## 2 Related Works
### MAC Layer
#### 2.2.2 MAC Protocols with Multi-Hop Routing Feature in WSNs  
Numerous researchers have spent considerable time developing novel routing methods to address multiple issues in WSNs [52]. The on-demand distance vector routing protocol (AODV) [53; 54] is a reactive routing protocol and is one of the most commonly utilized in the industry in which each node acts as a router. AODV obtains and maintains routes only when they are required [55]. The protocol's fundamental activities include route discovery, establishment of forward and reverse paths, routing table management, control of local connections, and path maintenance.  
In order to conserve energy, the Special Purpose Energy-Efficient Contention-based Hybrid MAC (SPEECH-MAC) [36] protocol incorporates the AODV routing protocol and the dual-hop approach. The sink node uses the dual-hop approach to collect data directly from the child nodes for any urgent packet and then collects data from the relay nodes where other child nodes can forward their data packets to the relay nodes. However, the protocol does not provide any energy-saving strategies for the child nodes that are going to use the dual-hop approach and might die rather soon because of their distance from the sink.  
Division-Multiple Access-MAC (MDA-SMAC) [35] is another protocol that is based on the AODV routing protocol. MDA-SMAC is an extension to the S-MAC protocol that incorporates two backoff techniques. The Fast-Binary Exponential Backoff and Conflict-Avoid-Binary Exponential Backoff algorithms are used to minimize data latency, which further lowers the chance of conflict. The duty cycle is constantly adjusted by the nodes based on the length of the buffer queue. MDA-SMAC achieves lower latency, higher throughput, and better residual energy than the S-MAC protocol by the combined efforts of backoff algorithms and micro-duty. Furthermore, the protocol utilizes the AODV routing protocol for multi-hop communication but does not take into account the QoS features of data packets.  
In EDS-MAC [37], two phases are provided in the proposed method: data transfer and cluster formation. In the first step, the VSSFFA algorithm generates energy-aware clusters by selecting the best cluster heads. Several factors go into the selection process: residual energy, intra-cluster distance, the number of potential cluster heads, and node degree. The data transmission stage is used for data transfer, which decreases latency, and control overhead. The protocol only considers two levels of data packet priority. The protocols analytical evaluation assumes that sender nodes which require minimum latency or minimum duty cycle to reach sink node will have the highest priority, which is not very realistic.  
Comparative analysis is shown in Table 2 for all the protocols mentioned above. On one hand, some QoS protocols lack multi-hop routing features, while the other protocols that provide multi-hop routing lack QoS features for data packets. Although a small number of protocols support both QoS and multi-hop routing, they lack higher packet priority for different kinds of emergencies. In addition, their performance assessments have not taken into account all performance metrics, such as energy efficiency, end-to-end packet latency, network throughput, or packet delivery ratio. As a result, there is a need to develop an energy-aware QoS MAC protocol that considers multi-hop communication for WSNs. It should have the ability to send data packets with multiple priority levels and can be deployed to cover a large area, with good energy efficiency.