Media Layers - Wireless Network Protocols

Wireless connections define core communication for IoT devices. A vast and growing amount of protocols, their variations and the dynamic IoT networking market all present a non-solid situation where old “adult” Internet protocols coexist along with new ideas, and IoT hardware and software platforms are more and more capable with every new generation; thus new concepts appear almost daily. Currently, many IoT networking protocols are defined for various layers of the protocol implementation stack, some compatible while others are concurring. Figure 1 presents some selected protocols existing for IoT. This covers only the most popular ones and gives a non-exhaustive view. We discuss them in more detail below.

Below is a list of the most popular wireless protocols for the lower ISO/OSI layers 1–2 (Physical and Media Access Control); some also implement layer 3 – Networking in a single component).

WiFi is the set of standards for wireless communication using the 2.4 GHz or 5 GHz band, slightly different spectrum in different countries. The core specification of the 2.4 GHz contains 14 channels with 20 MHz (currently 40 MHz) bandwidth. While there is no centralised physical layer controller, collisions frequently occur even more with a growing number of devices sharing the band. The collision is handled using CSMA-CA with a random binary exponential increase of repeating time.

With the high transmission speed and range usually not exceeding 100 m, it is widely used as the direct replacement of wired Ethernet in local area networks. It is an excellent choice when the amount of data to be transferred is larger, for example, video streams or assembled IoT streams delivered by gateways.
It can also be used in direct connectivity for smart sensors and other IoT elements, but the protocol is not designed to transmit small data packets. It is too energy-consuming for many IoT applications, especially battery-powered devices.
Moreover, WiFi itself offers only 1-to-1 (figure 2 or star-like, 1-to-many (figure 3 topologies of connections, where the central point is a WiFi Access Point. It does not provide mechanisms, e.g. self-reorganised, failure-tolerant mesh networks.

WiFi has become a more and more popular choice for not-so-constrained IoT devices because they need to implement a full TCP/IP stack, and those devices that are also not so constrained with power resources. A list of WiFi standards and related transmission speeds is present in table 1.

Bluetooth is a prevalent method of connecting various devices at short distances. Almost every computer and smartphone has a Bluetooth module built in. Standard has been defined by Bluetooth SIG (Special Interest Group), founded in 1998. Bluetooth operates in the 2.4 GHz band with 79 channels with automatic channel switching when interference occurs (frequency hopping). The single channel offers up to about 1Mbps (where around 700kbps is available for the user) bandwidth, and it provides communication within the range from up to 1 m (class 3, 1 mW) to up to 100 m (class 1, 100 mW). The most prevalent version is class 2, with a 10 m range (2.5 mW).

Every Bluetooth device has a unique 48-bit MAC address.

Bluetooth offers various “profiles” for multimedia, serial ports, packet transmission encapsulation (PAN), etc. The PAN (Personal Area Network) Profile and SPP (Serial Port) are the most useful for IoT devices.

Now Bluetooth covers two branches: BR/EDR (Basic Rate/Enhanced Data Rate) for high-speed audio and file transfer connections and LE (Low Energy) for short burst connections ^[1].

Classical (before BLE and 4.0) Bluetooth networks can create ad-hoc, so-called WPAN (Wireless Personal Area Networks), sometimes referenced as Piconets. Bluetooth Piconet can handle up to 7 + 1 devices, where 1 device acts as Master and can contact up to 7 Slave devices. Only the Master device can initiate a communication. Fortunately for the IoT approach, much Bluetooth hardware can act as Slave and Master simultaneously, constituting a kind of router; thus, devices can include a tree-like structure named a scatternet as presented in figure 4.

Bluetooth Low Energy (BLE) uses a simplified state machine implementation and thus is more constrained-devices friendly. It offers a limited range and is designed to expose the state rather than transmit streamed data. However, it provides a speed reaching up to about 1.4 Mbps (2 Mbps aerial throughput) if needed. It uses a 2.4 GHz band but is designed to avoid interference with WiFi AP and clients. Communication is organised into three advertising channels (located “between” WiFi) and 37 communication channels.

