K. Banerjee, P. Basuchaudhuri, D. Sadhukhan and N. Das A Fair Multiple-Slot Assignment Protocol for TDMA Scheduling in Wireless Sensor Networks K. Banerjee, P. Basuchaudhuri, D. Sadhukhan and N. Das

Organization WSN : Wireless Sensor Networks Collision Avoidance : TDMA Scheduling : Frame length minimization problem Distributed Protocol Performance Evaluation Conclusion WOMS 2008

What is Sensor Network? A collection of sensor nodes B E A D C F G SINK A collection of sensor nodes Engaged in data transmission, reception, aggregation and redirecting to a sink An ad-hoc network WOMS 2008

Major Applications Environmental Monitoring Habitat Monitoring Precision Agriculture Disaster Recovery Natural Calamity Prediction Defense Applications Assisted Living for aged & disabled Health Care WOMS 2008

Unique Constraints Large number of nodes Multi-hop network Streaming data No global knowledge about the network Frequent node failure Energy is the scarce resource Limited memory Autonomous WOMS 2008

Energy Consumers SENSORS CPU TX RX IDLE SLEEP RADIO Need to shutdown the radio if possible WOMS 2008

Communication in sensor network . A node broadcasts data packets and nodes within its transmission zone can receive those packets Communication uses a single channel over the same wireless medium Interference takes place when more than one transmission overlaps : Collision WOMS 2008

Primary Interference Primary interference occurs due to exposed terminals X Y Z

Secondary Interference Secondary interference occurs due to hidden terminals X W Y Z

Energy is the most scarce resource Collision Avoidance Collision causes retransmission : wastage of energy Energy is the most scarce resource Several collision avoidance methods are available while accessing the media- CSMA : listening also consumes energy FDMA : not suitable; generally single channel TDMA : best suited; nodes can sleep in idle times

Slot, Frame and Schedule Slot: Smallest time slice, in which a node can either transmit or receive Frame: A minimal sequence of slots is a frame Node→ R - 5 4 3 2 1 Slot A matrix is used to represent a schedule WOMS 2008

TDMA How to reduce the latency? Time is slotted : each node is assigned at least one collision-free slot in a frame; frames are repeated TDMA: Periodic listen and sleep transmit sleep listen Turn off radio when sleeping Reduce duty cycle to ~10% (e.g. 200 ms on/2s off) Increased latency for reduced energy How to reduce the latency?

Unique Slots Nodes within 1 hop neighborhood creates primary interference Nodes within 2 hop neighborhood (but not in 1 hop neighborhood) creates secondary interference So no two nodes within 2 hop neighborhood can be given same time slot for transmission Slots can be reused for nodes at more than 2-hop distance

Distributed solution is needed Problem Definition How to find a TDMA schedule with minimum frame length that assigns at least one conflict-free slot to each node? Can be modeled as a graph-coloring Problem NP-Complete Problem [Ephremides et al, 1990] Distributed solution is needed

The Problem : 3 4 2 1 WOMS 2008

Assumptions Revisited WSN consists of N static nodes Each node is assigned a unique id i, 1< i<N No global knowledge about network topology: each node knows N, the total number of nodes in the network A node can only be in one state at a time: broadcasting or receiving All the links are bi-directional WOMS 2008

Assigning Slots The easiest way to solve the problem is providing each node a particular time slot. But that leads to - Frame length = Number of nodes. Wastage of time slots.

Ephremides & Truong (IEEE Tr. Comm., 1990) 5 4 3 2 Table 1 Node→ R - 5 4 3 2 1 Slot R - R - WOMS 2008

Improvements over Previous Works Fairness – Even distribution of reserved slots Compaction – Reduction of number of slots in the schedule matrix, wherever possible WOMS 2008

Step – I : Initial-Schedule-Matrix 1 5 4 3 2 Table 2: The initial-schedule-matrix Node→ R - 5 4 3 2 1 Slot WOMS 2008

Step – II : Contention Matrix 1 5 4 3 2 Contention (Ci,j) = total number of 2-hop neighbors of nodei to which the slot Sj is available Table 3: The contention matrix Node→ X 1 5 4 3 2 Slot WOMS 2008

Step – III : Complete-Schedule-Matrix Parallel Execution Table 4: The complete-schedule-matrix after Fair-Reservation Node→ R - 5 4 3 2 1 Slot X WOMS 2008

Step – IV : Compact-Schedule-Matrix Table 5: The compact-schedule-matrix Node→ - R 3 2 1 5 4 Slot WOMS 2008

Simulation Environment Random graph generation Graph generation algorithms have been used Number of nodes may vary from 50-250 Randomly generated each time in Unix Environment WOMS 2008

Performance Evaluation : Frame Length Comparison based on frame length (L) WOMS 2008

Performance Evaluation : Fairness Comparison based on standard deviation of number of slots assigned to individual nodes WOMS 2008

Performance Evaluation : Throughput Tr = avg. # of slots reserved per node / frame length Comparison based on transmission rates (Tr)‏ WOMS 2008

Conclusion Proposed algorithm outperforms in terms of: frame length fairness and throughput Efficient for large networks with uniform traffic Distributed algorithm for compaction is to be studied WOMS 2008

References A. Ephremides and T. V. Truong, “Scheduling Broadcasts in Multihop Radio Networks,” IEEE Transactions on Communications, Vol. 38, No. 4, April 1990, pp: 483-495. I. F. Akyildiz, W. Su, Y. Sankarasubramaniam and E. Cayirci, “A Survey on Sensor Networks,” IEEE Communications Magazine, August 2002, pp: 102-114. S. Ramanathan and E. L. Lloyd, “Scheduling Algorithms for Multihop Radio Networks,” IEEE/ACM Transactions on Networking, Vol. 1, No. 2, April 1993, pp: 166-177. S. Ramanathan, “A Unified Framework and Algorithm for Channel Assignment in Wireless Sensor Networks,” Wireless Networks, Vol. 5, No. 2, 1999, pp: 81-94. I. Rhee, A. Warrier, J. Min and L. Xu, “DRAND: Distributed Randomized TDMA Scheduling for Wireless Ad-hoc Networks,” Proc. of MobiHoc ’06, May 2006, pp: 190-201. Y. Wang and I. Henning, “A Deterministic Distributed TDMA Scheduling Algorithm for Wireless Sensor Networks,” Proc. of International Conference on Wireless Communication, Networking and Mobile Computing, WiCOM 2007, pp: 2759-2762 S.  Gandham, M.  Dawande and R. Prakash, “Link scheduling in sensor networks: distributed edge coloring revisited,” Proc. of 24th Annual Joint Conference of the IEEE Computer and Communications Societies, INFOCOM 2005, pp: 2492- 2501. S. Bhattacharjee and N. Das, “Distributed Time Slot Assignment in Wireless Ad Hoc Networks for STDMA,” Lecture Notes in Computer Science (Springer), No. 3618, Proc. of the 2nd International Conference on Distributed Computing and Internet Technology (ICDCIT 2005), Dec. 2005, pp. 93-104. WOMS 2008