Wireless Sensor Networks UNIT-1

WIRELESS SENSOR
NETWORKS
UNIT –
Easy n Inspire
WIRELESS SENSOR
NETWORKS
1
Easy n Inspire

UNIT – 1
OVERVIEW OF WIRELESS SENSOR NETWORKS:
Introduction:
A Sensor is a device that is used to gather information about a physical process or a physical
phenomenon and translate it into electrical signals that can be processed, measured and analysed. The
term physical process used in the above definition of a Sensor can be any real-world information like
temperature, pressure, light, sound, motion, position, flow, humidity, radiation etc.
A Sensor Network is a structure consisting of sensors, computational units and communication
elements for the purpose of recording, observing and reacting to an event or a phenomenon. Such
Sensor Networks can be used for remote sensing, medical telemetry, surveillance, monitoring, data
collection etc.
If the communication system in a Sensor Network is implemented using a Wireless protocol, then the
networks are known as Wireless Sensor Networks or simply WSNs.
The wireless sensor networks is depends on a simple equation: Sensing + CPU + Radio = Thousands
of possible applications.
A Wireless Sensor Network (WSN) consists of base stations and a number of wireless sensors
(nodes).
 There are two types of WSNs
A. Structured model
B. Unstructured model
Structured WSN:
 All sensor nodes are deployed in pre designed manner.
 The benefit of structure wireless sensor network is that some nodes can be deployed with lower
network maintenance and management cost.
 Deployed in a pre-planned manner
 Fewer nodes
 Lower network maintenance
 Lower cost
 No uncovered regions

Unstructured WSN
 If we talk about unstructured so is a collection of sensor nodes.
 And these deployed in ad hoc manner into a region.
 Once deployed, the network is absent unattended perform monitoring and reporting functions.
 Densely deployed (many nodes)
 Randomly Deployed
 Maintenance is difficult
 Advantages of WSN:
 Network setups can be carried out without fixed infrastructure.
 Suitable for the non-reachable places such as over the sea, mountains, rural areas or deep forests.
 Flexible if there is random situation when additional workstation is needed.
 Implementation pricing is cheap.
 It avoids plenty of wiring.
 It might accommodate new devices at any time.
 It's flexible to undergo physical partitions.
 It can be deployed on a large scale.
 It can be developed according to the application.
 It can be monitored or accessed with remote location
The disadvantages of wireless sensor networks can be
 Less secure because hackers can enter the access point and obtain all the information.
 Lower speed as compared to a wired network.
 Easily troubled by surroundings (walls, microwave, large distances due to signal attenuation, etc).
 Still Costly (most importantly)
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 Wireless sensor network challenges and unique constraints:
WSN is an emerging area. It offers wide variety of applications and these applications can be
implement in real world. To implement them more efficient protocols and algorithms are needed.
Design a new protocol or algorithm addresses challenges of this field.
To design a better protocol or algorithm, it is necessary to first clearly understood challenges. These
challenges are summarized as
1. Physical Resource Constraints 4. Ad-hoc Deployment
2. Fault-Tolerance 5. Scalability
3. Quality of Service 6. Security

