## 4 The Architecture of IoT-Enabled Smart City
### Networking and Communication
#### 4.3.2 Cellular Mobile Networks  
Cellular mobile networks play a significant role in facilitating various services in smart cities. These networks are primarily designed to enable communication between human-to-human and human-to-machine interactions, such as telephony, texting services, multimedia downloading, and streaming [81]. The devices ecosystem also encompasses environment monitoring devices that gather data and require data exchange capabilities with the backend to receive feedback on the monitored data.  
The Radio Access Network (RAN) operates on a licensed network, while the Core Network (CN) includes various entities, such as user mobility and registration. For instance, the CN of the LTE system consists of several components, including the Serving Gateway (SGW), the Mobility Management Entity (MME), the Packet Data Network Gateway (PGW), the Home Subscriber Server (HSS), and the Policy and Charging Rules Function (PCRF) server [82]. In smart cities, the Internet of Things (IoT) has ushered in a new communication paradigm known as Machine-to-Machine (M2M) or Machine-Type Communications (MTCs). This paradigm relies on minimal or no human interaction, enabling devices to communicate directly with each other and exchange information without human intervention [83; 84].  
The characterization of M2M communications are featured distinctively concerning the \"legacy\" of human-to-human communications, and the end-user human is often replaced by a more significant number of devices, accessing the network periodically or irregularly [85; 86; 87]. Although 2G/3G (GSM, GPRS) cellular technologies are used for most M2M communications, the RAN and CN of mobile cellular networks face challenges due to the massive M2M data traffic [88]. Hence, extensive research is being conducted to effectively enhance cellular architectures to address the obstacles in M2M communications [87].  
As per the guidelines set by the Third Generation Partnership Project (3 GPP) [89], the main distinctive features of M2M concerning human-based communication include the following [80]:* The various market fields, such as smart grid or smart metering, environmental monitoring, and crowd monitoring applications, require different market scenarios to support their diverse applications.
* The end-user device should lower costs with powerful capabilities of handling energy efficiently.
* The number of communication terminals should be large enough to accommodate many devices.
* Every terminal should be able to efficiently handle traffic of varying sizes, ranging from small to large, that originates from the field devices or active smart devices and is transmitted to the network.  
In the IoT-based cellular ecosystems, the end-users play an essential role, and the main issues from the user's side are dealing with cost reduction and network assistive power-saving functionalities, which can improve the lifetime of the devices. From a network perspective, the most critical issues are improving the coverage area and defining lightweight data handling procedures for M2M devices to avoid data overloading difficulties [90]. It is imperative to take action to tackle traffic congestion in both the radio and core network layers [55] and manage the interplay between M2M communication by effectively allocating radio resources. The GSM [91] and LTE standards [92] are some of the initiatives launched by 3 GPP to address these challenges [93]. An outline of the primary characteristics of upcoming standardization endeavors in cellular IoT is provided in Table 2.