## 2 Related Works
### MAC Layer
#### 2.2.1 QoS MAC Protocols in WSNs  
For the purpose of providing QoS, a variety of MAC protocols have been proposed for WSN, such as QAEE-MAC [31], MPQ-MAC [30], PMME-MAC [32], RMP-MAC [33], EAMP-AIDC [28], etc. High-priority packets are sent first using the QoS Aware Energy-Efficient MAC (QAEE-MAC) protocol [31]. The protocol only takes into account two levels of data packet priority. The protocol's analytical evaluation assumes just a single node that sends high-priority packets, whereas the rest of the network's nodes produce low-priority packets, which may not be practical. Aside from that, QAEE-MAC does not provide multi-hop data packet transmission or other energy-saving strategies. Multi-priority QoS MAC (MPQ-MAC) [30] allows multiple levels of data packet priority. Because of this, considerable changes to the protocol were introduced to increase energy efficiency while maintaining packet priority in the network. The sink node keeps track of the waiting timer depending on the importance of the incoming packets in this method. Receiving several Tx-beacons (transmitter beacons) during the waiting period allows the receiver node to prioritize senders. While MPQ-MAC takes QoS into account, it does not provide multi-hop communication for data packets. Multi-event Priority MAC Protocol (PMME-MAC) [32] allows multiple priorities by assigning corresponding probability values for channel accessibility. High-priority nodes can expect reliable and low-latency from PMME-MAC, based on its mathematical analysis. PMME-MAC solves the issue of concurrent priority transmission of critical packets. However, the cost of the low latency comes in terms of high power consumption, and it also lacks multi-hop transmission. Receiver-Initiated and Multi Priority Backoff MAC (RMP-MAC) [33] protocol proposed adaptive backoff methods depending on the event priority, remaining energy level and the amount of data packets waiting to be sent. Energy usage is reduced because the transmitter adjusts its schedule depending on information from the receiver's wake-up time. Emergency data packets are delivered more quickly by taking the advantage of the adaptive backoff mechanism. Even though RMP-MAC considers the QoS characteristics, it does not allow multi-hop routing feature for data packets. \begin{table}
\begin{tabular}{c c c} \hline \hline
**Applications** & **Information** & **Priority QoS Requirements** \\ \hline \multirow{4}{*}{Monitoring in Smart Cities} & Gas Leakage, Health Monitoring, Smoke and Fire & Urgent \\ \cline{2-3}  & Flood, Security Monitoring & Very Important \\ \cline{2-3}  & Temperature Monitoring, Traffic Surveillance & Important \\ \cline{2-3}  & Air Quality Index & Normal \\ \hline \multirow{4}{*}{Environmental Monitoring} & Volcanic Eruption & Urgent \\ \cline{2-3}  & Forest Fire & Urgent \\ \cline{2-3}  & Under Water Monitoring & Important \\ \cline{2-3}  & Sea Water Level Monitoring & Important \\ \hline \multirow{4}{*}{Industry Systems} & Safety or Fire Emergency, Hazardous Gas Leakage & Urgent \\ \cline{2-3}  & Security Monitoring & Very Important \\ \cline{2-3}  & Equipment Malfunction & Important \\ \cline{2-3}  & Monitoring & Normal \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Priority QoS requirements for various applications. Energy-Aware MAC Protocol with Adaptive Individual Duty Cycle (EAMP-AIDC) [28] prioritizes data packets that need to be delivered quickly. The protocol's goal is to minimize energy waste via dynamic individual duty cycle. It attempts to maximize the active time of each node based on their energy budget, distance from the sink, and network congestion. The protocol uses a routing protocol, which allows each potential relay (PR) nodes to have a selectors list. The list contains nodes (cluster members) whose data packets can be relayed by the PR. Even though EAMP-AIDC considers multi-hop communication, data packets can only be relayed if the sensor nodes are on the PR list. Sensor nodes that are not on the list have to wait until other nodes complete their packet transmissions.