## 9 Conclusions  
This work has explored the role and current status of various MAC/PHY layer technologies and protocols, networking-layer protocols, and application-layer protocols in the development of smart cities. Technologies that include Wi-Fi, cellular networks, Bluetooth, RFID, NFC, Z-Wave, Zigbee, Sigfox, and LoRa support LoRaWAN, 6 Lo, BLE, 6 loWPAN, Wi-Fi, HaLow, etc. Based on the information collected through Table 3, we conclude that the general availability of Wi-Fi and cellular networks has remained unbeatable. Moreover, technologies such as Z-Wave, SigFox, and LoRa have the potential to reduce the responsibility of cellular networks for low-data-rate-based applications. The data security concerns in cellular and Wi-Fi networks are also adequately addressed in Z-wave and SigFox for wide area communication. On the other hand, these MAC layer protocols are responsible for handing internet protocols such as IPv4- and IPv6-based TCP and UDP connections. These networking protocols support application layer protocols such as MQTT, CoAP, XMPP, AMQP, DDS, LWM2M (lightweight M2M), OPC UA, STOMP, HTTP/HTTPS, etc. Operations like smart grid, smart building management, smart agriculture, smart healthcare, smart public safety, smart energy management and scheduling, smart waste management, smart water management, smart lighting, and smart transportation systems are only possible if the backbone of the city is supported by the aforementioned physical layer infrastructure, MAC layer, and application-layer-protocols-based networking schemes.  
We conclude that the application-layer protocols, MAC layer, physical-layer protocols, and network-layer protocols play importance roles in enabling smart cities. The application layer enables the infrastructure to develop and implement specific applications such as smart lighting, supply chain management, energy management, traffic management, etc. Such applications directly improve smart cities. Likewise, the MAC layer enables efficient communication for the stated applications for which the physical layer is responsible for the provision of the required infrastructure for wireless and wired communications. The need of 6G is also realized with time as its development aids the expansion of smart technologies and smart environments for its growing connectivity and enhanced data rates.  
Despite this, the current advancements in IoT for smart cities, such as the use of massive connectivity environments, large inputs, and utilization of big data analytics, edge computing and machine learning, are helping to gain command over the challenges faced in the expansion of smart cities. This way, smart cities will become more intelligent, productive, and sustainable.  
Conceptualization, J.A., C.H. and M.H.Z.; methodology, J.A.; software, J.A.; validation, J.A., C.H. and M.H.Z.; formal analysis, J.A.; investigation, J.A.; resources, S.R.H., R.A.; data curation, J.A.; writing--original draft preparation, J.A., C.H.; writing--review and editing, J.A., C.H., R.A., S.R.H. and M.H.Z.; visualization, J.A.; supervision, C.H., M.H.Z.; project administration, M.H.Z.; S.R.H. and R.A.; funding acquisition, R.A. All authors have read and agreed to the published version of the manuscript. The authors acknowledge the internal research start-up funding, reference: 1012606 FA1, from the University of East Anglia (UEA), Norwich, UK. The authors declare no conflict of interest.