1. Collision-free Time Slot Reuse in Multi-hop Wireless Sensor Networks
Lodewijk van Hoesel and Paul Havinga
Department of Electrical Engineering, Computer Science and Mathematics,
University of Twente
Intelligent Sensors, Sensor Networks and Information Processing Conference, 2005.IEEE

2. Outline Introduction Related Work
Lightweight Medium Access Protocol (LMAC)
Localized Algorithm for assigning Time Slots
Simulation
Conclusion

3. Introduction
Motivation
To ensure a long-lived network of wireless communicating sensors
To prevent energy-wasting effects like idle listening or collision of packets.
To limit latency
It is key issue to keep the number of time slots in a frame

4. Related Work- Lightweight Medium Access Protocol (LMAC)
frame
frame
frame
frame
frame … …
timeslot CM DM
CM: Control Message
DM: Data Message

5. Related Work- Lightweight Medium Access Protocol (LMAC)
A node always transmits a control message in its own slot
All neighbors should receive the control message
CM of a node as RTS message
The ACK of reception of another node in its occupied slots field as CTS message

6. Related Work- Lightweight Medium Access Protocol (LMAC)
A node always starts its time slot by sending out a CM
-Even if it does not have any data to send.
Addressing other nodes, CM is need of synchronization
-Resolving collisions and the operation of the distributed time slot scheduling
The data message (DM) does not have a fixed length and is even omitted when a node does not have any message to send

7. Related Work- Interference Regions
{A,C,D} :blocking by CTS of Node B
Aint :blocking by Carrier Sense of Node A
{B,C} :blocking by RTS/DATA of Node A E
RTS B A D
{E,F} :Not blocked during
DATA transmit F C
Interference range typical larger than transmit range

8. Localized Algorithm for assigning Time Slots
Initialization(I)
Network setup
Active(A)
Wait(W)
Time slot collision
Discover(D)

9. Localized Algorithm for assigning Time Slots
Detect other neighboring nodes
When a neighboring node is detected,
the node synchronize
After a frame,nodes switch to the wait state W
Initialization state (/) 1 2 3 4
Nodes wait randomness time W
between synchronization and choosing of a free time slot
After the random wait time,
the node continues with the discover state D.
Wait state (W)

10. Localized Algorithm for assigning Time Slots
Discover state (D)
Every node broadcasts a bit vector of occupied slots in its CM
The node collects second order (2 hops) neighborhood information
during one entire frame
When a node receives a CM successfully, it updates the vector
Logical '1' at the position of the time slot
Otherwise a '0' will be inserted
The node chooses a time slot and advances to the active state A
To reduce the probability of collisions, nodes randomly choose one
from the set F
'OR'-operation between all received bit vectors

11. Localized Algorithm for assigning Time Slots
M={1,2,3,4,5,6,7,8} 1
ACK
F={5,6,7,8} 1
timeslot CM 2 3
F :the set of non-interfering time slot
A similar distance min |Tint -T|> rint
Minimum distance between receiver R and interfering transmitter Tint is min |Tint - R| > r 4

12. Localized Algorithm for assigning Time Slots – Choosing one time slot from the free ones
'OR'-operation
Time slots are only reused after three
hops or more
Free time slot:1,2,7
This node control time slot 6 … … … 3 2 4 … … … 7 8 ? 6 … … 1 5
Bit vector of occupied time slots

13. Localized Algorithm for assigning Time Slots
Active state (A)
The node transmits a CM (and DM if necessary) in its own time slot.
It listens to other time slots and accepts data from neighboring nodes
The nodes cannot detect the collision by themselves
Need to be informed by their neighboring nodes
These neighboring nodes use their own time slot to inform
By using a special field in the CM
When a node is informed a collision has by neighbor’s CM
It will give up its time slot and back to the wait state W

14. Localized Algorithm for assigning Time Slots- Resolving Collisions A C CM
collide B
When a node is informed a collision has by neighbor’s CM
It will give up its time slot and back to the wait state W

15. Simulation Sensor nodes:500
Divide topology into sets with Max △
△:local maximum connectivity of node in network
Localized Algorithm compares with Random
Localized Algorithm for assigning Time Slots
Node with ID '0' is assigned time slot '1' and is put into the active state (A).
Other nodes are put in the initialization state (I)

16. Simulation
△ = 8, in 70% of the topologies a node-conflicting schedule could be create with 9 time slots
Number of time slots necessary in random networks for different maximal connectivities

17. Simulation
Number of time slots necessary in random networks for different maximal connectivities using the localized algorithm
△+1≦Ntimesolts ≦ △²- △+1

18. Conclusion Localized time slot assignment algorithm
Allows nodes to be self-configuring
A mechanism allows nodes to discover which time slots can be used.
Reuse time slots to reduce the length of time slots
Our method does not rely on central managers or hierarchical relations in the network

19. Thanks for Your Listening!!

20. Localized Algorithm for assigning Time Slots

21. Localized Algorithm for assigning Time Slots