Wireless Sensor Networks(WSN) - An Introduction

Wireless sensor networks are a trend of the past few years, and they involve deploying large number of small nodes. The nodes then sense environmental changes and report them to other nodes over flexible network architecture. Sensor nodes are great for deployment in hostile environments or over large geographical areas.

Sensor network is composed of a large number of sensor nodes, which are densely deployed either inside the phenomenon or very close to it. The position of sensor nodes need not be engineered or pre-determined. This allows random deployment in inaccessible terrains or disaster relief operations. On the other hand, this also means that sensor network protocols and algorithms must possess self-organizing capabilities. Another unique feature of sensor networks is the cooperative effort of sensor nodes. Sensor nodes are fitted with an on-board processor. Instead of sending the raw data to the nodes responsible for the fusion, sensor nodes use their processing abilities to locally carry out simple computations and transmit only the required and partially processed data.

Since large number of sensor nodes is densely deployed, neighbor nodes may be very close to each other. Hence, multi hop communication in sensor networks is expected to consume less power than the traditional single hop communication. Furthermore, the transmission power levels can be kept low, which is highly desired in covert operations. Multi hop communication can also effectively overcome some of the signal propagation effects experienced in long-distance wireless communication. One of the most important constraints on sensor nodes is the low power consumption requirement. 

Sensor nodes carry limited, generally irreplaceable, power sources. Therefore, while traditional networks aim to achieve high quality of service (QoS) provisions, sensor network protocols must focus primarily on power conservation. They must have inbuilt trade-off mechanisms that give the end user the option of prolonging network lifetime at the cost of lower throughput or higher transmission delay.


Usage of sensor networks:
Sensor networks have been useful in a variety of domains. The primary domains at which sensor are deployed follow:-

• Environmental observation. Sensor networks can be used to monitor environmental changes. An example could be water pollution detection in a lake that is located near a factory that uses chemical substances. Sensor nodes could be randomly deployed in unknown and hostile areas and relay the exact origin of a pollutant to a centralized authority to take appropriate measures to limit the spreading of pollution. Other examples include forest fire detection, air pollution and rainfall observation in agriculture.
• Military monitoring. Military uses sensor networks for battlefield surveillance; sensors could monitor vehicular traffic, track the position of the enemy or even safeguard the equipment of the side deploying sensors.
• Building monitoring. Sensors can also be used in large buildings or factories monitoring climate changes. Thermostats and temperature sensor nodes are deployed all over the building’s area. In addition, sensors could be used to monitor vibration that could damage the structure of a building.
• Healthcare. Sensors can be used in biomedical applications to improve the quality of the provided care. Sensors are implanted in the human body to monitor medical problems like cancer and help patients maintain their health.

Network model

A wireless sensor network consists of hundreds or thousands of low cost nodes which could either have a fixed location or randomly deployed to monitor the environment. Due to their small size, they have a number of limitations, an issue that I will discuss later. Sensors usually communicate with each other using a multi hop approach. The flowing of data ends at special nodes called base stations (sometimes they are also referred to as sinks). A base station links the sensor network to another network (like a gateway) to disseminate the data sensed for further processing. Base stations have enhanced capabilities over simple sensor nodes since they must do complex data processing; this justifies the fact that bases stations have workstation/laptop class processors, and of course enough memory, energy, storage and computational power to perform their tasks well. Usually, the communication between base stations is initiated over high bandwidth links.
Keep in mind that one of the biggest problems of sensor networks is power consumption, which is greatly affected by the communication between nodes. To solve this issue, aggregation points are introduced to the network. This reduces the total number of messages exchanged between nodes and saves some energy. Usually, aggregation points are regular nodes that receive data from neighboring nodes, perform some kind of processing, and then forward the filtered data to the next hop. Similar to aggregation points is clustering. Sensor nodes are organized into clusters, each cluster having a “cluster head” as the leader. The communication within a cluster must travel through the cluster head, which then is forwarded to a neighboring cluster head until it reaches its destination, the base station. Another method for saving energy is setting the nodes to go idle (into sleep mode) if they are not needed and wake up when required. Of course, the challenge is to find a pattern at which energy consumption is made evenly for all the nodes in the network.

Comparing Sensor Nodes to Ad-hoc Wireless Networks

Wireless sensor networks share similarities (and differences) with ad-hoc wireless networks. The main similarity is the multi-hop communication method. The differences among the two types of networks are listed below:

• More nodes are deployed in a sensor network, up to hundred or thousand nodes, than in an ad-hoc network that usually involves far fewer nodes.
• Sensor nodes are more constrained in computational, energy and storage resources than ad-hoc.
• Sensor nodes can be deployed in environments without the need of human intervention and can remain unattended for a long time after deployment.
• Neighboring sensor nodes often sense the same events from their environment thus forwarding the same data to the base station resulting in redundant information.
• Aggregation and in-network processing often requires trust relationships between sensor nodes that are not typically assumed in ad-hoc networks.

Sensor Node Hardware Architecture

Sensor nodes are minute. The following figure presents the Mica2 sensor node, which is the most popular research platform at the moment.

As you can see, the main components of a typical sensor node include an antenna and a radio frequency (RF) transceiver to allow communication with other nodes, a memory unit, a CPU, the sensor unit (i.e. thermostat) and the power source which is usually provided by batteries. The operating system running on sensor nodes is called TinyOS and was initially developed at theUniversityofCalifornia,Berkeley. TinyOS is designed to run on platforms with limited computational power and memory space.The programming language of TinyOS is stylized C and uses a custom compiler called NesC. Though it may work on other platforms, the supported platforms are Linux RedHat 9.0, Windows 2000, and Windows XP. Further information you may obtain from the official TinyOS websitehttp://webs.cs.berkeley.edu/tos/.

Due to sensors’ limited capabilities, there are a lot of design issues that must be addressed to achieve an effective and efficient operation of wireless sensor networks.

• Energy saving algorithms. Since sensor nodes use batteries for power that are difficult to replace when consumed (remember that often sensor nodes are deployed in remote and hostile environments), it is critical to design algorithms and protocols in such a way to utilize minimal energy. To do so, implementers must reduce communication between sensor nodes, simplify computations and apply lightweight security solutions.
• Location discovery. Many applications can tracking an object require knowing the exact or approximate physical location of a sensor node in order to link sensed data with the object under investigation. Furthermore, many geographical routing protocols need the location of sensor nodes to forward data among the network. Location discovery protocols must be designed in such a way that minimum information is needed to be exchanged among nodes to discover their location. Since sensor nodes are energy constrained, solutions like GPS are not recommended. After all, cost is another factor that influences design; manufacturers try to keep the cost at minimum levels since most sensor nodes are usually needed for many applications. If the cost is high, the adoption and spread of sensor technology will be prohibited.
• Security. Is it possible to introduce a new technology without addressing security? Of course not! However, as all other technologies, security is not the top priority when designing something new. This approach is acknowledged by almost everyone, and it is erroneous but they keep doing it anyway. Security solutions are constrained when applying them to sensor networks. For example, cryptography requires complex processing to provide encryption to the transmitted data. Secure routing, secure discovery and verification of location, key establishment and trust setup, attacks against sensor nodes, secure group management and secure data aggregation are some of the many issues that need to be addressed in a security context. Future articles will analyze some of these issues to give you a good understanding of security related issues in wireless sensor networks.   

This article was to familiarize you with fundamental issues of sensor networks. Sensor area is growing very fast, attracting more and more people to its use. Of course, to my knowledge, research on sensor security is very recent and a lot of work needs to be done in many areas that are not addressed so far.