Communication Stack
IoT devices are not separated from the global networking environment that nowadays is highly integrated, connecting various wired and wireless transmission standards into one network called the Internet. Indeed, some networks are separated because of security and safety reasons and regulations, yet they usually share the same standards as global Internet networks.
The XXI century brought wide acceptance of wireless connections. They became prevalent even in so-called pico-networks, like your PAN (Personal Area Network)[1], implemented using, e.g. Bluetooth Connection, where your smartphone in the left pocket of your jeans is hosting a server. There are many wireless devices connected to it: your wireless headset/audio device that you're listening to music, wireless HID controller, your Smartwatch, AR glasses, etc. All those devices constitute a personal (PAN-Personal Area Network) - a wireless network cell, usually also routed via NAT (Network Adress Translation) [2] to the Internet, through some other wireless connection gateway or router, using WiFi or mobile data (3G/4G/LTE).
Many IoT devices share standard wireless protocols, models and ideas, but some are not powerful enough to integrate with the Internet. On the other hand, wireless connections are natural to IoT devices, where they are expected to operate in remote destinations, usually without any wired infrastructure. Because of it, some Internet standards were adapted to their constraints or networks of constrained devices are separated and gatewayed through more powerful devices, where protocol translation occurs. Those models are discussed below in the following chapters.
In a similar way to regular Internet networking, IoT networking is implemented using a (usually simplified) layered stack, similar to the regular ISO/OSI 7-layer networking stack well known to all IT students [3], where the lowest 3 levels constitute so-called Media layers of the stack (recommendation X.200).
Level 1 is a Physical Layer (PHY). On top of it, level 2 is the Data Link Layer with Media Access Control and Logical Link Control (MAC/LLC). Level 3 is the Networking layer (NET) where packets are formed and routed as presented in figure 1. ISO-OSI model was designed and implemented for dedicated network controller chips and powerful processors; thus, not all IoT devices can fully implement this model, mainly because of constrained RAM and storage memory sizes. Also, this model requires an instant connection to the remote node (PHY dependent), so it strongly impacts the battery drain in the case of constrained-power devices.
A quick overview of popular communication technologies for the Internet is presented in figure 2. Note wide distance range between nodes of the Wireless devices regarding protocol used (primarily because of the PHY nature) where it varies from some meters in case of piconets up to some 180–2000 km when considering LEO (Low Earth Orbit) satellites, e.g. communicating through Iridium network and even up to about 35 786 km in case of the use of the geostationary satellites [4].
Another factor is the communication bandwidth. Fortunately, IoT devices usually do not require high bandwidth – a couple of kbps is enough; thus, almost all protocols apply here.
In many cases, IoT remote, distant nodes do not need constant communication, e.g. weather sensing would better communicate on a datagram communication model (UDP rather than TCP) [5]. In such cases, IoT devices utilize a simplified IoT stack, as presented in figure 3.
