Transcript Document

MANAGEMENT ISSUES
FOR WIRELESS SENSOR NETWORKS,
AN OVERVIEW:
[email protected]
By Antonio Ruzzelli
Examples of WSNs applications
environmental data collection:
temperature light, humidity, pressure,
solar radiation.
This network has given biologists new view
into the nesting patterns of Storm
Petrels.
Fire prevention
Wind Response
Of Golden Gate Bridge
By Antonio Ruzzelli
Cooperation of applications
Gateway
Nodes
By Antonio Ruzzelli
Prototypes of low cost wireless sensors
EYES sensor prototype, University of Twente (NL)
MICA mote sensor prototype, University of Berkeley (US)
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 efficiency
often 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 measurements
application 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 complexity
Increase 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
•Machine/User
interaction issues
•Language syntax
issues
•Transmission
control e.g. TCP/IP
•For routing
issues
•For medium
access control
issues
•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 architecture
An example:
•The most suitable network
architecture for WSNs is
still an open issue
•Each layer has its own task
•Any layer try to achieve
the task using the smallest
amount of energy possible
Application
Data aggregation
Sensing coverage
Localization
Routing
Cross layer interaction
•Researchers are evaluating
how to best use the cross
layer interaction
Sensing
devices
MAC
Physical
Antenna
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 TX
?
TX
node3
node2
TX
?
node1 node2
TX
node3
By Antonio Ruzzelli
Wireless Medium Access Control (MAC)
fundamental mechanisms (3):
General classes of MAC:
•Frequency Division Multiple Access (FDMA)
•Code Division Multiple Access (CDMA)
Applied on Sensor Networks:
node1 TX
TX
?
node3
node2
•Carrier Sense Multiple Access (CSMA)
•Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA)
•Time Division Multiple Access (TDMA)
•TDMA/CSMA hybrid approach
By Antonio Ruzzelli
New Concepts of Wireless Sensor
Networks MACs:
MAC protocols tailored for WSNs have
introduced novel concepts:
The node duty Cycle and Wakeup concepts
-To reduce the energy consumption nodes
alternate periods of radio activity
and inactivity
Sleep period
Wakeup period
Listening
period
New issues arises
a result!
Time
By Antonio Ruzzelli
New issues of Wireless Sensor
Networks MACs:
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
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 TX
TX
node3
node2
By Antonio Ruzzelli
Mechanisms applied in Wireless Sensor
Networks MACs:
The CSMA approach
Advantages: Flexibility as nodes works independently
Low packet delay
Drawbacks: The Hidden Terminal Problem (HTP)
TX
?
node1 node2
TX
node3
The CSMA does not
prevent packet
collisions if
transmitters are not
within the same range
By Antonio Ruzzelli
Mechanisms applied in Wireless Sensor
Networks MACs:
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
Transmitter
Receiver
RTS
CTS
time
Data
ACK
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
Recv.
CTS
By Antonio Ruzzelli
Mechanisms applied in Wireless Sensor
Networks MACs:
The CSMA/CA approach
Advantages: Avoid the Hidden Terminal problem
Good Flexibility
Disadvantages: High latency of messages
Control packet overhead
External Terminal Problem ETP
TX
node0 node1 node2
node3
Node2 could theoretically
transmit to node0 but the
CSMA blocks it while
node2 transmission is in
progress
By Antonio Ruzzelli
Mechanisms applied in Wireless Sensor
Networks MACs:
The TDMA approach
-Time is divided into frames
-Frames are divided into slots
- Each node within the same range
owns
one slot for transmission
TxN1
Node2
Node4
slot
TxN2
TxN3
TxN4
TxN5
TxN1
TxN2
TxN3
…
frame
Node1
Node3
Node5
By Antonio Ruzzelli
Mechanisms applied in Wireless Sensor
Networks MACs:
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)
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
S
L
CSMA
A
O
T
DATA
B
Zone 1 Zone 2 Zone 3 Zone 4 Zone 5
A
B
By Antonio Ruzzelli
Conclusion
The seminar
showed just
some of WSNs
protocol issues.
WSNs are still
a World wide
work in
progress that
can be very
useful for a
great number
of applications.
Gateway
Nodes
By Antonio Ruzzelli