If we consider IoT trends, they show that there are a lot more IoT devices than predicted. Based on 2019 data, there are 9,5 billion IoT assets, compared to the estimated 8,3 billion. The number of IoT devices could reach an estimated 28 billion by 2025 [1]. Surveys show that the vast majority of these devices are related to smart city applications. Researchers among the ten most relevant trends are also among the smart city solutions in 2020 [2]. LoRaWAN is also one of the IoT, LPWAN communication technologies [3]. LoRaWAN technology is wireless, low-power, long-range radio communication that works reliably even in weak signal propagation conditions [2] [4]. Because of these properties, they are preferred for use in smart city solutions. With the help of LoRaWAN devices, which are connected to the network, large areas, especially cities, can be easily covered. The following conclusion addresses LoRaWAN devices that are suitable for smart city solutions. The grouping of smart city solutions and areas is possible from several aspects. Six core areas can be distinguished: environment, life, people, mobility, economy, and government [5] [6] [7]. These include, for example, trackers, consumption meters, environment sensors, and parking sensors with LoRaWAN technology. LoRaWAN communication is one of the most significant technologies used in smart city solutions. This is shown by the presence of available LoRaWAN networks in 162 countries [8]. Numerous research and development projects deal with LoRaWAN due to its topicality. An important aspect when applying a new technology is the set of requirements and challenges it has to meet and the issues it may need to solve. Each technology has its own limitations that need to be considered in such studies; this is also true for LoRaWAN. In addition, similar, competing LPWAN technologies also present challenges. The LoRa Alliance is responsible for the LoRaWAN specification, which is constantly being developed, improved, and upgraded with newer versions [8] [9]. These specifications also raise some issues. The application environment of a LoRaWAN solution (indoor, outdoor, fixed, mobile) presents additional challenges. In addition, it is important to highlight that certified LoRaWAN products can be categorized by functionality. The categories contribute to research and findings. The structure of the article is as follows: The introduction is followed by a description of LoRaWAN technology. Next is a summary of the grouping of certified LoRaWAN products, as well as the significant challenges arising from the technology, followed by a discussion of the aspects of LoRaWAN specifications. The challenges posed by the nature of the application are also summarized. The review ends with a conclusion, which includes the possibility of additional study areas.

LoRaWAN network technology is based on LoRa technology developed by Semtech [4] [10]. The first issued tool appeared around 2013-2014. The name LoRa is an abbreviation of ‘long-range’, which refers to the long data transmission distance that the technology is capable of. The LoRa Alliance was founded in 2015 to regulate the network operation of this technology [11]. The essence of the technology is to be able to build large-scale wireless networks, even in unfavourable field conditions, all with low energy consumption. Considering that we are talking about IoT technology, we must not forget about mobility and that it is possible to establish secure two-way communication. The standard, which is continuously updated by the LoRa Alliance, allows developers and users flexible networking and use. The essential features of LoRa and LoRaWAN communications related to the topic are highlighted. The long range of LoRa technology is due to the use of special frequency modulation, called chirp spread spectrum (CSS) modulation. The LoRaWAN specification defines the lower two layers of the OSI model: the physical layer and the MAC sublayer of the data link layer; and the application layer. Communication in Europe takes place at 868 MHz and 433 MHz, but 868 MHz is typical. Communication is two-way, with multiple channels. The devices are characterized by low power consumption; 10-year operating time is also available with a button cell. The data transmission distance in the city is a few kilometres, but it can be up to 10-15 km in less populated areas. The data transfer rate is relatively low, at a maximum of around 50 kbps. During communication, the messages are encrypted. The communication uses AES-128 encryption [11] [12]. The features can be summarized as: • chirp spread spectrum (CSS) modulation, • 868 MHz and 433 MHz frequencies in Europe, • wireless radio two-way communication, • low consumption, • 1 % transmission time, • low consumption, • 10-15 km bridgeable distance, • 0,3-50 kbps data transfer rate, • AES-128 encryption, • star-of-stars network topology. Network participants include end-devices, gateways, the network server, and application servers [12]. Communication between nodes and gateways takes place via LoRa. The connection between a gateway, a network server, and an application server is IP based. Nodes are usually simple devices equipped with LoRaWAN communication capability and some sensors. Gateways are connected to end-devices. There can be multiple nodes and gateways within a network. Gateways, in addition to network end-nodes, communicate with network servers. The application server and the network server are connected to each other and can be physically present on one device. The network server is responsible for monitoring and managing the network format. The application server decodes the data and displays it in a way that the user can understand. Depending on usage, multiple applications can be connected to a network. This classic network architecture is based on the 1,0 specification; other LoRaWAN specifications may result in a different network architecture [13] [14] [15] [16]. Nodes can be divided into three classes according to their operation mode: Class A, Class B, or Class C. In all three cases, two-way communication is supported. Class A operation is optimized for consumption, with Class A devices having the lowest power consumption. Class A devices can only receive messages after they are sent, after which they open two reception windows. It is possible to send messages to the end-devices from the network server side at any time, but they can only be received after the end-device transmission period. Class B nodes operate with extra, scheduled reception windows in addition to the previous ones. In the case of Class C devices, no waiting time is to be expected; they are capable of continuous reception. This type has the highest consumption. There are uplink and downlink messages regarding the direction of data communication. An uplink message transmits from the end-device to the network server; the downlink message’s direction is the opposite. The uplink message can be an unconfirmed data message or confirmed data message [12]. Identifiers and keys are used during network communication. The most significant of these are the following. Device Address is a 32-bit identifier used to identify devices [12]. This ID is unique within each network and is part of the messages. Also worth mentioning the Network Session Key and the Application Session Key, which are 128-bit AES encryption keys for encrypting and decrypting data and messages [12]. The connection procedure is essential for the network operation of the nodes. This can be ABP (Activation by Personalization) or OTAA (Over-the-Air Activation) [12] [17]. In the case of ABP connection mode, a request is received from the end-devices to the network server once before starting the network operation. If the IDs and keys match, the connection is established. In OTAA connection mode, after a successful connection, a handshake connection is established between the nodes and the network server. The keys are exchanged continuously, ensuring a much more secure connection [17].