 Physical Resource Constraints: The most important constraint in sensor network is the limited
battery power of sensor nodes. Sensor nodes are left in unattended environment where recharge and
replacement of battery is not possible. Sensor node’s lifetime dependents on battery power. Thus
effective lifetime of sensor network is directly dependent on battery. Hence the energy consumption
is main design issue of a protocol. Limited computational power and memory size is another
constraint due to that individual sensor node can store and process less amount of data. So the
protocol should be simple and light-weighted. Limited bandwidth is also a constraint due to this
communication delay can be high.
 Ad-hoc Deployment: Sensor nodes are randomly deployed in required monitoring field without any
infrastructure. For an example, for fire detection in a forest the nodes are typically dropped in to the
forest from a plane. Sensor nodes itself create connections with other nodes and form an
infrastructure. Hence new protocol or algorithm should be able to handle this ad-hoc deployment.
 Fault-Tolerance: Sensor nodes are prone to failure because of unattended environment. A sensor
node may fail due to hardware or software problem or energy exhaustion. If few of sensor nodes fail,
working protocol should handle all type of failures to maintain connectivity and prolong lifetime of
network. For example, routing or aggregation protocol, must find suitable paths or aggregation point
in case of these kinds of failures.
 Scalability: In monitoring field, number of sensor nodes deployed could be in order of hundreds,
thousands or even more. It depends upon the application. It may possible that initially deployed
sensor nodes are not enough to monitor the environment. In this situation, protocol that is working
upon network should be scalable and able to accommodate large number of sensor nodes.
 Quality of Service: Some applications like multi-media or time critical needs QoS. Multi-media
application requires enough good quality of contents (video, audio and image). In time critical
application, the data should be delivered within a certain period of time from the moment it is sensed;
otherwise the data will be useless. New protocols which are designed for such applications should
handle QoS.
 Security: In sensor networks, security is another important and challenging parameter. An effective
and efficient compromise should be achieved, between security demands for secure communication
and low bandwidth required for communication in sensor network. Whereas in traditional networks,
the focus is on maximizing channel throughput with secure transmission.
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 Wireless sensor network driving applications:
Wireless sensor network has a lots of applications like security, monitoring, biomedical research,
tracking etc. Basically these application are used emergency services. Theoretically speaking, the
possible applications of Wireless Sensor Networks are unlimited.
1. Environmental Data Collection
 In environmental data collection application, WSNs are used collect various sensor data in a period
of time.
 If a data to be meaningful so collecting sensor data at regular interval and the nodes would remain at
known locations.
 In the environmental data collection application, a large number of nodes continuously sensing and
transmitting data back to a set of base stations that store the data using traditional methods.
 In typical usage scenario, the nodes will be evenly distributed over an outdoor environment.
2. Military Applications
 Distributed Sensor Networks (DSN) and the Sensor Information Technology form the Defence
Advanced Research Project Agency (DARPA), sensor networks are applied very successfully in the
military sensing
 Now wireless sensor networks can be an integral part of military command, control, communications,
computing, intelligence, surveillance, reconnaissance and targeting systems.
 In the battlefield context, rapid deployment, self-organization, fault tolerance security of the network
should be required.
 The sensor devices or nodes should provide following services: like Monitoring friendly forces,
equipment and ammunition, Battlefield surveillance, Targeting, Battle damage

3. Security Monitoring
 A key difference between security monitoring and environmental monitoring is that security
networks are not actually collecting any data.
 This has a significant impact on the optimal network architecture.
 Each node has to frequently check the status of its sensors but it only has to transmit a data report
when there is a security violation.
 The immediate and reliable communication of alarm messages is the primary system requirement.
 These are “report by exception” networks.
 It is confirmed that each node is still present and functioning.
 If a node were to be disabled or fail, it would represent a security violation that should be reported
4. Node tracking scenarios
 In which wireless sensor network is the tracking of a tagged object through a area of space monitored
by a sensor network.
 There are many condition where one would like to track the location of important assets or personnel.
 Current inventory control systems attempt to track objects by recording the last checkpoint that an
object passed through.
 However, with these systems it is not possible to determine the current location of an object. For
example, UPS tracks every shipment by scanning it with a barcode whenever it passes through
routing centres.
5. Health Applications
 Sensor networks are also widely used in health care area.
 In some modern hospital sensor networks are constructed to monitor patient physiological data, to
control the drug administration track and monitor patients and doctors and inside a hospital.
 In spring 2004 some hospital in Taiwan even use RFID basic of above named applications to get the
situation at first hand.
 Long-term nursing home: this application is focus on nursing of old people.
 In the town farm cameras, pressure sensors, orientation sensors and sensors for detection of muscle
activity construct a complex network.
 They support fall detection, unconsciousness detection, vital sign monitoring and dietary/exercise
monitoring. These applications reduce personnel cost and rapid the reaction of emergence situation.
6. Home Application
 Many concepts are already designed by researcher and architects, like “Smart Environment: Some
are even realized.
 Let’s see the concept “the intelligent home”:After one day hard work you come back home.

