By Antonio Ruzzelli

MANAGEMENT ISSUES

FOR WIRELESS SENSOR NETWORKS,

AN OVERVIEW:

ruzzelli@ucd.ie

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

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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