*4.1. Communication Protocols*

The general architecture of IoMT is reported in Figure 1. The provided architecture is composed of three main layers: (1) the things/devices layer, where the WBAN is installed, along with the gateway devices, (2) the fog layer, and (3) the cloud layer [53,54]. Different communication technologies can be identified within each layer, which enable transmission of the data from the end devices to the end user.

In the first layer, different sensor nodes are installed on the body, which build the WBAN. Sensor nodes can be implantable, wearable or mobile, placed for example in the hand of the patient. In this type of network the communication between sensors is carried out within a short range of 2 to 3 m. These devices, basically, require small power sources with respect to the safety and security of the user. Therefore, with respect to the low power specification and small communication range, the standard Industrial Scientific and Medical (ISM) band is sufficient to cover the installed nodes [55,56]. Various communication technologies are supported in the ISM band, such as ZigBee, Bluetooth and Wi-Fi. Moreover, alternative communication technology is introduced such as the Intra-Body Communication (IBC) technology [57,58]. Through IBC, the human body is used as a transmission medium, enabling power-saving, and thus improving the robustness and security of communications. Due to these advantages, IBC has been included as a third physical layer in the IEEE 802.15.6 standard for wireless body area networks designated as Human Body Communication (HBC) [59]. A central device, refereed also as base station, is responsible for collecting sensor data and forwarding them to the next communication layer. Accordingly, intermediate devices are installed as a bridge between the small interconnected WBAN and the exterior local network, namely the Wireless Local Area Network (WLAN). In this case, local gateways are used such as mobile devices, access points or simple midlayer gateways. Typically, they provide a bridge between the IoT edge devices and the fog and cloud servers. They enable the passing of data from the discrete sensor network to the other cloud and application layers. On one side, common communication technologies can be initiated between the WBAN' nodes and the intermediate gateways, such as the Wi-Fi and Bluetooth. In the other side, communication with the fog and cloud server can be realized through 5G, Wi-Fi or GPR [60,61]. To this end, data communication and storage are carried out over this layer, whereas in the IoT layer, installed wireless devices are periodically transmitting information. Sensor devices remain awake for a specific time frame from time to time to transmit the required information.

Within the second layer, local servers and gateway devices for the fog network are placed. These devices enable the processing of the collected data. Excessive and complex processing and data-mining algorithms can be carried out at this stage. Later, the collected data are redirected to the cloud layer for further processing. In the case of the cloud or fog layer, more powerful and long-range protocols are required, namely the LoRaWAN, Sigfox, NB-IoT and LTW-M [62], which ensure a better coverage range with a minimum of 1 km in urban deployment and 10 km in rural deployment. Moreover, the fog layer is in connection with healthcare experts responsible, which permits a reduction to the time delay of the interpretation and execution of specific tasks and decisions. In the third layer, powerful data storage and computation resources are installed. In this instance data analysis, decision-making and urgent intervention can be recognized. In addition, the cloud layer permits the incorporation of various and heterogeneous healthcare systems, which enables a real-time and continuous access to the current patient, equipment and planned tasks supervision and monitoring. Basically, this layer consists of cloud-based resources that will store the data generated by the medical infrastructure and be used to perform analytical work as needed in the future [54]. An overview of the common communication technologies used in WBANs is presented in Table 3 [63,64].

**Table 3.** Comparison between communication technologies used in the WBANs.