 At the front door the sensor detects you are opening the door, then it will tell the electric kettle to boil
some water and the air condition to be turned on.
 You sit in the sofa lazily. The light on the table and is automatically on because the pressure sensor
under the cushion has detected your weight. The TV is also on.
 One sensor has monitored that you are sitting in front of it. “I’m simply roasting. The summer time in
Asia is really painful.” You think and turn down the temperature of the air condition.
 At the sometime five sensors in every corner in the room are measuring the temperature. Originally
there is also sensor in the air condition.
 But it can only get the temperature at the edge of the machine not the real temperature in the room.
 So the sensors in the room will be detecting the environment. The air condition will turn to sleep
mode until all the sensors get the right temperature. The light on the corridor, in the washing groom
and balcony are all installed with sensor and they can be turned on or turn out automatically. Even
the widows are also attached with vibratory sensors connected to police to against thief. How nice!
You become nurse and bodyguard at the same time.
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 ENABLING TECHNOLOGIES FOR WIRELESS SENSOR NETWORKS:
Building such wireless sensor networks has only become possible with some fundamental advances
in enabling technologies.
 First technology is the miniaturization of hardware.
 Smaller feature sizes in chips have driven down the power consumption of the basic
components of a sensor node to a level that the constructions of WSNs can be planned.
 This is particularly relevant to microcontrollers and memory chips and the radio modems
which are responsible for wireless communication.
 Reduced chip size and improved energy efficiency is accompanied by reduced cost.
 Second one is processing and communication and the actual sensing equipment is the third
relevant technology.
 Here, however, it is difficult to generalize because of the vast range of possible sensors.

These three basic parts of a sensor node have to accompany by power supply. This requires,
depending on application, high capacity batteries that last for long times, that is, have only a
negligible self-discharge rate, and that can efficiently provide small amounts of current.
Ideally, a sensor node also has a device for energy scavenging (collecting), recharging the battery
with energy gathered from the environment – solar cells or vibration-based power generation.
Such a concept requires the battery to be efficiently chargeable with smallamounts of current, which
is not a standard ability.
The counterpart to the basic hardware technologies is software.
This software architecture on a single node has to be extended to a network architecture, where the
division of tasks between nodes, not only on a single node, becomes the relevant question-for
example, how to structure interfaces for application programmers. The third part to solve then is the
question of how to design appropriate communication protocols.
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 ARCHITECTURES:
 SINGLE-NODE ARCHITECTURE:
 Hardware components / Sensor Node Hardware components:
A basic sensor node comprises five main components such as Controller, Memory, Sensors and
Actuators, Communication devices and Power supply Unit.Choosing the hardware components for a
wireless sensor node, obviously the applications has to consider size, costs, and energy consumption
of the nodes.
Memory:
 Memory is used to store programs and intermediate data.
 In WSN there is a need for Random Access Memory (RAM) to store intermediate sensor readings,
packets from other nodes, and so on. While RAM is fast, its main disadvantage is that it loses its
content if power supply is interrupted.
 Program code can be stored in Read-Only Memory (ROM) or, more typically, in Electrically
Erasable Programmable Read-Only Memory (EEPROM) or flash memory (the latter being similar to
EEPROM but allowing data to be erased or written in blocks instead of only a byte at a time).
 Flash memory can also serve as intermediate storage of data in case RAM is insufficient or when the
power supply of RAM should be shut down for some time.

