By Antonio Ruzzelli MANAGEMENT ISSUES FOR WIRELESS SENSOR NETWORKS, AN OVERVIEW: [email protected].
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Transcript of By Antonio Ruzzelli MANAGEMENT ISSUES FOR WIRELESS SENSOR NETWORKS, AN OVERVIEW: [email protected].
By Antonio Ruzzelli
Examples of WSNs applications
This network has given biologists new view into the nesting patterns of Storm Petrels.
Fire prevention
Wind ResponseOf Golden Gate Bridge
environmental data collection: temperature light, humidity, pressure, solar radiation.
By Antonio Ruzzelli
Cooperation of applications
Gateway
Nodes
By Antonio Ruzzelli
Prototypes of low cost wireless sensors
MICA mote sensor prototype, University of Berkeley (US)
EYES sensor prototype, University of Twente (NL)
By Antonio Ruzzelli
How can we make such applications properly working?
Let’s identify some issues:
• One application for WSN can include thousands of sensors
Sensors must be cheap
• Cheap nodes Limited memory capabilities Limited processing capabilities
Limited power capabilities Maybe not very reliable
By Antonio Ruzzelli
Some other issues are:
•If the radio of a node is always on, the node will be depleted in few days (e.g. Mobile phone) Node depletion
•If a node receives more than one message at the same time, it will not able to understand the content Packet Collision
•Long distance transmission implies very high power consumption : Pc ≈ dⁿ, n≥2. Limited transmitting power
•Identification of useful messages to deliver Efficient type of service
By Antonio Ruzzelli
Sensor network Design Considerations (1)
• Energy efficiencyoften difficult recharge or replace batteries:prolonging the network life expectancy is the MAIN TASK
• Scalability Network may change in size, node density
and topology. Nodes may die over time and new nodes may
join later
By Antonio Ruzzelli
Sensor network Design Considerations (2)
• Event detection and periodic measurementsapplication dependently
• Multihop wireless communication to save energy by relaying messages to close
neighbors
• Good maintainability due to hash environment condition
• Network self configuration as network condition change over time
By Antonio Ruzzelli
How can we address such issues to fulfil the requirements?
• Issues are addressed in different layers referred to as network protocols
Adv: Decrease of complexityIncrease of flexibility
Dis: Increase of memory requirements
• The whole set of network protocols is known as network architecture (e.g. the ISO/OSI architecture)
By Antonio Ruzzelli
Internet: The ISO/OSI architecture
•For routing issues
•Machine/User interaction issues
•Language syntax issues
•For medium access control issues
•Transmission control e.g. TCP/IP
•Radio issues
Some layers are not suitable for sensor networks!
By Antonio Ruzzelli
How do protocols for a sensor network differ from traditional ones?
Protocols for ad hoc networks to (e.g. WiFi)are aimed to obtain:
• High bandwidth Utilization• Good fairness• Low latency of packets• High throughput
These are generally the primary concerns in traditional wireless voice and data networks But in sensor networks they are secondary!
By Antonio Ruzzelli
Wireless sensor network architectureAn example:
Antenna
Sensingdevices
Application
Data aggregation
Routing
MAC
Physical
Sensing coverage
Cross layer interaction
Localization
•The most suitable network architecture for WSNs is still an open issue
•Researchers are evaluating how to best use the cross layer interaction
•Each layer has its own task
•Any layer try to achieve the task using the smallest amount of energy possible
By Antonio Ruzzelli
Relevant existing MAC protocol for traditional wireless networks
•Bluetooth -Very short computer communication-Used in Personal Area Networks (PAN) e.g. in conference rooms
•IEEE 802.11 also known as WiFi-Short range computer communication-Used for internet in a Local Area Network (Lan), e.g. the CS department.
•Such protocols require nodes to be constantly powered on VERY HIGH ENERGY CONSUMPTION!
By Antonio Ruzzelli
Wireless Medium Access Control (MAC)
protocol fundamentals (1):•The MAC protocol coordinates the times at which nodes access the wireless channel
General Duties:•Packet collision avoidance
•Packet collision detection
• Efficient channel utilization
•Low access delay
•Node access fairness
By Antonio Ruzzelli
Wireless Medium Access Control (MAC)
protocol fundamentals (2):•Identification of collision
node1 node2 node3
?TXTX
node1
node2
node3?
