In the Web of Things (WoT), special communication
networks composed of sensor nodes, actuator nodes and service nodes form the
basis for new types of web applications, which are directly connected to the
real world via sensors and actuators, and can be private, such as smart home
apps, personal robotics apps and factory control applications, or public,
such as air pollution monitoring systems and city parking management
systems.
An Internet-of-Things (IoT) system is a communication network
consisting of sensor nodes, actuator nodes and service nodes, such that at
least one node is connected to the Internet. A sensor node consists of a
controller to which one or more sensors and a communication unit are
attached. An actuator node consists of a controller to which one or more
actuators, zero or more sensors and a communication unit are attached.
A WoT system is an IoT system that is built with web technologies. These
technologies do not only include the classical web technologies HTTP(S),
HTML, CSS and JavaScript, but also the more recent web technologies
Server-Sent Events, Web Sockets, and the Constrained
Application Protocol (CoAP), which is a kind of downsized version of
HTTP.
There are three recent trends promoting the WoT. First, the web's
infrastructure has progressed dramatically 1) by extending the internet's
address space with IPV6, 2) by continuously increasing the speed and
bandwidth of internet connections, and 3) by improving the speed of HTTP
with HTTP 2.0 as well as introducing near-real-time web protocols like Web
Sockets. Second, the widespread use of smartphones and tablets, containing
various sensors, has created a large pool of sensing and computing resources
for the WoT. Third, the increasing availability of many kinds of cheap
sensors, actuators and other electronics components has led to the
development of a large Do It Yourself (DIY) robotics and WoT community,
creating lots of open source software and hardware, and publishing a great
variety of DIY projects (e.g., on the instructables
website). The availability of all these resources, and, in particular,
of low-cost hardware, creates new opportunities for WoT and robotics-related
research and education.
For instance, a simple WoT project can be the temperature monitoring of a
room by using a cheap temperature sensor, like the Texas Instruments LM352
(available for about 1 Euro), attached to an Arduino microcontroller
(available for about 5 Euro) connected to the Internet via an ESP8266 Wi-Fi
module (for about 5 Euro). More complex WoT systems, like a home security
and monitoring system or a home robot that is able to move around and talk
to people, can be built with hardware costs of a few hundreds Euro, only,
possibly using a no-longer-needed smartphone as the control computer and
exploiting its (GSM/3G and WiFi) communication and its (GPS, microphone,
camera) sensing capabilities.
We should distinguish between the following three impotant cases of WoT
architectures:
The office/factory WoT architectureThe office/factory WoT architecture: WoT systems that do
not have the limitations implied by constrained resource devices. These
systems can use ordinary networking and web technologies such as Ethernet
or IEEE 802.11 for wireless networking, SOAP via HTTPS for
application-level messaging, and SOAP-based web
service co-ordination and security techniques.
The home/DIY WoT architectureThe home/DIY WoT architecture: Home (possibly
do-it-yourself) WoT systems based on constrained resource devices having
unlimited power supply (not using batteries), such that power consumption
is not a concern. These systems need an alternative software/technology
stack that is adapted to the limited main memory, storage and processor
speed of the constrained resource devices. Ethernet (or IEEE 802.11) can
still be used for (wireless) networking, but only CoAP or an HTTP subset,
and no HTTPS, can be used for application-level messaging.
The deployment-in-the-wild WoT architectureThe deployment-in-the-wild WoT architecture: WoT systems
based on constrained resource devices that are battery-powered, requiring
low-energy wireless networking technologies, such as IEEE 802.15.4,
possibly using a mesh network for overcoming larger distances to a collection point, and small
footprint software technologies, such as CoAP for application-level
messaging. These systems often have higher packet error rates and a lower
throughput (say, of only tens of kbit/s).
The issue of using the new Internet Protocol (IP) version 6 (IPv6) instead
of the established version 4 (IPv4) is orthogonal to the WoT. The main issue
solved by IPv6, allowing a greater address space than IPv4, is not
necessarily an issue for WoT systems, which can, in many cases, be built
with either of them. Of course, the increasing use of IoT apps will
contribute to the increasing demand for IP addresses. But since most IoT/WoT
devices will not have to be reachable via an IP address, the expected
explosive growth of the IoT/WoT will not imply a similar explosion of the IP
address space.
The following are desirable features of a WoT system:
self-configuration: the dynamic composition of a WoT system by nodes
joining and leaving the network at any time
self-diagnosis: automatic discovery of failures and faults of the
components of a WoT system
self-optimization of constrained energy (battery-based) WoT systems:
automatic monitoring and on/off-time control of components
For the success of the WoT, it will be crucial to develop simple and robust
solutions for these features, in addition to establishing reference
architectures based on low-cost standard components such as the Arduino
micro-controller or the Raspberry Pi single-board computer.
