A Fair Multiple-Slot Assignment Protocol for TDMA Scheduling in Wireless Sensor Networks

1
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

3
What is Sensor Network?

• A collection of sensor nodes
• Engaged in data transmission, reception,
  aggregation and redirecting to a sink
• An ad-hoc network

4
Major Applications

• Environmental Monitoring
• Habitat Monitoring
• Precision Agriculture
• Disaster Recovery
• Natural Calamity Prediction
• Defense Applications
• Assisted Living for aged disabled
• Health Care

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

6
Energy Consumers

• Need to shutdown the radio if possible

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

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Primary Interference

• Primary interference occurs due to exposed
  terminals

X
Y
Z
9
Secondary Interference

• Secondary interference occurs due to hidden
  terminals

X
Y
W
Z
10
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?
A matrix is used to represent a schedule
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TDMA
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

• 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
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
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The Problem
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Assumptions Revisited

• WSN consists of N static nodes
• Each node is assigned a unique id i, 1lt iltN
• 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

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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.

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Ephremides Truong (IEEE Tr. Comm., 1990)
Table 1
Node?
-
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Improvements over Previous Works
Fairness Even distribution of reserved
slots Compaction Reduction of number of
slots in the schedule matrix, wherever possible
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Step I Initial-Schedule-Matrix
Table 2 The initial-schedule-matrix Node?
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Step II Contention Matrix
Contention (Ci,j) total number of 2-hop
neighbors of nodei to which the slot Sj is
available
Table 3 The contention matrix Node?
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Step III Complete-Schedule-Matrix
Parallel Execution
Table 4 The complete-schedule-matrix after
Fair-Reservation Node?
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Step IV Compact-Schedule-Matrix
Table 5 The compact-schedule-matrix Node?
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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

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Performance Evaluation Frame Length
Comparison based on frame length (L)
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Performance Evaluation Fairness
Comparison based on standard deviation of number
of slots assigned to individual nodes
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Performance Evaluation Throughput
Tr avg. of slots reserved per node / frame
length
Comparison based on transmission rates (Tr)?
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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

29
References

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