Controller:
 A controller to process all the relevant data, capable of executing arbitrary code.
 The controller is the core of a wireless sensor node. It collects data from the sensors, processes this
data, decides when and where to send it, receives data from other sensor nodes, and decides on the
actuator’s behaviour.
 It has to execute various programs.
 It is the Central Processing Unit (CPU) of the node.
 For General-purpose processors applications microcontrollers are used.
 These are highly overpowered, and their energy consumption is excessive. These are used in
embedded systems.
 A specialized case of programmable processors are Digital Signal Processors (DSPs).
 In a wireless sensor node, such a DSP could be used to process data coming from a simple analog,
wireless communication device to extract a digital data stream.
 An ASIC is a specialized processor, custom designed for a given application such as, for example,
high-speed routers and switches.
Power supply:
 As usually no tethered power supply is available, some form of batteries are necessary to provide
energy.
 Sometimes, some form of recharging by obtaining energy from the environment is available as well
(e.g. solar cells).
 There are essentially two aspects: Storing energy and Energy scavenging.
 Storing energy: Traditional batteries: The power source of a sensor node is a battery, either
nonrechargeable (“primary batteries”) or, if an energy scavenging device is present on the node, also
rechargeable
 Energy scavenging: Depending on application, high capacity batteries that last for long times, that is,
have only a negligible self-discharge rate, and that can efficiently provide small amounts of current.
Ideally, a sensor node also has a device for energy scavenging, recharging the battery with energy
gathered from the environment
Communication Device:
 Turning nodes into a network requires a device for sending and receiving information over a wireless
channel.
 Choice of transmission medium: The communication device is used to exchange data between
individual nodes.
 In some cases, wired communication can actually be the method of choice and is frequently applied
in many sensor networks.

 The case of wireless communication is considerably more interesting because it include radio
frequencies. Radio Frequency (RF)based communication is by far the most relevant one as it best fits
the requirements of most WSN applications.
 Transceivers: For Communication, both transmitter and receiver are required in a sensor node to
convert a bit stream coming from a microcontroller and convert them to and from radio waves. For
two tasks a combined device called transceiver is used.
Sensors and actuators:
 The actual interface to the physical world: devices that can observe or control physical parameters of
the environment.
Sensors can be roughly categorized into three categories as
 Passive Omnidirectional sensors: These sensors can measure a physical quantity at the point of the
sensor node without actually manipulating the environment by active probing – in this sense, they are
passive.
 Passive, narrow-beam sensors: these sensors are passive as well, but have a well-defined notion of
direction of measurement.
 Active sensors This last group of sensors actively probes the environment, for example, a sonar or
radar sensor or some types of seismic sensors, which generate shock waves by small explosions.
These are quite specific – triggering an explosion is certainly not a lightly undertaken action – and
require quite special attention.
Actuators: Actuators are just about as diverse as sensors, yet for the purposes of designing a WSN
that converts electrical signals into physical phenomenon.
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 Energy consumption of sensor nodes:
 Energy supply for a sensor node is at a premium: batteries have small capacity, and recharging by
energy scavenging is complicated and volatile.
 Hence, the energy consumption of a sensor node must be tightly controlled.
 The main consumers of energy are the controller, the radio front ends, to some degree the
memory, and, depending on the type, the sensors.
 One important contribution to reduce power consumption of these components comes from chip-
level and lower technologies: Designing low-power chips is the best starting point for an energy-
efficient sensor node.
 But this is only one half of the picture, as any advantages gained by such designs can easily be
squandered when the components are improperly operated.

