## 4 Application layer
### IoT Connectivity  
Connectivity is a crucial aspect of IoT networks. Relevant parties must determine the optimal connectivity to satisfy the demands of the applications. There are numerous market-available IoT connectivity solutions; however, each has characteristics that fit certain applications. Deploying various interfaces ensures full connectivity and empowers IoT networks [84]. The IoT should offer seamless connectivity and secure Internet access everywhere. Heterogeneous IoT networks support two main types of connectivity: short- or long-range communication technologies. Figure 9 illustrates the common long- and short-range communication technologies used in IoT networks [84; 85]. Long-range technologies, including 5G and low power wide-area network (LPWAN), are intended for outdoor applications, while short-range technologies, on the other hand, such as Wi-Fi, Bluetooth, Zigbee, and near-field communication (NFC), are widely used for indoor IoT applications [85]. 1. Short-range technologies Short-range communication interfaces provide coverage for a maximum of tens of meters. It is primarily suitable for indoor IoT applications. In the following, we consider the most common short-range interfaces [84; 85; 86]. 1. Bluetooth  
2. Bluetooth has long been recognized for its ability to stream enormous amounts of information. It is the optimal solution for personal IoT devices, such as activity trackers and wearables. Additionally, Bluetooth is made with low-power IoT gadgets. It is ideally suited for tools that offer small data values. These systems are configured to save power if data are not transmitted. Due to the narrow communication range and heavy battery consumption, it is used in a restricted number of industrial projects. However, these cons have been tackled in Bluetooth Low Energy (BLE). Figure 8: Structure of IoT networks. ZigBee Zigbee is a wireless communication technology designed for low-power, short-range, and low-data-rate applications. It is based on the IEEE 802.15.4 standard, which defines the protocol's physical (PHY) and medium access control (MAC) layers. This standard operates in the 2.4 GHz band, although some regional variations use other frequencies. It fits well for many indoor IoT applications, including smart homes and smart lighting systems. 3. IEEE 802.11 (Wi-Fi) Wi-Fi is the optimal choice when transferring massive files, with a transfer rate 20 to 30 times higher than Bluetooth. Wi-Fi was not made for IoT networks; however, there are two currently improved IEEE standards, namely, 802.11 ah and 802.11 ax, for IoT applications. Sensors, which are critical components of IoT networks, run on batteries and send small quantities of data across vast areas. As a result, they require a different type of connection. 4. NFC NFC has a very short communication range, typically around a few centimeters. This close proximity requirement ensures that NFC communication is intentional and secure. It is a vital solution for secure and straightforward communication between electronic devices, e.g., smartphones. NFC operates at radio frequencies of 13.56 MHz, which is within the high-frequency (HF) band. This frequency band is unlicensed. NFC is commonly used for contactless payments, enabling users to make transactions by tapping their NFC-enabled payment cards, smartphones, or wearable devices on a compatible point-of-sale (POS) terminal. 5. Long-range technologies With the rapid growth of IoT technologies, numerous use cases in numerous sectors, such as security, asset monitoring, agriculture, smart metering, smart cities, and smart homes, have been identified [84]. Long range, low energy consumption, and cost effectiveness are all important characteristics of IoT applications. Popular short-range radio technologies, such as ZigBee and Bluetooth, are not intended for long-distance transmission. Thus, long-range radio technologies have evolved in response to the needs of current IoT applications [84; 85].