Latest Bluetooth implementations (protocol version 5.0 and newer, implemented in mid-2017) offer a Bluetooth mesh network extending ubiquitous connectivity via a many-to-many communication model dedicated to IoT devices, lighting, Industry 4.0, etc. The Bluetooth mesh is layer-organised, and since there is no longer a Master-Slave model used, but messages are relayed through the mesh, it is considered to be no longer the Scatternet because of its flat structure ^[2]. Sample Bluetooth Mesh Network idea is presented in figure 5 and a review of the Bluetooth protocols in table 2.

Improvements introduced in the 5.1, 5.2, and 5.3 versions focused on localising neighbouring nodes better, on audio use, and improving power efficiency. Version 5.4 introduces a new feature that allows nodes to send encrypted data in the advertising frames (Encrypted Advertising Data). Another improvement is also focused on advertising frames, making it possible to respond to such a frame (Periodic Advertising with Responses). Both extensions are beneficial in the IoT world, allowing nodes to send small, encrypted packets using an advertising mechanism. Bluetooth 5.4 enables connectionless, bidirectional, secure communication with many low-power end nodes in the star topology.

Cellular (mobile/GSM) networks are viable options for IoT communication because of their omnipresence and long-range communication capabilities. Those networks use orthogonality in frequency and time spaces. Cellular networks are presented by the subsequent generations (G) – currently up to 5G on the market and 6G in the experimental phase (introduced in years 2025-2029, country-dependent). Typical GSM network technology, sometimes referenced as an era, runs out within about 10–15 years. It is pretty close but still less than expected end-of-life for classes of IoT devices (15-25 years). GSM hardware was backwards compatible, enabling users to access older, even before 2G GSM networks with the latest chips. Still, the presence of old-generation networks becomes sparse, particularly in metropolitan areas, where recent generations provide better coverage and capacity.

Figure 6 presents GSM network evolution over time and generations. Cellular networks use different frequencies in different countries, yet available radio implementations nowadays can usually handle all of them.

Figure 7 presents sample GSM hardware (separate module and ready shield for the Arduino platform).

GSM protocols are proprietary, complex (including advanced ciphering) and require dedicated hardware. Documentation and standards are not publicly available because of security considerations (e.g., voice transmission ciphering details).
On the one hand, the GSM network seems to be a good solution for extended distant IoT networks. They have many disadvantages, however: they require operators' infrastructure, as GSM bands are not free, and GSM signalling requires quite decent energy.

Besides limited access constraints, one more particularly important exists: GSM modems use quite a significant amount of energy when establishing a connection because they need to broadcast their existence as far as possible to gain a link with a possibly distant-located base station. It requires tremendous power and drains the battery (up to 10 W peak); thus, cellular solutions are unsuitable for IoT devices that use frequent data communication and are constrained on energy resources.

ZigBee protocol is prevalent in Smart House but also in Industry appliances. Zigbee is a wireless technology developed as an open standard to address the needs of low-cost, low-power wireless machine-to-machine networks. However, it is more popular in the industry because of the relatively higher equipment cost than WiFi, Bluetooth or other RF modules.
The Zigbee standard operates on the radio bands 2.4 GHz for smart home applications, 915 MHz in the US and Australia, 868 MHz in Europe and 784 MHz in China. The advantage of ZigBee is the possibility of forming mesh networks where nodes are interconnected with others, so there are multiple paths connecting each pair of nodes. Connections are dynamically updated, so when one node turns off, the path going through that node will be automatically rerouted via another route.
Transmission speed is up to 250 kbps, with a theoretical range of up to 100 m but usually to some 10–30 m.
ZigBee does not provide direct, unique IP-addressing on the Networking layer like 6LowPAN or Thread do. A single ZigBee network can handle up to 65000 devices.

Z-Wave is a protocol similar in principle to the ZigBee, but hardware is cheaper; thus, it is more towards inexpensive home automation systems. Like in ZigBee, Z-Wave operates on different frequencies depending on the world region, usually between 865 MHz and 926 MHz. The transmission speed is up to 200 kbps, and the range is 100m. A single Z-Wave network is pretty limited in the number of concurrent devices in one network, that is, only 232 devices. Each Z-Wave network has a unique ID, and each node (device) in a network has a unique 8-bit identifier.

