Wireless Sensor Networks (WSNs)

A WSN is a collection of sensor nodes distributed in a selected geographical area in order to monitor certain aspects of the environment. It incorporates techniques from 3 areas, i.e., sensing, communication and computing. WSN can be used to monitor space, objects, and the interaction between objects with each other and the encompassing space.

A sensor node (also referred to as a mote or probe) performs the following functions:

• Senses environmental physical parameters,
• Processes raw data locally to extract characteristic features of interest,
• Temporary storage of data,
• Uses the wireless link to transmit data to its neighbours.

The IEEE 1451 standard defines smart sensors as sensors with small memory and standardized physical connection to enable the communication with processor and data network. Smart sensors are a combination of a sensor with signal conditioning, embedded algorithms and digital interface.

Elements of a Sensor

Examples of applications of WSN:

• Monitoring of a habitat without requiring heavy and bulky equipment.
• GlacsWeb project monitors ice caps and glaciers to gather information about global warming and climate change.
• Motion monitoring, e.g. monitoring of physical structure (such as machines and airplanes), where variations in behaviour indicates wear, fatigue or other mechanical changes.

Depending on the application, sensor nodes may be placed in areas where they are not easily accessible. Therefore, it is imperative that the nodes must be robust, energy efficient and require minimal maintenance. Among the requirements for WSN are:

• Low cost – many may be required to monitor an environment,
• Unobtrusive but durable,
• Energy efficient,
• Robust to withstand errors and failures,
• Low polluting,
• Maintenance-free for years at a time,
• Reliably connected with a cyber infrastructure that permits frequent network access for data upload, device programming, and management.

There are three architectures for sensor networks:

1. Flat architecture, where data is transferred from static sensor nodes to the sink in a multi-hop fashion,
2. Two-layer architecture, where static and mobile sink nodes are used to collect data from sensor nodes,
3. Three-layer architecture, where multiple sensor networks are connected together over the Internet — this is the architecture for IoT.

WSNs may be built using any wireless technologies, such as WPAN (e.g., Bluetooth), WLAN (e.g., Wi-Fi), WMAN (e.g., WiMAX), WWAN (e.g., 3G network)and satellite network (e.g., GPS). WSN may use non-IP based protocol (e.g., ZigBee) and IP-based protocol (e.g., NanoStack and IPv6).

There are three categories of sensor networks for IoT, i.e., body sensor network (BSN), object sensor network (OSN) and environment sensor network (ESN).

Perera et al (2014) identified six layers in a sensor network:

1. Layer 1 consists of low-end sensor nodes,
2. Layer 2 consists of high-end sensor nodes,
3. Layer 3 consists of sink nodes, which could be static or mobile,
4. Layer 4 consists of low-end computational device, such as a mobile phone,
5. Layer 5 consists of high-end computational device, and
6. Layer 6 is the Internet / cloud infrastructure.

Capability (which refers to the processing, memory, communication and energy capacity) increases from Layer 1 to Layer 6. An ideal system should understand the capability differences and perform data management accordingly. For example, performing processing at the lower layers would reduce data transfers, but devices at these layers have lower processing power and may have low energy capacity.

What are Wireless Sensor Networks?

Reference:

C. Perera, A. Zaslavsky, P. Christen, D. Georgakopoulos, “Context Aware Computing for the Internet of Things: A Survey”, IEEE Communications Surveys & Tutorials, Vol. 16, No. 1, 2014