 The crucial observation for proper operation is that most of the time a wireless sensor node has
nothing to do, hence, it is best to turn it off.
 Naturally, it should be able to wake up again, on the basis of external stimuli or on the basis of
time.
 Therefore, completely turning off a node is not possible, but rather, its operational state can be
adapted to the tasks at hand.
 Introducing and using multiple states of operation with reduced energy consumption in return for
reduced functionality is the core technique for energy-efficient wireless sensor node. for example,
the Advanced Configuration and Power Interface (ACPI)
 For a controller, typical states are “active”, “idle”, and “sleep”.
 The usual terminology is to speak of a “deeper” sleep state if less power is consumed. The usual
assumption is that the deeper the sleep state, the more time and energy it takes to wake up again to
fully operational state (or to another, less deep sleep state).
 Hence, it may be worthwhile to remain in an idle state instead of going to deeper sleep states even
from an energy consumption point of view
 At time t1, the decision should be taken to reduce power consumption from Pactive to Psleep.
 If it remains active and the next event occurs at time tevent, then a total energy of
Eactive = Pactive(tevent − t1) has be spent uselessly idling.
 Putting the component into sleep mode, on the other hand, requires a time τdownuntil sleep
mode has been reached; as a simplification, assume that the average power consumption during
this phase is (Pactive + Psleep)/2.
 Then, Psleep is consumed until tevent.
 In total,τdown(Pactive + Psleep)/2 + (tevent − t1 − τdown)Psleep energy is required in sleep mode as
opposed to (tevent− t1)Pactive when remaining active.
 The energy saving is thus
Esaved =(tevent − t1)Pactive − (τdown(Pactive + Psleep)/2 + (tevent − t1 − τdown)Psleep).
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 Operating systems and execution environments:
An operating system (OS) is system software that manages computer hardware and software
resources i.e acts as an intermediary between programs and the computer hardware.
 Embedded operating systems:
 An embedded system is some combination of computer hardware and software, either fixed in
capability or programmable, that is specifically designed for a particular function.
 Embedded operating systems (EOS) are designed to be used in embedded computer systems.
 EOS are able to operate with a limited number of resources. They are very compact and
extremely efficient by design
 TinyOS:
 TinyOS is an open-source, flexible and Application-Specific Operating System for wireless
sensor networks.
 WSN consists of a large number of tiny and low-power nodes, each of which executes
simultaneous and reactive programs that must work with strict memory and power constraints.
TinyOS meets these challenges.
Salient features of TinyOS are:
 Has Event-based architecture.
 TinyOS’s component library includes network protocols, distributed services, sensor drivers,
and data acquisition tools.
 TinyOS’s event-driven execution model
 Programming paradigms and application programming interfaces:
 Concurrent Programming:
 Concurrent processing is a computing model in which multiple processors execute instructions
simultaneously for better performance. It is said to be synonymous with parallel processing.
 WSNs have to handle data communing from arbitrary sources – for example, multiple sensors
or the radio transceiver – at arbitrary points in time
 Tasks are broken down into subtasks that are then assigned to separate processors to perform
simultaneously.
 This is also known as sequential programming model

 Process-based concurrency:
 Most modern, general-purpose operating systems support concurrent (seemingly parallel)
execution of multiple processes on a single CPU.
 Hence, such a process-based approach would be the first to support concurrency in a sensor node
 Also, each process requires its own stack space in memory, which fits ill with the stringent
memory constraints of sensor nodes.
 Event-based programming
 For these reasons, a somewhat different programming model seems preferable.
 The idea is to embrace the reactive nature of a WSN node and integrate it into the design of the
operating system.
 The system essentially waits for any event to happen, where an event typically can be the
availability of data from a sensor, the arrival of a packet, or the expiration of a timer.
 Such an event is then handled by a short sequence of instructions that only stores the fact that
this event has occurred and stores the necessary information
 STRUCTURE OF OS AND PROTOCOL STACK:
 Layering is the traditional approach to communication protocol structuring.
 Individual protocols are stacked on top of each other, each layer only using functions of the
layer directly
 This layered approach has great benefits in keeping the entire protocol stack manageable, in
containing complexity, and in promoting modularity and reuse.
 But it is not clear whether such a strictly layered approach will serve for WSN.
 A protocol stack refers to a group of protocols that are running concurrently that are employed
for the implementation of network protocol suite.
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 NETWORK ARCHITECTURE:
This concept has discussion on turning individual sensor nodes into a wireless sensor network and
Optimization goals of how a network should function.
o Sensor network scenarios
o Optimization goals and figures of merit
o Gateway concepts
 SENSOR NETWORK SCENARIO:
Types of sources and sinks:
 Source is any unit in the network that can provide information(sensor node).
 A sink is the unit where information is required, it could belong to the sensor network or
outside this network to interact with the another network or a gateway to another larger
Internet
 Single-hop versus multi-hop networks
 Single Hop:
 Because of limited distance the direct communication between source and sink is not always
possible.
 In WSNs, to cover a lot of environment the data packets taking multi hops from source to the
sink.
 Multi-hopping improves the energy efficiency of communication as it consumes less energy to
use relays instead of direct communication.
Three types of sinks in a very simple, single-hop sensor network
 Multiple Hop
 In many cases, multiple sources and multiple sinks present.
 Multiple sources should send information to multiple sinks.
 Either all or some of the information has to reach all or some of the sinks.