Another standard ^[3] works using the same 802.15.4 radio and is based on IPv6. There are some differences in the protocol, like address allocation. Like the Z-Wave mentioned above and Zigbee, Thread uses mesh network topology. It incorporates encryption, authentication, and secure key management to protect communication between devices on the network. It is also energy efficient, allowing devices constituting a mesh network to fall asleep and awake when only needed for communication. Those mechanisms cover (among others) asynchronous communication, scheduled sleep, routing concerning the devices' energy resources, adaptive data rates, wake-on-radio, and paging mechanisms (waking up only a selected group of devices).

NFC (Near Field Communication) is a technology that enables two-way interactions between electronic devices. One device mustn't have to be equipped with the power source – the receiving radio signal powers it. That's why NFC is used in contactless card technology, enabling devices to exchange data at a distance of less than 4 cm. Transmission speed varies between 100–420 kbps, the range between active devices is up to 10 cm, and the operating frequency is 13.56 MHz.

Sigfox ^[4] is the idea to connect objects with sub 1 GHz radio frequency. It uses the 900 MHz frequency range from the ISM band. The range is about 30–50 km (open space) and 3–10 km (urban environments). This standard uses a technology called Ultra Narrow Band (UNB). It has been designed to transmit data with deficient speed – from 10 to 1000 bps. Thanks to small data packets, it consumes only 50 mW of power. It is intended to create public networks only, so using Sigfox requires a subscription plan. The Sigfox network covers many (but not all) European countries.

LoRa (Long Range) is the technology for data transmission with a relatively low speed (20 bps do 41 kbps) and a range of about 2 km (new transceivers can transmit data up to 15 km). It uses CSS (Chirp Spread Spectrum) modulation in the 433 MHz or 868 ISM radio band.

A chirp signal is characterized by a continuous frequency sweep over time. This means that the frequency of the transmitted signal starts at some lower frequency and continuously increases throughout the transmission of a single symbol. In LoRa the starting frequency differs depending on the symbol encoded, and while the modulated signal achieves the maximal value of the frequency starts from the minimal one. It means that each chirp uses the whole available bandwidth. Chirp Spread Spectrum modulation makes LoRa signals less susceptible to interference and noise and allows LoRa to achieve long-range communication. LoRa modulation is characterized by two parameters:

• Spreading Factor determines the speed of the signal frequency change over time. Higher spreading factors result in a longer communication range but lower data rates. It also defines the number of bits encoded by one chirp.
• The Bandwidth of the LoRa signal determines the amount of spectrum occupied by the transmitted signal. It can be 125, 250 or 500 kHz. It also specifies the sampling frequency of the signal in the receiver.

Having these parameters it is possible to calculate the efficient data rate (in bps). Because the range of LoRa communication is long, transmitters can interfere, so some rules for the maximum time of occupation of the channel were introduced. In the European Union, the maximum percentage of transmission time known as the Duty Cycle is 1%. This gives a maximum transmission time of 864 seconds per day. Transmission should be as short as possible, and the delay between following transmissions should last a few minutes. The duty cycle together with bandwidth and spreading factor makes it possible to calculate the maximum payload of the frame and the bitrate. Some online calculators help set LoRa parameters to fulfil the local regulations ^[5].

The cell topology is the star, with the gateway at the central point. End devices use one-hop communication with the gateway. A LoRaWAN gateway is usually connected to the standard IP network with a central network server. The LoRa technology is supported as LoRaWAN by LoRa Alliance ^[6] designed as Sigfox for public networks. Still, it can also be used in private networks that do not require a subscription. LoRaWAN uses simplified messaging, where collisions are solved at the server level.
The major assumption for the LoRaWAN network is each end-node device is within a range of at least one LoRaWAN gateway.
There are 3 classes of devices in LoRa:

• Class A: where downlink is active only after the device uses uplink in a particular time window (twice). It has the greatest energy efficiency among other classes. Downlink opportunity appears asynchronously, so this class is for scenarios where low latency is not a critical requirement.
• Class B: with scheduled receive window, where the downlink is synchronised; thus, the LoRa device listens to the downlink periodically. This causes increased energy use, however.
• Class C: is a class where the device listens to the downlink communication almost continuously. This brings the lowest latency in communication and the highest energy demand compared to the other classes.