TX
TX
By Antonio Ruzzelli
General classes of MAC:
•Frequency Division Multiple Access (FDMA)
•Code Division Multiple Access (CDMA)
Applied on Sensor Networks:
•Carrier Sense Multiple Access (CSMA)
•Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA)
•Time Division Multiple Access (TDMA)
•TDMA/CSMA hybrid approach
Wireless Medium Access Control (MAC)
fundamental mechanisms (3):
node1
node2
node3?
TX
TX
By Antonio Ruzzelli
MAC protocols tailored for WSNs have introduced an novel concept:
The node duty Cycle and Wakeup concepts
-To reduce the energy consumption nodes alternate periods of radio activity and inactivity
New Concepts of Wireless Sensor Networks MACs:
Wakeup period Sleep periodListening
period
Time
New issues arises a result!
By Antonio Ruzzelli
The Synchronization issue:-In order to communicate, nodes
have to synchronize their internal clock
-The low-cost technology of sensors make things worse:
-Clock Skew and Offset need to be compensated for
periodically
New issues of Wireless Sensor Networks MACs:
Periodically nodes must broadcast their time to neighbours and synchronize clocks together
By Antonio Ruzzelli
Mechanisms applied in Wireless Sensor Networks MACs:
The CSMA approach (Carrier sense multiple access)
•Any node that has a packet to transmit listen to the channel for a period to sense any ongoing transmission in progress
•Nothing is sensed Channel free assumed Transmit the packet•Something is sensed Channel busy Transmission postponed and node switches off the radio
node1
node2
node3?
TX
TX
By Antonio Ruzzelli
The CSMA approach
Advantages: Flexibility as nodes works independently Low packet delay
Drawbacks: The Hidden Terminal Problem (HTP)
node1 node2 node3
?TXTX The CSMA does not
prevent packet collisions if transmitters are not within the same range
Mechanisms applied in Wireless Sensor Networks MACs:
By Antonio Ruzzelli
The CSMA/CA approach (CSMA with collision avoidance)
Any node that has a message to transmit applies CSMA then if nothing is sensed:
-The node transmits a Request To Send
RTS packet (very short packet) -The node waits for a Clear To Send
CTS Packet - Once CTS is received, the node is
enabled to transmit the message -The correct message reception is
followed by an acknowledgment ACK
RTS
CTS
Data
Transmitter Receiver
time
ACK
Mechanisms applied in Wireless Sensor Networks MACs:
By Antonio Ruzzelli
Mechanisms applied in Wireless Sensor Networks MACs:
The CSMA/CA removes the Hidden Terminal Problem
-The RTS and CTS contain: message length, transmitter ID, receiver ID.
- The CTS notifies the neighbourhood of an imminent transmission -The neighbourhood can switchs off the radio to save energy
Transm.RTS
CTS
Recv.
By Antonio Ruzzelli
The CSMA/CA approach
Advantages: Avoid the Hidden Terminal problem Good Flexibility
Disadvantages: High latency of messages Control packet overhead External Terminal Problem ETP
Mechanisms applied in Wireless Sensor Networks MACs:
node1 node2 node3
TXNode2 could theoretically transmit to node0 but the CSMA blocks it while node2 transmission is in progress
node0
By Antonio Ruzzelli
TxN1
The TDMA approach
-Time is divided into frames -Frames are divided into slots - Each node within the same range owns one slot for transmission
Mechanisms applied in Wireless Sensor Networks MACs:
Node1
Node2
Node3
Node4
Node5
frameslot
TxN2 TxN4TxN3 TxN5 TxN1 TxN2
TxN3 …
By Antonio Ruzzelli
The TDMA approach
Advantages: -Collision free mechanism-Both HTP and ETP are solved
Disadvantages: -High packet overhead (all neighbours have to notify their presence before dividing the
resource- Low flexible (e.g. for late joining node)
Mechanisms applied in Wireless Sensor Networks MACs:
By Antonio Ruzzelli
Mechanisms applied in Wireless Sensor Networks MACs:
The TDMA/CSMA hybrid approach, the MERLIN protocol as an example
By Antonio Ruzzelli
MERLIN CSMA/TDMA Hybrid: Transmission Mechanism
• Every slot has a contention period to perform CSMA;• Nodes pick up a random time “t” in Contention Period;• Start listen to the channel at “t”;
– Channel is Free Start sending the packet ;– Channel is busy Turn off the radio until the next
scheduled Slot
Zone 1 Zone 2Zone 3
Zone 4Zone 5
CSMA DATA
A
A B
B
S L O T
By Antonio RuzzelliBy Antonio Ruzzelli
Conclusion
The seminar showed just some of WSNs protocol issues.
WSNs is still a World wide work in progress that can be very useful for a great number of applications.
Gateway
Nodes