# IoT-Enabled Smart Cities: A Review of Concepts, Frameworks and Key Technologies
## 2 IoT Technologies and Architecture
### Transportation/Network Layer  
The transportation or network layer provides the function of data routing and transmission. Network gateways/brokers can be used as mediators for integrating different IoT nodes, allowing them to transmit and receive data to and from different sensors for M2M connectivity. Regarding data transmission, different technologies and protocols are used. Proximity communication protocols include Bluetooth, radio-frequency identification (RFID) tag technology and near-field communication (NFC). Larger coverage networks exploit wireless technologies, such as Wi-Fi, Zigbee, long range wide-area network (LoRaWAN), Sigfox and 5G. Bluetooth and Bluetooth Low Energy [16] are low-power wireless communication technologies designed for personal area networks (PANs), which are suitable for low-bandwidth data transmission among mobile devices over a short range of up to 10 m. They are usually employed in smart contexts for connecting personal devices. RFID exploits radio frequencies for data communication and is usually used to uniquely identify objects, people, vehicles, etc. Communication is established between a reader device and a tag device, which can be passive or active. NFC [17] is similar to RFID, but it has an even shorter range for communication (up to few centimeters). Unlike RFID, NFC does not implement a reader/tag hierarchy and can be used for two-way communication. It is typically used for mobile payments and access control operations. Wi-Fi is based on the IEEE 802.11 standard and uses wireless frequencies (operating at 2.4, 5 and 60 GHz bands) to provide high-speed internet connectivity (from 1 Mb/s to 6.75 Gb/s) within a limited distance (up to 100 m) [18]. It is designed for wireless local area networks (WLAN). WiMAX (Worldwide Interoperability for Microwave Access) is based on the IEEE 802.16 standard, operating at 2-66 GHz bands and providing data rates from 1.5 Mb/s to 1 Gb/s [18]. It can support broadband wireless access for up to 50 km for fixed stations and between 5-15 km for mobile stations [19]. Zigbee is based on the IEEE 802.15.4 standard. It is a low-power and low cost protocol for wireless sensor networks (WSNs). It usually supports star, tree and mesh network topologies. The Zigbee protocol operates at 2.4 GHz and provides a data rate of about 250 kbit/s [20]. The data transmission range is similar to that of Wi-Fi (from 10 to 100 m). LoRaWAN is a low-power wide-area network (LPWAN) that is capable of transmitting over long ranges (about 10 km) and also supports multitenancy and multi-domain networks [14]. It consists of several gateways that can be added when the network size increases and a higher capacity is required. Narrowband IoT (NB-IoT) is an LPWAN protocol that operates on LTE licensed frequency bands, with a data rate of about 200 Kb/s [3]. The bandwidth of NB-IoT is about 180 KHz, and this protocol allows the connection of the order of hundreds of thousands of devices [21]. The NB-IoT technology also optimizes energy consumption, providing power control and power saving modalities [22]. LTE-M is another LPWAN protocol, operating at a 1.4 MHz bandwidth [23]. It was introduced with the aim of supporting a massive number of devices, providing 10 year battery life support [24] and supporting data rates of up to 1 Mbit/s, which is enough to satisfy the higher data rate requirements for devices such as video cameras and wearables [25]. The features supported in LTE-M include handover management, extended discontinuous reception and the suspension/resumption of radio resource control connection. Z-Wave is a low-power protocol typically used in wireless home area networks, operating at 868 MHz and 900 MHz frequencies [7]. Z-Wave devices cannot directly connect to the Internet or other mobile devices. Therefore, the Z-Wave network is based on the use of a controller, which acts as a gateway, in order to manage all connected devices and allow them to interact with other mobile devices via the Internet or local networks [26].