 Multiple sinks and sources:
 Three types of mobility:
In wireless sensor networks, mobility can appear in three main forms:
o Node mobility: The wireless sensor nodes themselves can be mobile
o Sink mobility: The information sinks can be mobile.
o Event mobility: The objects to be tracked can be mobile
Node mobility: The wireless sensor nodes themselves can be mobile. The meaning of such mobility is
highly application dependent. In examples like environmental control, node mobility should not happen;
but it is possible in livestock surveillance (sensor nodes attached to cattle, for example), it is the
common rule.
In the face of node mobility, the network has to reorganize itself frequently enough to be able to
function correctly. It is clear that the balance between the frequency and speed of node movement on the
one hand and the energy required maintaining a desired level of functionality in the network on the other
hand.
Sink mobility The information sinks can be mobile. While this can be a special case of node mobility,
the important aspect is the mobility of an information sink that is not part of the sensor network, for
example, a human user requested information via a PDA while walking in an intelligent building.
In a simple case, such a requester can interact with the WSN at one point and complete its interactions
before moving on.

Object/Event Mobility
In applications like event detection and in particular in tracking applications, the cause of the events or
the objects to be tracked can be mobile. In such scenarios, it is (usually) important that the observed
event is covered by a sufficient number of sensors at all time. Hence, sensors will wake up around the
object, engaged in higher activity to observe the present object, and then go back to sleep. As the event
source moves through the network, it is accompanied by an area of activity within the network – this has
been called the Frisbee model.
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 Optimization goals and figures of merit:
Considering the different scenarios, the applications and the different options for networking for
WSNs, makes it almost impossible to obtain a general optimization that fits all previous
considerations. In this manner, the objective perspectives that are distinguished as more
appropriate to design are
 How to optimize a network and How to get the solutions?
• How to decide which approach is better?
• How to turn relatively inaccurate optimization goals into measurable figures of merit?
• For all the above questions the general answer is obtained from
o Quality of service
o Scalability
o Energy efficiency
o Robustness
Quality of service:
WSNs differ from other conventional communication networks in the type of service they offer.
These networks essentially only move bits from one place to another. Possibly, additional requirements
about the offered Quality of Service (QoS) are made, especially in the context of multimedia
applications. Such QoS can be regarded as a low-level, networking-device-observable attribute –
bandwidth, delay, jitter, packet loss rate
Some generic possibilities are
 Event detection/reporting probability:
What is the probability that an event that actually occurred is not detected or, more precisely, not
reported to an information sink that is interested in such an event? For example, not reporting a
fire alarm to a surveillance station would be a severe shortcoming.
 Event classification error
If events are not only to be detected but also to be classified, the error in classification must be
small.

