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TDMA based MAC protocol to assist in Wide area Sensor Network Deployment
Nuwan Gajaweera
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Outline
Wireless Sensor Networks Background & Objectives Literature Survey Technology Choices TDMA MAC protocol Remaining Work
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Wireless Sensor Network
Collection of low power computer With integrated sensors Networked via short range radio transceivers
Application Areas Environmental Monitoring Defense & security Health etc
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Outline
Wireless Sensor Networks Background & Objectives Literature Survey Technology Choices TDMA MAC protocol Remaining Work
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Background
Monitoring of a large area WSN deployment over a large area Large number of repeater nodes needed
Alternative Use a Data Mule
Data Mule – Mobile mote that traverse the area of the sensor network Random Path: Motes mounted on people, livestock or
vehicles Deterministic Path: Robots
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Background
Area to be monitored
Gateway
Data Mule
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Issue
Multiple Data Mules at the Base stationEfficient use of radio resources – i.e. minimize
packet collisionsFair bandwidth allocation to all data mulesCollect maximum data volume from data mule
in given time
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Objectives
Develop suitable MAC protocolSingle hop networkTraffic pattern: data flows from data mule to
base stationMaximize throughputMinimize delay
Develop storage engineData collected from leaf nodes should stored so
that fast retrieval is possible
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Outline
Wireless Sensor Networks Background & Objectives Literature Survey Technology Choices TDMA MAC protocol Remaining Work
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Literature Survey
Wireless sensor network platforms
Different MAC protocols in existence
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Wireless sensor network platforms- Research TinyOS
Popular event driven OS for deeply embedded systems
SOS Mantis Contiki
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Wireless sensor network platforms- Commercial Crossbow Moteiv Ember Corp Dust Networks – TDMA-FDMA based
MAC
TinyOS
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WSN MAC protocols
Random access schemesLPL, B-MAC, X-MAC802.15.4 MAC
Slotted schemesS-MAC, S-MAC/AI, T-MAC
TDMA based schemes Hybrids protocols
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TDMA MAC protocols
Pros High channel utilization due to absence of
packet collisions Cons
Complexity in building a scheduling Need for node synchronization Inability to handle mobile nodes, due the
frequent need to reschedule
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TDMA MAC protocols
PEDMACS LMACS Cluster Based
PACTEMACLEACH – routing protocolBMA (utilizes LEACH for clustering)
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Outline
Wireless Sensor Networks Background & Objectives Literature Survey Technology Choices TDMA MAC protocol Remaining Work
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Technology Choices
Wireless Sensor Network Platform TinyOS
Mote MICA2
MAC Protocol TDMA MAC
Storage engine Under review
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Why TDMA?
CSMA Channel capacity wasted due to packet collisions
TDMA Channel capacity wasted due to control traffic Maximize throughput Energy conservation is a not a priority
“Develop demand assigned TDMA MAC Minimizes control traffic”
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D-Lab Mote
MICA2 Clone Developed at Dialog UoM Lab (D-Lab Mote) Exact copy of MICA2
Currently working on developing D-Lab Mote V2 Small modifications to original MICA2 GPS, GSM Modules RTC Flash Memory (MMC card)
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Outline
Wireless Sensor Networks Background & Objectives Literature Survey Technology Choices TDMA MAC protocol Remaining Work
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MAC protocol development process
Concept Development
Write TinyOS Code
Simulate(Arvora)
Analyze ResultsRefine Concept
Execute onMICA2 motes
SatisfactoryResults
UnsatisfactoryResults
Analyze Results
UnsatisfactoryResults
Process End
SatisfactoryResults
Completed In progress To be done
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TDMA MAC
Base station(Gateway)
Radio rangeof BS
mote #1(Data Mule)
mote #2
mote #3
TransmissionGroup
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TDMA MAC
Contention Periods Motes contend for membership of the tx group
Transmission Periods Motes in the tx group transmit data in allocated slot
Time
…
Transmission period
Contention Period
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Transmission Period
Time
…
Uplink Downlink
tx start packet tx end packet
Guard period
Time
1 …
Uplink Downlink
2 3 1 2 3
0 1 2 3 4 5Slot
tx start packet tx end packet
Also serves as ack packet
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Contention Period
Time
…
Uplink Downlink
RTS
con start packet
con end packet + CTS
Tb
Tb,maxTp Tb,max - Max back-off
Tp - Packet timeTb - Actual b-off
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Contention Period
Motes are only told if they were added to the tx group or not
The time slot is only advertised during the tx start packet
When the membership of the tx group changes, the following tx start packet will carry a uplink map that gives the time slot allocation.
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TDMA MAC Simulation
Base station wakes up at simulation start motes wake up after random delay The motes that are awake contend for
membership of the transmission group After transmitting ‘N’ packets, the mote removes
itself from the tx group. (N = 32, 64, 128, ∞)
The said mote again starts to contend for tx group membership
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Throughput vs Number of Nodes
4000
5000
6000
7000
8000
9000
10000
11000
0 2 4 6 8 10 12 14 16
Number of Nodes
Th
rou
gh
pu
t (b
ps) 32 PPC
64 PPC
128 PPC
inf PPC
BMAC
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Delay vs Number of Nodes
0
1
2
3
4
5
6
7
8
0 2 4 6 8 10 12 14 16
Number of Nodes
De
lay
(s
)
32 PPC
64 PPC
128
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Outline
Wireless Sensor Networks Background & Objectives Literature Survey Technology Choices TDMA MAC protocol Remaining Work
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Storage Engine
Mote will contain SRAM Flash Memory (MMC Card)
Mote (Data mule) will collect data from leaf nodes and store data in flash memory
When in range of the base station the mote will read data from the (slow) flash into the SRAM in fixed size blocks
The mote will then attempt to become a member of the tx group and upload this block of data to the base station
Once the block is transferred to the BS, the mote will remove itself from the group and repeat the process
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Remaining Work
Select/Develop storage engine Build MICA2 Clone (V2) Integration of storage engine & TDMA
MAC Uploading base station data to a server
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Conclusion