collision-free time slot reuse in multi-hop wireless sensor networks lodewijk van hoesel and paul...

Collision-free Time Collision-free Time Slot Reuse in Multi-Slot Reuse in Multi-

hophopWireless Sensor Wireless Sensor

NetworksNetworksLodewijk 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

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OutlineOutline

Introduction Related Work

Lightweight Medium Access Protocol (LMAC)

Localized Algorithm for assigning Time Slots

Simulation Conclusion

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IntroductionIntroduction

MotivationMotivation 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

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Related Work-Related Work- Lightweight Medium Access Protocol Lightweight Medium Access Protocol

(LMAC)(LMAC)

frame …

…timeslot

CM DM

frame frame frame frame

• CM: Control Message• DM: Data Message

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Related Work-Related Work- Lightweight Medium Access Lightweight Medium Access

Protocol (LMAC)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

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Related Work-Related Work- Lightweight Medium Access Protocol Lightweight Medium Access Protocol

(LMAC)(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

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{B,C} :blocking by RTS/DATA of Node A

Related Work-Related Work-Interference RegionsInterference Regions

ABRTS

C

E

D

F{E,F} :Not blocked duringDATA transmit

Aint :blocking by Carrier Sense of Node A

{A,C,D} :blocking by CTS of Node B

Interference range typical larger than transmit range

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Localized Algorithm for Localized Algorithm for assigning Time Slotsassigning Time Slots

Initialization(I)Initialization(I)

Wait(W)Wait(W)

Active(A)Active(A)

Discover(D)Discover(D)

Network setup

Time slot collisionTime slot collision

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2 3

1

4

Nodes wait randomness time W between synchronization and choosing of a free time slotAfter the random wait time, the node continues with the discover state D.

Wait state (W)

Detect other neighboring nodesWhen a neighboring node is detected, the node synchronizeAfter a frame,nodes switch to the wait state W

Initialization state (/)

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Every node broadcasts a bit vector of occupied slots in its CMThe node collects second order (2 hops) neighborhood information during one entire frameWhen a node receives a CM successfully, it updates the vector Logical '1' at the position of the time slot Otherwise a '0' will be insertedThe 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

Discover state (D)

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0 0 0 00 0 0 0

timeslot

2 3

1

01000000 00100000

10000000

CM

ACK

11000000

4

11000000

01010000000010000

10100000

11100000

M={1,2,3,4,5,6,7,8}

F={5,6,7,8}

1 1 1 01 0 0 0

timeslot

F :the set of non-interfering time slotA similar distance min |Tint -T|> rintMinimum distance between receiver R and interfering transmitter Tint is min |Tint - R| > r

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Localized Algorithm for assigning Localized Algorithm for assigning Time Slots –Time Slots –

Choosing one time slot from the free Choosing one time slot from the free onesones

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11000010…

4

001111001…

2

11100011…

3

01110001…

1

11000011…

5

00011101…6

00011100…

8

11111001…

??

Bit vector of occupied time slots

This node control time slot 6

00111101000111010001110000111101

'OR'-operation

Free time slot:1,2,7Time slots are only reused after three

hops or more

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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 nodesThe nodes cannot detect the collision by themselvesNeed to be informed by their neighboring nodesThese neighboring nodes use their own time slot to inform By using a special field in the CMWhen 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

Active state (A)

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Localized Algorithm for assigning Time Localized Algorithm for assigning Time Slots-Slots-

Resolving CollisionsResolving Collisions

A

B

C

01000000

10000000

10000000

collide

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

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SimulationSimulation

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)

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Simulation Simulation

Number of time slots necessary in random networks for different maximal connectivities

△ = 8, in 70% of the topologies a node-conflicting schedule could be create with 9 time slots

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SimulationSimulationNumber of time slots necessary in random networks for

different maximal connectivities using the localized algorithm

△+1≦ Ntimesolts ≦ ²- +1△ △

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ConclusionConclusion

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

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Thanks for Your Thanks for Your Listening!!Listening!!

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collision-free time slot reuse in multi-hop wireless sensor networks lodewijk van hoesel and paul...

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