The LoRa Alliance is responsible for the certification of LoRaWAN products. The number of certified devices published on their website was 177 at the time of writing the review [18]. If LoRaWAN products are categorized, additional tests can be performed. Besides, it is necessary to describe the specific categories. There are eight categories based on the function categorization of certified LoRaWAN devices. These are as follows: • module: a PCBA card, a basic circuit set that complies with the LoRaWAN specification and features long-range, low-power, bidirectional, wireless communication. The LoRaWAN module is not a finished product, and it is the main chip of LoRaWAN application development and finished product production [18] [19], • development board: a printed circuit board that fundamentally contains a LoRaWAN module and the necessary additional logic associated with it, used for learning and prototyping [18] [20]. The LoRaWAN development board can include a microcontroller or can be connected to microcontroller development boards via a typical connection line, • sensor node: a finished product which consists of three central units: a sensor unit, a processing unit and a LoRaWAN module [18]. The sensor node can detect specific changes in the environment, react to them and transmit the processed data over the LoRaWAN network [21] [22], • actuator node: a finished product which consists of three central units: an actuator unit, a processing unit and a LoRaWAN module [18]. The actuator can make a specific change in its environment [23], depending on the messages received on the LoRaWAN network, • sensor interface unit: a finished product which consists of three central units: a sensor connection, a processing unit and a LoRaWAN module. The sensor interface unit connects to sensor devices [18]. It can collect data from them and then transmit the data on the LoRaWAN network after processing [24], • meter interface unit: a finished product which consists of three central units: a standard meter connector or a standard communication interface, a processing unit and a LoRaWAN [18]. The meter interface unit can be connected to consumption meters and can collect data from them and then transmit them on the LoRaWAN network after processing, • tracker: a finished product which consists of three central units: a locating unit, a processing unit and a LoRaWAN module [18]. The tracker is suitable for tracking persons/animals/objects, and the location data are transmitted via the LoRaWAN network [25], • tester: a complex device that monitors and diagnoses the LoRaWAN network parameters, especially for end-devices and gateways. Another feature may be RF power testing. [18] It is also possible to create categories according to other logic. By this logic, the ninth group would be gateways, but there is no authentication procedure for these. The formed groups can be further divided into subgroups according to further aspects. Table 1 shows the grouping of LoRaWAN products by functionality [18]. There may be devices with properties that fit several groups. In these cases, the main functionality applies.

For a smart city, the additional findings apply to the following categories: sensor node, actuator node, sensor interface unit, measuring interface unit and tracker. These categories represent a total of 127 different products. The study does not consider devices that use LoRaWAN technology but are not on the list of certified devices. These devices can be directly or indirectly part of smart city solutions.

