## 2 Related Studies
In [PERSON] et al., 2014 [3], the authors reviewed a number of opportunistic routing protocols, wherein they focused on all WSN issues and discussed the contribution of the opportunistic routing protocol to these issues. However, they excluded a crucial point by not discussing the MAC mechanism underlying the protocols. Moreover, the protocols reviewed in the paper were assumed to be active all the time, which means they did not take into account the duty-cycle. On the other hand, all opportunistic routing protocols have been established based on a specific MAC protocol. The routing decision they made for their process depends on the underlying MAC protocol because, since the beginning, the main concept of opportunistic routing has been a combination of MAC and a routing mechanism, as represented in Figure 2. Several survey articles [22; 23; 24; 25; 26; 27] have studied the existing MAC protocols in the literature, focusing on their contributions to preserving the energy and lifetime of the network. [PERSON] et al. [22] surveyed MAC protocols in the literature, focusing on the energy efficiency, latency and throughput factors of WSNs. This paper classified the existing MAC protocols based on the underlying mechanisms they worked with. Depending on classification characteristics, they covered a large number of protocols and have explained them specifically with regard to different aspects. [PERSON] et al. [23] mainly classified protocols focusing on the WSN issues that they were working on. This article covers several protocols and explains their functionalities in order to solve these issues. This survey also points out some of the hardware factors of the protocols. It may therefore help readers obtain a brief view of almost all the types of characteristics of the protocols. [PERSON] et al. [24] not only classified protocols, but also explained the course of motivation that drove the evaluation of the protocols. This paper points out the necessity and characteristics of the issues and solutions that these protocols were trying to address. The main focus of the asynchronous MAC protocols discussed here was time management and the sender and receiver waiting time. [PERSON] et al. [25] conducted a study on the latest asynchronous MAC protocols at the time, and explained how sensor nodes communicate with each other using their mismatched duty-cycles. They mainly focused on the energy consumption factors of the protocols. [PERSON] et al. [26] discussed both synchronous and asynchronous MAC protocols and their respective issues in the MAC layer for WSNs. Here, the authors examined their performances considering a linear/chain-type WSN. [PERSON] et al. [27] reviewed MAC protocols based on throughput, efficiency, stability, fairness, low access delay, low transmission delay and low overhead. The article did not specifically focus on  
Figure 2: Decision-making hierarchy of opportunistic routing protocol. duty-cycling or synchronous and asynchronous characteristics; rather, it evaluates the protocols by taking into account the specifications mentioned above. Thus, each previous study either reviewed only opportunistic routing protocols or only MAC protocols. They did not provide any survey discussing both protocols, and did not describe their compatibility and adjustments to each other while focusing on preserving energy. In this study, we aimed to discuss energy efficiency, and fixed our focus on the load-balancing of asynchronous duty cycles combined with opportunistic routing. Hence, protocols related to this specific issue are described here. Among them, ORW [11], ORD [12], ORR [13], MOR [14], MORR [15], LORA [16] and LEOR [17] are arguably the most significant opportunistic routing protocols that have efficiently improved the aforementioned issues, while combining with unscheduled duty-cycled MAC protocols and balancing energy consumption effectively. Additionally, we have provided a review of the evolution of asynchronous duty cycle MAC protocols in WSNs from their early years to the present time.