1. 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

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

3. What is Sensor Network? A collection of sensor nodesB 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

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

5. 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

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

7. 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

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

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

10. 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

11. 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

12. 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?

13. 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

14. 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

15. The Problem : 3 4 2 1
WOMS 2008

16. 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

17. 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.

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

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

20. Step – I : Initial-Schedule-Matrix1 5 4 3 2
Table 2: The initial-schedule-matrix
Node→ R - 5 4 3 2 1
Slot
WOMS 2008

21. Step – II : Contention Matrix1 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

22. 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

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

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

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

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

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

28. 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

29. References
A. Ephremides and T. V. Truong, “Scheduling Broadcasts in Multihop Radio Networks,” IEEE Transactions on Communications, Vol. 38, No. 4, April 1990, pp:
I. F. Akyildiz, W. Su, Y. Sankarasubramaniam and E. Cayirci, “A Survey on Sensor Networks,” IEEE Communications Magazine, August 2002, pp:
S. Ramanathan and E. L. Lloyd, “Scheduling Algorithms for Multihop Radio Networks,” IEEE/ACM Transactions on Networking, Vol. 1, No. 2, April 1993, pp:
S. Ramanathan, “A Unified Framework and Algorithm for Channel Assignment in Wireless Sensor Networks,” Wireless Networks, Vol. 5, No. 2, 1999, pp:
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:
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:
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:
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
WOMS 2008