Traditionally, we use IP addressing (usually masked by DNS to be more user-friendly) when accessing Internet resources. IoT devices may also benefit from this approach. However, constrained devices require special “editions” of the conventional protocols, which are lightweight. The networking layer implements the basic communication mechanisms on the packet level, like routing, delivery, proxying, etc. Many IoT, lightweight implementations of the protocols presented below benefit or at least inherit ideas from regular “adult” implementations. Please note that some protocols implement more than one layer, as illustrated in image 1. We also provide a short reference of the IPv4 and IPv6 to show advantages and drawbacks.

Internet Protocol v4 (1981) is perhaps the most widespread networking protocol. The predecessor of the IPv4 protocol, originally called IP, was introduced in 1974 and supported up to 2^8 hosts, organised in 2^4 subnetworks (RFC 675).

In IPv4 (RFC 760/RFC791), the logical addressing space was extended to 2^32 devices, which seemed to be quite much in 1981, but now we struggle with a lack of free addressing space. This number is less because some addresses are reserved, e.g. for broadcasting and due to the existence of different classes of addresses and their pools ^[7]. Sample IPv4 address is, for example, 157.158.56.1.

Some relief to the suffocating Internet was brought as an ad-hoc solution with the NAT (Network Address Translation) introduction. NAT-enabled subnetworks are those where one public address represents a set of devices hidden behind the router. However, that limits usability because of the lack of direct access and unique identification in the global network level of the devices sharing private address spaces. Even so, more than 29 billion IoT devices are expected to be connected to the Internet by the end of 2030, according to Statista forecast ^[8]. They all need to be uniquely addressed!

IPv6 is the next generation of the IPv4 protocol. It is supposed to replace IPv4. The transition process is not as quick as expected because many Internet and intranet services implement IPv4 only and would become inoperable if IPv4 were unavailable.
IPv6 brings addressing space large enough to cover all existing and future needs as it is possible to forecast. The number of possible addresses is 2^128. Addresses are presented by 8 groups of 4 hexadecimal values, e.g. 2001:0db8:0000:0042:0000:8a2e:0370:7334.

This brings the capability to uniquely identify any device connected to the Internet using its IPv6 address. Regarding IoT, implementations have many drawbacks (IPv4 also has them). IPv6 network is star-like, whereas IoT networks can benefit from the mesh model. IPv6 network requires a controller providing free addresses (a DHCP server) – devices must contact it to obtain the address. Every IoT device needs to keep a list of devices it corresponds with (ARP) to resolve their physical address. Moreover, full IPv6 stack implementation requires large RAM when used.

The name is the abbreviation of “IPv6 over Low-Power Wireless Personal Area Networks” ^[9] and, as it says, it is the IP-based network.
This protocol was introduced as a lightweight version of full IPv6, IoT-oriented.
This feature allows connecting 6LoWPAN networks with other networks using a so-called Edge Router. Thus, every node can be visible on the Internet and uniquely addressable, as stated in the IoT principles. This standard has been developed to operate on the radio channel defined in 802.15.4 (as ZigBee, Z-Wave). It creates the adaptation layer that allows the use of IPv6 over the 802.15.4 link. 6LoWPAN has been adopted in Bluetooth Smart 4.2 standard as well.

6LoWPAN supports two addressing models: 64-bit and 16-bit. The former limits the number of devices connected to one network to 64000 nodes. The primary frame size is just 127 bytes (compared to full IPv6, where it is 1280 bytes at least). 6LoWPAN supports unicast and broadcast. It also supports IP routing and link-layer mesh (802.15.5) that introduces the fail-safe redundant, self-organising networks because the link-layer mesh can have more than one Edge Router. 6LoWPAN uses autoconfiguration for neighbour device discovery, so it does not require a DHCP server. It also supports ciphered transportation using AES 128 (and AES 64 for constrained devices).

6LoWPAN devices can be just nodes (Hosts) or nodes with routing capability (Routers) as presented in figure 8.

The Edge Router implements a gateway between 6LoWPAN and the regular IPv6 (IPv4) network. It aims to translate “compressed” IPv6 addresses to ensure bi-directional communication between the Internet and 6LoWPAN nodes. Note – the network structure of the 6LoWPAN is logically flat (star/mesh with single addressing space), and devices have unique MAC addresses to be recognisable by the Edge Router device.