Goal: The goal of the sensor project is to set up a shortest-path spanning tree over the nodes reachable from a central node.  After the whole network converges to the static topology, the network should support the following features:

• Static Scenario: Each mote in the network displays its hop-distance from the central node on its LED.  The LED would only allow a value up to 7 hops, but we are not going to test it on networks of larger diameter.
  
• Dynamic Scenario: Once you manually change the motes placement, say remove a certain mote, the network should recover to static state within a short time, say 1 minute.  And the related motes should update their number of hops  on their LEDs.
  
• Path to the root: Each mote is capable of sensing the TSR value. Once the value is below a threshold predefined in your program, the mote sends an alarm back to the central mote. And all motes along the route from this mote back to the central mote should turn on an LED, so that the route could be displayed.
• Network size: Your code should work correctly on networks of up to 8 nodes and up to 4 hops.
  

Deliverables:

• A zip file containing the code, including a brief description of the algorithm.
• A demo to the TA and instructor.

Extra Credit:

• Larger networks: If your code works correctly with larger networks (say, 15 nodes or more), then some extra credit will be given.  One can demonstrate this using the simulator TOSSIM or a real demo with our motes.
• Additional features: E.g., implementing a routing mechanism over the spanning tree which can be demonstrated, displaying the (potentially changing) spanning tree using a GUI, implementing a broadcast or multicast protocol.
  

Hints:

• Collisions: One problem you may have to deal with is collisions among transmissions, especially in a dense neighborhood of sensors.  A simple strategy is to randomize the times at which nodes transmit.  This may prevent collisions to some extent, but would not guarantee reliable delivery for large networks.
• Retransmissions: For more reliable delivery, you can implement a simple retransmission protocol in which the node retransmits up to a MAX (say, 5) number of times, after waiting a random period of time between retransmissions. You can set up random waiting times using a timer.  I would recommend trying this approach only for large networks.
  
• Demo: When you work with the actual sensors, you may notice that the default transmission range is quite high (about 100 ms).  You can use CC2420Control to configure the transmission power.   If you use the radio with minimum transmission power,which has a range of around 0.5 meter.  This would make it easier to test and demonstrate your program in a small area.