## 1 Introduction  
Secure and ease in human lifestyle have led to a healthy increase in the adoption of smart cities in the last decade. In a smart city, its residents enjoy smart services, such as IoT-based healthcare services, intelligent farming, live surveillance, smart industry, and intelligent transportation systems [1, 2]. Improved communication and advancement in information technologies help in provisioning quality implementation of these smart city applications by ensuring secure and reliable data delivery [3, 4, 5, 6]. Smart city applications save their data on cloud servers for analysis and take proactive measures to avoid any disturbance. However, due to remotely placed cloud servers with increased propagation delay and with the emergence of fog-computing nodes, which are created by placing computing nodes in the near vicinity of smart city applications, fog-computing nodes are now becoming a permanent part of IoT-based smart cities, as shown in Figure 1.  
Smart cities rely on IoT applications, which use numerous sensors to transmit data to other nodes by sharing the medium. Different medium access control (MAC) protocols have been designed for wireless sensor networks. MAC protocols are differentiated into contention-based and contention-free access. In contention-based MAC protocols, data-sending nodes have to contend with other nodes to transmit their data within the medium [7; 8]. However, in contention-free MAC protocols, data-sending nodes are assigned dedicated slots to transmit their data without contending with other nodes [9; 10; 11; 12]. The increased number of nodes increases the probability of collisions in contention-based MAC protocols; hence, contention-free MAC protocols are preferred in such environments. Wireless mediums in IoT networks are vulnerable to malicious nodes causing network collision, denial of service, and unfair allocation [13]. The presence of malicious nodes raises a question as to the trust level of the nodes' requests, and a trust level of the networking nodes is required, resulting in compromised QoS in the networks [14; 15; 16; 17]. Attacks during the contention period are hard to identify because there are already chances of collision, medium busyness, and unfairness due to system occupation by other nodes. Thus, contention-free MAC protocols are preferred over contention-based MAC protocols because there are dedicated slots for each node and because interference of any other node can easily be determined. However, in contention-free MAC protocols, malicious nodes acting as member nodes are allocated dedicated slots in each communication session and disturb the medium, leading to reduced medium utilization. In smart cities, different applications require sensor nodes to collect and transmit data in various time intervals. These data requirements may change based on the situation and need to be adaptable. A classic TDMA-based MAC protocol with fixed-duration time slots for all nodes is not recommended because each node is assigned a fixed-duration time slot in each communication session. If a node wants to transmit more data than the  
Figure 1: Smart city applications with fog nodes. assigned allocation, it may not be possible during that communication session. To meet these adaptive data requirements, an adaptive TDMA-based MAC protocol with enhanced medium utilization with secure and reliable data communication is required. This can be achieved by implementing a trust management algorithm that verifies the authenticity of nodes [18,19]. This research work proposes a time-slot-based medium access control protocol to meet adaptive data requirements (\(TMPAD\)) of trusted nodes for fog-enabled smart city applications. \(TMPAD\) distinguishes between malicious and trusted nodes, and guaranteed time slots (GTSs) are only assigned to the trusted nodes. The salient features of \(TMPAD\) are mentioned below. 1. A trust mechanism for fog nodes to differentiate the data forwarded from the legitimate and malicious nodes. 2.