## 4 The Architecture of IoT-Enabled Smart City
### Communications in Multi-Tier Architectures  
Multi-tier architectures [99] rely on a layered approach to collect and process sensing data and establish a network infrastructure in a multi-hop manner. The sensor data are typically transmitted to central gateways, which then transmit the data to the internet using various communication technologies. The multi-hop transmission technique [100; 101] compensates for the limited communication ranges by introducing various radio technologies. This technique also helps to use an extremely low-powered device, which is essential for specific applications. On the other hand, a number of devices can be utilized due to the limited radio technologies needed to ensure connectivity. Table 4 compares several short-range communication standards for their use in multi-tier architectures [102]. Such architectures are based on a layered design and utilize multi-hop routing to establish a network infrastructure. The IEEE 802.15.4 standard [103] is designed for low-cost, low-data rate, and low-power consumption personal wireless networks. The protocol stack comprises PHY and MAC layers, and it supports operation on three unlicensed frequency bands, namely 868 MHz, 915 MHz, and 2.4 GHz. The standard uses the direct sequence spread spectrum (DSSS) modulation scheme, enabling 20, 40, and 250 kbps data rates on the three frequency bands. Over time, the IEEE 802.15.4 standard [103] has undergone various enhancements to improve its performance, especially in the PHY layers. The MAC layers employ carrier-sense multiple access with collision avoidance (CSMA-CA) mechanisms to facilitate channel access, synchronization, PAN association, and beacon transmission of devices. Numerous solutions have emerged from the IEEE 802.15.4 standard [103], such as ZigBee, 6 LoWPAN, WI-SUN, and ULP, which can be used for diverse applications in personal wireless networks. The Z-Wave Alliance developed the Z-Wave protocol stack for home automation applications. It utilizes Gaussian frequency-shift keying (GFSK) modulation and mesh network architecture for routing and security. The Z-Wave protocol stack defines all the available layers, and one company operates it to enable interoperability between companies. Variable data rates of 9.6/40/100 kbps are possible, and its communication range is similar to IEEE 802.15.4-based solutions. The Metering Bus (M-Bus) is a specialized bus-based transmission technique for metering utility services such as gas, electricity, and water. It is essential for developing smart utility services and can operate in three frequency bands (169/433/868 MHz) with star or mesh network topologies. It employs synchronized time-division multiple access (TDMA) routing protocols and can communicate up to a range of 300 m with high data rates of 100 kbps. Bluetooth Low Energy (BLE) has recently been very popular in audio streaming and is suitable for IoT applications. It is a low-powered protocol stack where the BLE devices consume low energy and can be used for a long time. The first modulation scheme is GFSK, with a 1 Mbps data rate. There are 40 different channels; 3 are used for advertising, and 37 are data channels. The BLE can integrate with IPv6 and supports the packet fragmentation technique with basic security features. BLE's major disadvantage is that it only supports mesh topologies, which limits the applications to real-life scenarios. Wi-Fi Low Power increases Wi-Fi range by using less power and operating in the 900 MHz frequency spectrum. As a result, it is appropriate for IoT applications. The data rate is