Wireless Sensor Networks

Mote

•A very low cost low power computer•Monitors one or more sensors•A Radio Link to the outside world•Are the building blocks of Wireless Sensor Networks (WSN)

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Sensor

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Wireless Sensor Network

“A wireless sensor network (WSN) is a wireless network consisting of spatially distributed

autonomous devices using sensors to cooperatively monitor physical or environmental conditions, such as

temperature, sound, vibration, pressure, motion or pollutants, at different locations.”

- Wikipedia

Wireless Sensor Networks

• Formed by hundreds or thousands of motes that communicate with each other and pass data along from one to another

• Research done in this area focus mostly on energy aware computing and distributed computing

Super Node

Links to Other networks or Similar Super Nodes

Motes

WSN Applications•Environmental/Habitat monitoring •Acoustic detection •Seismic Detection •Military surveillance •Inventory tracking •Medical monitoring •Smart spaces •Process Monitoring

WSN vs Ad-HocWire Sensor Network Ad-Hoc Networks

Large number of nodes Lesser number of nodes in comparison with WSN

Nodes are more prone to failure and energy drain, batteries are not replaceable or rechargeable

Power source can be replaced or recharged

Sensor nodes may not have unique global identifiers

Unique addressing is possible

Data Centric and do not have unique global identifiers

Address Centric and have unique addresses

Data fusion to reduce BW No data fusion

Routing protocols in ad-hoc cannot be directly ported to sensor networks due to limitation in memory, power, and processing capabilities in the sensor nodes and the non scalable nature of the protocols.

Issues and Challenges• Nodes are randomly deployed and no regular topology, so

setup and maintenance should be autonomous.• Network is infrastructure less so all routing and routing

algorithms must be distributed.• Energy constraint is there so power efficient protocols are

needed.• Hardware design of sensor nodes should be energy efficient.• Sensor nodes must be synchronized with each other in

distributed manner so that TDMA schedules can be imposed.• Sensor networks must be adaptive to changing connectivity due

to failure of nodes and powering-up of new nodes.• Routing protocols must be dynamic.• Communication security must be there.

Classification of Sensor Network Protocols

Sensor Network Architecture• Layered Architecture: Single powerful base station(BS) and layers of sensor

nodes around it. Nodes of each layer have the same hop count to BS. Used with in-building wireless backbone and military such

as Multi-hop Infrastructure Network Architecture (MINA). In the in-building scenario BS acts as an access point to a

wired network. Advantage is that each node is involved only in short

distance, low power transmissions to nodes of neighboring nodes.

Layered Architecture

Unified Network Protocol Framework (UNPF)

• UNPF: Set of protocols for complete implementation of layered architecture.

• which includes»Network initialization and Maintenance

Protocol»MAC Protocol»Routing Protocol

Network initialization and Maintenance Protocol

• Uses broadcast capability of BS.• BS broadcasts its identifier (ID) using CDMA code.• Nodes after hearing this record the BS ID and send a beacon

signal with their own IDs and low default power levels.• The nodes BS can hear form Layer-1 as they are a single hop

away.• BS again broadcasts a control packet with all Layer-1 node

IDs.• All nodes send a beacon signal again.• Layer-1 nodes record the IDs which they hear and these form

Layer-2 as they are one hop away from layer-1 nodes.• In the next round Layer-1 nodes inform about the Layer-2

nodes to the BS.• So layers are built with successive round of beacons.

MAC Protocol

• During data transmission phase Distributed TDMA Receiver Oriented Channel assignment (DTROC) MAC protocol is used.

• Two steps of DTROC are:• Channel Allocation (assignment of channels)• Channel Scheduling (sharing of the channel by

neighbors)• BS assigns a reception channel for each node with channel

reuse.• Nodes schedule transmission slots for all the neighbors and

broadcast the schedule.• It enables collision free transmission and saves energy• Nodes can turn off when they are not in send/receive

operation.

Routing Protocol• Downlink from BS is by direct broadcast on the

control channel.

• With the layered architecture nodes forward the data to BS through multiple hops.

• The node to which a packet is to be forwarded is selected as per the remaining energy

Clustered Architecture The Sensor nodes are organized into clusters each

governed by a cluster-head Each node provide information to their cluster-head

and these heads send messages to the BS (BS connected to a wired network)

Cluster architecture is useful for the sensor networks because of its inherent suitability for data fusion.

The data gathering by all the members of the cluster can be fused at cluster-head, and only the resulting information needs to be communicated to the BS

LEACH (Low-Energy Adaptive Clustering Hierarchy) Protocol governs the formation of cluster-head and entire working of cluster architecture.

LEACH (Low-Energy Adaptive Clustering Hierarchy) Protocol

LEACH is a clustering-based protocol that minimizes energy dissipation in sensor networks.

It randomly selects cluster-heads and performs periodic re-election, so that the high energy dissipation experienced by cluster-head in communicating with BS is spread across all the nodes of the network.

Two Phases: (1) Set-Up and (2) Steady In set-up phase, each sensor node chooses a random

number between 0 & 1, if this is lower than threshold for a node, the sensor nodes become the cluster-head.

Each node then chooses their cluster-heads on the basis of signal strength received from them.

In steady state, data fusion takes place at cluster head and then forwarded to the BS.

Data Dissemination• The process by which queries or data are routed in the sensor

network i.e, to the BS or to the other nodes.• Source: A node that generates data.• Event: The information to be reported.• Sink: A node which is interested in an event and seeks

information.• Data dissemination is done by Dissemination Models which

may be periodic or on-demand.• Data Diffusion: Consists of Interest Propagation and Data

Propagation.• Interest Propagation: particular kind of data or event that a

node is interested such as temp, intrusion or presence of Bio agents.

• Data propagation: Shortest path and strongest path is selected.

Flooding• Each node which receives a packet broadcasts it if the

maximum hop count of the packet is not reached or the node itself is not the destination.

• Disadvantages of Flooding:

• Implosion (duplicate messages are sent to same node)

• Overlap (same event sensed by more node due to overlapping of coverage regions)

• Resource blindness (Flooding protocol doesn’t consider the available energy at the nodes and result in many redundant transmission)

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Gossiping• Modified version of Flooding.

• Nodes don’t broadcast a packet but send it to a randomly selected neighbor.

• It avoids the problem of Implosion.

• Problem is that it takes a longer time for a message to propagate throughout the network

• Doesn’t guarantee all nodes of the network will receive the message.

Rumor Routing

Rumor Routing

Sequential Assignment Routing

DATA GATHERING

Direct Transmission

Power Efficient Gathering for Sensor Information Systems (PEGASIS)

Binary Scheme

Chain Based Three Level Scheme

Thank you..