 Event detection delay
What is the delay between detecting an event and reporting it to any/all interested sinks?
 Missing reports In applications that require periodic reporting, the probability of undelivered
reports should be small.
 Approximation accuracy- For function approximation applications, the average/maximum
absolute or relative error with respect to the actual function.
 Tracking accuracy - Tracking applications must not miss an object to be tracked, the
reportedposition should be as close to the real position as possible, and the error should be
small.
Scalability:
 The ability to maintain performance characteristics irrespective of the size of the network is
referred to as scalability.
 With WSN potentially consisting of thousands of nodes, scalability is an obviously essential
requirement
 The need for extreme scalability has direct consequences for the protocol design
 Often, a penalty in performance or complexity has to be paid for small networks
Robustness:
 Wireless sensor networks should also exhibit an appropriate robustness
 They should not fail just because a limited number of nodes run out of energy, or because their
environment changes and severs existing radio links between two nodes
 If possible, these failures have to be compensated by finding other routes.
Energy efficiency:
 In wireless sensor networks and that energy efficiency should therefore make an evident
optimization goal.
Energy per correctly received bit: How much energy, counting all sources of energy consumption
at all possible intermediate hops, is spent on average to transport one bit of information.
Energy per reported (unique) event: similarly, what is the average energy spent to report one
event? Since the same event is sometimes reported from various sources.
Network lifetime: The time for which the network is operational or, put another way, the time during
which it is able to fulfil its tasks.
1. Time to first node death when does the first node in the network run out of energy or fail and stop
operating?
2. Network half-life When have 50% of the nodes run out of energy and stopped operating? Any
other fixed percentile is applicable as well.
3. Time to partition When does the first partition of the network in two (or more) disconnected parts
occur.

All these metrics can of course only be evaluated under a clear set of assumptions about the energy
consumption characteristics of a given node, about the actual “load” that the network has to deal
with (e.g. when and where do events happen), and also about the behavior of the radio channel.
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 Gate way concepts:
 Need for gateways:
 For practical deployment, a sensor network only concerned with itself is insufficient.
 The network rather has to be able to interact with other information devices for example to read
the temperature sensors in one’s home while traveling and accessing the Internet via a wireless
 Wireless sensor networks should also exhibit an appropriate robustness
 They should not fail just because of a limited number of nodes run out of energy or because of
their environment changes and breaks existing radio links between two nodes
 If possible, these failures have to be compensated by finding other routes.
The types are mentioned below:
 Internet to WSN communication
 WSN to Internet communication
 WSN tunnelling:
1. Internet to WSN communication
It is the case of an Internet-based entity trying to access services of a WSN. This is fairly simple if
this requesting terminal is able to directly communicate with the WSN, for example, a mobile
requester equipped with a WSN transceiver.
In the above fig. Requesting sensor network information from a remote terminal entails choices
about which network to address, which gateway node of a given network, and how and where to
adapt application-layer protocol in the Internet to WSN-specific protocols. The requesting terminal
can instead send a properly formatted request to this gateway, which acts as an application-level
gateway of sensor nodes that can answer this request.

2. WSN to Internet communication:
When a sensor node wants to deliver an alarm message to some Internet host. For example, a sensor
node wants to deliver an alarm message to some Internet host. The first problem to solve is ad hoc
networks, namely, how to find the gateway from within the network. Basically, a routing problem to
a node that offers a specific service has to be solved, integrating routing and service discovery. The
gateway then has to extract this information and translate it into IP packets. An ensuing question is
which source address to use here – the gateway in a sense has to perform tasks similar to that of a
Network Address Translation (NAT) device
3. WSN tunnelling:
In addition to these scenarios describing actual interactions between a WSN and Internet terminals,
the gateways can also act as simple extensions of one WSN to another WSN. The idea is to build a
larger, “virtual” WSN out of separate parts, transparently “tunnelling” all protocol messages between
these two networks and simply using the Internet as a transport network
This can be attractive, but care has to be taken not to confuse the virtual link between two gateway
nodes with a real link; otherwise, protocols that rely on physical properties of a communication link
can get quite confused. Such tunnels need not necessarily be in the form of fixed network
connections, even mobile nodes carried by people can be considered as means for intermediate
interconnection of WSNs

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