1. A Low-Latency and Energy-Efficient Algorithm for Convergecast in Wireless Sensor Networks Authors Sarma Upadhyayula, Valliappan Annamalai, Sandeep Gupta Presented by Bin Wang Arizona State University

2. Presentation Flow Introduction Problem Description System Model Algorithm Results Conclusions and Future Work References

3. Introduction Wireless Sensor Networks (WSN) – long life expectancy Power Anemic Communication consumes maximum energy Data aggregation (convergecast) is a frequent operation in WSN – important to minimize its energy consumption Prior Work: PEGASIS [Step02], LEACH [Wendi00], CCTCCA [Valli03] Typically, convergecast follows broadcast – broadcast tree for convergecast. [Bhas02]

4. Introduction Contd... Prior work concentrate on energy efficiency alone. We have dual objective – Energy-Efficiency and Low-Latency Conventional approach not necessarily the best approach

5. Problem Description n nodes in the network Data from all the nodes to be collected at a central node Single Hop or Multi-Hop communication Energy consumed for communication is proportional to distance (, between 2 and 4) [Wendi00] Objective # 1: Find a route connecting all nodes to central node consuming minimum energy. Objective # 2: Minimum latency

6. System Model Assumptions Nodes are static and clocks are synchronized Every node has only one transceiver A node can transmit or receive at a time but not both Intermediate nodes concatenate the data they receive during upstream transmission Intermediate nodes wait until it receives data from all the nodes in whose path it lies.

7. System Model Energy Model [Wendi00] is the electrical energy required on the circuit of transceiver is the amplification energy required to transmit a unit of data over unit distance k is the size of the data packet transmitted by a node r is the distance between communicating nodes

8. System Model Latency Model [Valli03] Let be time taken to transmit longest data packet Latency is the total time required to transmit data from all the nodes to the central node

9. System Model Latency Model Balanced trees increases possibility of multiple simultaneous transmissions : number of children per node where is a positive integer If, due to the rule, a node will be left out of the tree – overlook the rule.

10. Algorithm (CCA) Rationale for Tree Construction Broadcast trees may not be suitable for convergecast BroadcastConvergecast Same data packet is transmitted to all nodes Different data packets are collected Latency depends on longest tree path Latency depends on # of parallel transmissions

11. Tree Construction Algorithm Constructs tree following greedy approach A set of nodes chooses closest neighbors as its children – subject to This process is followed iteratively until all the nodes in the network join the tree

12. Tree Construction Algorithm Network Tree

13. Channel Allocation Algorithm A fixed number of CDMA codes are given Each node is assigned a triplet (Transmission Code, Reception Code, Transmission Time Slot) Reception code of a node and Transmission code of all its children are same A node uses a time slot and a code for transmission if Its parent is receiving using same code Choosing the code and time slot will avoid any collisions with all of its neighbors

14. Channel Allocation Algorithm Network (Transmission Code, Reception Code, Transmission Time Slot)

15. Results Energy for Convergecast ( = 3) [Valli03] and [CCA] consumes almost same amount of energy for convergecast [CCA] gains upto 8% over [Imrich87] for network of size >150 nodes

16. Results Latency for Convergecast ( = 3) [CCA] is almost 4 times faster than [Valli03] and 2 times faster than [Imrich87]

17. Conclusions and Future Work Proposed a tree construction and channel allocation algorithm for convergecast satisfying two objectives Showed that broadcast trees are not efficient for convergecast The proposed work should be studied for distributed manner Cluster based convergecast can be studied in future work

18. References [Valli03] V. Annamalai., S.K.S. Gupta and L. Schwiebert “On Tree- Based Convergecasting in Wireless Sensor Networks”. IEEE Wireless Communications and Networking Conference 2003, New Orleans 2003. [Imrich87] I. Chalmatac. and S. Kutten “Tree-Based Broadcasting in Multihop Radio Networks”. IEEE Transactions on Computers Vol. C-36, No. 10, Oct 1987. [Wendi00] W. R. Heinzelman, A. Chandrakasan and H. Balakrishnan “Energy-Efficient Communication Protocol for Wireless Micro Sensor Networks”. Proceedingsof the Hawaii International Conference on System Science, Jan 2000.

19. Reference [Step02] S. Lindsey, C. Raghavendra, K. M. Sivalingam “Data Gathering Algorithms in Sensor Networks Using Energy Metrics”. IEEE Transactions on Parallel and Distributed Systems, Vol. 13, No. 9, Sept 2002. [Bhas02] B. Krishnamachari, D. Estrin and S. Wicker “Impact of Data Aggregation in Wireless Sensor Networks”. International Workshop on Distributed Event-Based Systems (DEBS, ‘02) Vienna, Austria, July 2002.