Network Sensor: The Invisible Super Cop

The day when the potential of Wireless Sensor Networks (WSNs)

was realized in remote monitoring, WSNs have gained a lot of attention as an

important research domain.

Typical Wireless Sensor Network



A typical WSN is a multi-hop wireless network consisting of small sensor
devices that are capable of sensing, processing (computing), and communicating.

These devices sense environmental variations like temperature, pressure,

vibration, acoustics, etc, and process the sensed data, often in a distributed

fashion. Later, this processed data is communicated wirelessly to a base

station.

For WSNs to be viable, the physical size should be small and

cost of the sensor devices low. Consequently, sensor devices are severely

constrained by resources. A typical sensor node is built of a 8-bit micro

controller, 4-16 MHz crystal oscillator, few hundred kilobytes of program

memory, 4-10 kb of data memory (RAM), radio-chip with data rates between 10-250

Kbps, typically a 10-bit ADC, and two AA batteries. Because of these

constraints, even simple protocols and algorithms may not perform to the

expectation when actually implemented on the sensor devices-this is the major

challenge driving all the current research in WSNs.

Security Avenues



Though WSNs have been proposed for a wide variety of applications, most of
these are confined to military use. A very few commercial sensor applications

have been explored till date. For WSNs to be truly ubiquitous, many more

commercial sensor applications have to be explored. In an effort toward that, we

are exploring the potential of using WSNs for handling certain security and

safety issues in residential layouts.

A residential layout is typically vulnerable to problems such as

burglary, fire accidents, and environmental pollution. WSNs could be employed to

address these safety issues. There are five important issues, and a Residential

Wireless Sensor Network (RWSN) and Central Facility (CF) could be set up to

address these issues.

RWSN is a wireless network of location-aware tiny embedded

sensor nodes (motes) placed at strategic spots within a residential layout. It

provides a common networking infrastructure to deal with all the identified

issues. The CF is a geographical location where the information conveyed by the

RWSN is monitored. Let's understand how to address these issues by employing

WSNs.

Burglary: A burglary is reported almost every day. The good

news is that this menace can be thwarted by adding the human intrusion detection

capability to RWSN. Detection, classification, and tracking of human targets are

basic surveillance applications, and hence have received a considerable amount

of research interest. In the past few years, a number of human target detection

and classification techniques (based on sensors like vibration, acoustic,

magnetic, etc) have been proposed.

For this solution to be robust, it is essential to differentiate

between a resident and an intruder/stranger. A wireless wearable mote having a

very limited radio range can serve this purpose. Basically, a mote periodically

transmits a beacon consisting of identification codes. Every resident is

provided with a wearable identity mote that periodically sends out a beacon

containing a unique identification code corresponding to the resident. On

hearing to the beacon, an RWSN node forwards it to the CF, so that simultaneous

reception of human detection information and a valid identification code from

the same location confirms that the detected person is a resident.

Domicile Surveillance: Using RWSNs to remotely monitor

houses is another significant aspect. For example, equipping electricity and

water billing meters with sensors capable of sensing and transmitting allows a

meter reader to collect meter readings at the CF, thereby eliminating the need

to visit individual domiciles. Similarly, by furnishing domiciles with the

sensors capable of detecting fire accidents, LPG leakage, etc enables staff at

the CF to detect accidents and take appropriate actions, even when the residents

are away from home.

Patients Monitoring: Fitting medical sensors capable of

communicating to RWSNs with residential patients allow doctors to remotely and

continuously monitor the real-time physiological status of the patients. It

helps greatly for emergency treatment. This is true irrespective of the location

of doctors and patients as long as both are in the vicinity of the RWSN.

Furthermore, collecting and maintaining the physiological database of the

patients is remarkably useful in diagnosis, prognosis, and forestalling

potential health problems.

Tracking Children and Pets: Children and pets can be tracked

with the help of wireless identity motes. On its reception, an RWSN node route

beacons to the CF along with the node's location information. As identity

motes are tuned to have a limited radio transmission range, the location

information conveyed in the beacon can be considered as location of the wearer

corresponding to the received identification code. This also allows tracing the

trajectory of the movement of children and pets.

Air Pollution Monitoring: In an attempt to control air

pollution and prevent health hazards, a pollution monitoring capability should

be added to RWSN. By equipping gas sensors (eg, metal oxide sensors,

electrochemical sensors, and QCM sensors) on RWSN enables to monitor air

pollution levels and take corrective action in the case of pollution exceeding

the limit.

The Challenges



As WSNs are on the path to widespread deployment, securing them becomes a
central concern. One of the common attacks on WSNs is node capture attack, where

an adversary gains full control over sensor nodes through direct physical

access.

It is a common notion that WSNs are highly susceptible to node

capture attack. But, any unauthorized physical access to the sensor nodes could

be detected and communicated to CF. So, it is not easy for an adversary to lay

hand on the sensor nodes.

Methods for tightly attaching the wearable identity mote to the

wearer needs to be studied. One such simple and reliable method is to compel

residents to inform CF on loosing his/her identity mote, so that a pre-assigned

identification code, corresponding to the resident, could be deactivated.

The employed network protocols of RWSN must support real-time

requirements. For instance, in case of intrusion detection, the intrusion should

be instantaneously notified to CF and then collected and delivered. Intrusion

data must still be valid at the time of responding, thereby necessitating a

real-time communication.

As WSNs are severely constrained by power supply, it is

essential to employ power-aware routing, medium access, and physical layer

protocols. Most of the network protocols proposed for WSNs are associated with a

well-known tradeoff between real-time requirements and power management;

therefore, it is necessary to choose well-balanced protocols that are

appropriate to RWSN.

The RWSN nodes must be location-aware, but it is not feasible to

have a Global Positioning System (GPS) receiver on every RWSN node, as GPS is an

expensive solution. In the past several years, a number of location discovery

schemes have been proposed to eliminate the need for having a GPS receiver on

individual sensor nodes.

There is still a long way to realizing the proposed system,

which needs comprehensive study of system design, realization challenges,

efficient algorithms, etc.

In the proposed RWSN, only the static backbone nodes are to be

aware of their location. Moreover, only approximate location information can

serve the application requirements. This enables employing a location scheme

that is easy-to-implement, inexpensive, and reliable by incorporating support

for mobile nodes and highly accurate solutions like GPS.

D Manjunath and SV Gopalaiah, IISc





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