People sometimes talk about the Internet of Things (IoT) as if it simply means the networking of digital devices, but that type of machine-to-machine communication has been in factories for a long time.
However, industry is increasingly moving to internet-based IoT protocols, making it easier and cheaper to connect different sensors and devices.
The IoT doesn’t use a single application layer protocol; earlier implementations used standard internet protocols, but dedicated IoT protocols are now becoming more common.
Digital communications hide the inner workings of each device or software layer behind various abstraction layers. This keeps the communication as simple as possible. Internet protocols are a four-layer model of communications. It is founded on the Transmission Control Protocol (TCP) and the Internet Protocol (IP) and is therefore often referred to as TCP/IP. Within this model there are four abstraction layers:
The physical network or link layer is about the transmission of raw data and specifies things such as pin layouts, voltage levels, data rates and line impedance. Ethernet is a common physical layer protocol.
The internet layer handles connections between devices, error correction and data packets. It uses IP addresses to identify hosts on the network.
The transport layer enhances the reliability and security of communications by transferring data sequences. In TCP/IP it generally consists of either the Transmission Control Protocol (TCP) or the User Datagram Protocol (UDP). TCP is used for human interactions such as email and web browsing. It provides logical connections, acknowledgement of packets transmitted, retransmission of lost packets and flow control. However, for embedded systems UDP can provide a lower overhead and better realtime performance. UDP is used for Domain Name Servers (DNS) and Dynamic Host Configuration Protocol (DHCP), and is now being applied to IoT applications.
The application layer is the highest level, and in TCP/IP its functions include what may be considered the session layer and presentation layer. Abstraction layers have at their heart the idea that knowledge of the underlying layers is not required. For most purposes it is, therefore, only necessary to consider the application layer.
Application layer protocols in TCP/IP
Different application layer protocols may offer different levels of bandwidth, realtime capability and hardware requirement. It can be tempting to use protocols that staff already understand. Many internet protocols are intended for humans to interact with information, such as File Transfer Protocol (FTP), Hypertext Transfer Protocol (HTTP), and Simple Mail Transfer Protocol (SMTP). IoT systems can be created using these web technologies, but this can have security issues.
Various protocols have been specifically designed for IoT or adapted for it. MQTT was originally created as a telemetry protocol for devices with a small memory and processor footprints, making it well suited to the IoT. It is standardised under ISO/IEC 20922 and its openness can make it easy to implement. However, many features are not included in the standard, it has no error handling and is not as well suited to low-power and high-security connections as more IoT-focused protocols.
The Constrained Application Protocol (CoAP) is well suited to IoT applications. It allows devices with minimal memory and processing power to communicate with each other on a low-power, lossy network. It uses methods such as POST, GET, DELETE and PUT, and easily works with HTTP, XML and JSON for integration with other web functions, much like using a Web API.
The Lightweight Machine-to-Machine (LwM2M) protocol, created by the Open Mobile Alliance, is based on CoAP. It includes a wide range of clearly defined and maintained standard objects, connectivity monitoring and remote device actions. It also has automatic firmware upgrades built in, which makes the management of devices much easier. LwM2M can also run strong DTLS security with minimal overheads.
The Data Distribution Service (DDS) is used for highly reliable, secure and high-performance connections in applications such as power generation, defence and air-traffic control.
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