Each node listens to its parent forward packets. forward packets. N: total number of source N: total number of source Token: Parents forward Token: Parents forward N packets. N packets. Once transmission cost one Once transmission cost one token. token. i Sink Pi S1 Source (S1, S2) N=2 S2

16. Mitigating Congestion Prioritized MAC layer Prioritized MAC layer Network layer mechanisms cannot always react to congestion fast enough to prevent buffer losses. Network layer mechanisms cannot always react to congestion fast enough to prevent buffer losses. Standard CSMA MAC gives all sensors to transmit an equal chance of success. Standard CSMA MAC gives all sensors to transmit an equal chance of success. Congestion contention window=1/4 non-congestion contention window Congestion contention window=1/4 non-congestion contention window i C1 C2 C3 C4 C5 Pi High fan-in scenario High priority access

17. Mitigating Congestion The hidden terminal problem The hidden terminal problem RTS/CTS exchange RTS/CTS exchange Data packets are usually small in sensor network. Data packets are usually small in sensor network. “ Transmission Control Scheme for Media Access in Sensor Networks. ” “ Transmission Control Scheme for Media Access in Sensor Networks. ” Alleviating hidden terminals in tree-based topologies. Alleviating hidden terminals in tree-based topologies. When a node overhears its parent finish sending a packet, it waits for one packet-time plus a guard time, to avoid a likely hidden terminal collision with its grandparent. When a node overhears its parent finish sending a packet, it waits for one packet-time plus a guard time, to avoid a likely hidden terminal collision with its grandparent.

18. Experimental Evaluation Environment Environment 55-node indoor wireless sensor network testbed. 55-node indoor wireless sensor network testbed. Each node is a Crossbow MICA2. Each node is a Crossbow MICA2. Area: 16076 square feet on one floor of office building Area: 16076 square feet on one floor of office building Power level: -10dbm Power level: -10dbm Packet size: 36 bytes Packet size: 36 bytes Retransmission: 3 times Retransmission: 3 times Queue Size: 8 Queue Size: 8

19. Experimental Evaluation Summary of congestion control strategies Summary of congestion control strategies

20. Experimental Evaluation Fusion

21. Experimental Evaluation Fusion

22. Experimental Evaluation No congestion control

23. Experimental Evaluation Periodic workload: Network efficiency Periodic workload: Network efficiency

24. Experimental Evaluation Periodic workload: Imbalance (4 pps) Periodic workload: Imbalance (4 pps)

25. Experimental Evaluation Periodic workload: Throughput and Fairness Periodic workload: Throughput and Fairness

26. Experimental Evaluation Periodic workload: Throughput and Fairness (2pps) Periodic workload: Throughput and Fairness (2pps)

27. Experimental Evaluation Periodic workload: Throughput and Fairness Periodic workload: Throughput and Fairness

28. Experimental Evaluation Periodic workload: Latency Periodic workload: Latency

29. Experimental Evaluation Periodic workload: Sources of loss Periodic workload: Sources of loss

30. Experimental Evaluation Periodic workload: Sources of loss Periodic workload: Sources of loss

31. Experimental Evaluation Correlated-event workload: Network efficiency Correlated-event workload: Network efficiency

32. Experimental Evaluation Correlated-event workload: Drop rate Correlated-event workload: Drop rate

33. Experimental Evaluation Correlated-event workload: Latency Correlated-event workload: Latency

34. Conclusion This paper presents an experimental evaluation of three complementary congestion control strategies for WSNs. This paper presents an experimental evaluation of three complementary congestion control strategies for WSNs. Our results show that hop-by-hop flow control with a simple queue occupancy-based congestion detection method offers substantial efficiency improvements for all types of workloads and utilization levels. Our results show that hop-by-hop flow control with a simple queue occupancy-based congestion detection method offers substantial efficiency improvements for all types of workloads and utilization levels. Implementing a rate-limiting policy results in substantial improvements to fairness. Implementing a rate-limiting policy results in substantial improvements to fairness. MAC enhancements support the operation of hop-by- hop flow control. MAC enhancements support the operation of hop-by- hop flow control.

35. Conclusion We present Fusion, a congestion control mechanism that combines rate limiting, hop- by-hop flow control, and a prioritized MAC. We present Fusion, a congestion control mechanism that combines rate limiting, hop- by-hop flow control, and a prioritized MAC. Our results show the efficacy of Fusion under a variety of workloads on a 55-node deployment. Our results show the efficacy of Fusion under a variety of workloads on a 55-node deployment.