Considering the challenges of LoRaWAN technology, the following three main issues can be highlighted: competing technologies, technological limitations and network operation. Closely related to these are the aspects of the specifications, which are discussed in a separate section. LoRaWAN technology belongs to LPWA networks; there are many other technologies in this area. If we highlight two major LPWAN technologies – Sigfox, and NB-IoT – the LoRaWAN technology performs below regarding the following features. The properties of the three technologies are summarized in Table 2.

The maximum data rate of LoRaWAN technology is not the lowest, at 50 kbps. The length of the payload is 1600 bytes for NB-IoT and 243 bytes for LoRaWAN. It belongs to the middle field in terms of range (5-15 km). It is outstanding that LoRaWAN technology is actively immune to interference and uses AES-128 encryption during communication; the adaptive data rate is implemented, and built-in positioning is used. These technological characteristics affect the areas of application. It should be emphasized that a great advantage of LoRaWAN technology, compared to similar technologies, is that private networks can be implemented with it. The private network avoids dependence on the service provider. Also, the cost of LoRaWAN network equipment and the spectrum is the lowest. Some of the technology limits measured above have been presented. LoRaWAN end-devices implement at least Class A operation options, in addition to which Class B and C operations are optional [13]. The operation mode is vital in the case of bi-directional communication. Class A operation results in the nodes being able to receive only for a limited time after transmission. This way, messages sent to the end-device only arrive after the transmission period of the end-device, which can reduce the application areas. This problem does not exist for Class C operation. Communication takes place at 868 MHz or 433 MHz in Europe, in the ISM band [13]. As a result, the transmission time is also limited. The classic LoRaWAN network consists of end-devices, gateway(s), network- and application server(s) [13]. End-devices communicate with servers through gateways. It follows that the gateway is the weakest point in the network. There can be multiple gateways in a network, which increases the robustness of the communication. In addition to the implementation of private networks, it is also possible to use a service provider network and a network service. The conclusions to be drawn for smart city solutions are as follows. This technology is not the right choice in cases: • at high data rates, • in case of transmission of large data, • in case of transmission of large amounts of data, • continuous two-way communication is essential, • and any combination thereof. In the case of collecting small measurement data that needs to be transmitted continuously but at infrequent intervals, this technology can be the right choice. A typical area of application is consumption metering. Unfortunately, developers also have to deal with changing user needs. Previously, it was appropriate to have meter readings taken monthly. In the case of a consumption meter with wireless communication, there are demands that users want to see the data continuously. Comparing the monthly meter readings with the continuous data provision, this seems unreasonable. Of course, the technology allows data to be provided more frequently than monthly.

The LoRa Alliance is responsible for LoRaWAN specifications and certification. Newer specifications are issued mainly for improvements, to correct bugs, and for clarifications [28] The specifications include everything that the LoRaWAN devices and networks must comply with. Compared to the 1,0 specification published in 2015, the next version (1.0.1) was released in early 2016 [12] [14]. This version mostly includes spelling corrections and clarifications. The next version, 1.0.2, was released in mid-2016 with some minor changes and fixes [15]. Subsequently, specification 1.1 was released in 2017, which resulted in significant changes [15]. In addition to clarifications, it contained several functional modifications. The changes that appeared were related to the following areas: frame counters, device classes, activation modes, keys and server structure. Version 1.0.3 was released in 2018 and took over the parts for Class B devices from Specification 1.1. Previously, Class B (beacon) operations could only be encountered on a theoretical level [13].

LoRaWAN specification	1.0	1.0.1	1.0, 1.0.2	1.0.2	1.0.3	1.sij
Number of devices	6	38	0	82	1	0

Due to several innovations in the 1.1 specification, it is not popular with developers and operators. It is less understandable compared to the 1.0.x series [28]. If we look at the comparison tables (Table 3 and Table 4), it turns out that most of the products that meet the 1.0.2 specification are in all categories. Only one product meets the latest specification (1.0.3). Of course, the specifications are backwards compatible with each other. The conclusion is that using the latest specifications can avoid bugs and misunderstandings that can be found in older versions. It is recommended to use the 1.0.3 specification.

Specification	Sensor node	Actuator node	Sensor interface unit	Meter interface unit	Tracker
1.0	5	0	0	1	0
1.0.1	22	1	2	7	6
1.0 and 1.0.2	0	0	0	0	0
1.0.2	55	2	4	16	5
1.0.3	1	0	0	0	0
1.sij	0	0	0	0	0