## 2 Related Studies  
In this section, we present an at-a-glance review of studies that are related to the evolution of opportunistic routing protocols with asynchronous duty-cycled MAC in WSNs. WSNs, in which nodes share a single channel or multiple channels and contend for their transmission, have both advantages and disadvantages. Opportunistic routing, on the other hand, uses the shared channel characteristic as an advantage with regards to forwarding data. Using the broadcasting mechanism, it searches for available and suitable nodes dynamically in order to forward the data to the target sink node. In this way, this protocol can distribute the energy load from one node to other usable nodes. In a dense network, where it is costly to maintain a topology and to run synchronization processes repeatedly, opportunistic routing with asynchronous duty cycles plays a significant role. The concept of opportunistic routing, also known as ExOR, was first proposed by [PERSON] and [PERSON] in 2005 [4]. This protocol proved to be robust, increasing throughput by almost 35% compared to conventional protocols. Nevertheless, the authors only presented simulations describing the throughput, but did not discuss the issue of energy consumption. The advantages of OR grabbed the attention of researchers as it would now be possible to improve loopholes and make routing more energy-efficient, thus boosting the performance of WSNs. Subsequently, using the principles of ExOR, many other opportunistic routing protocols have been proposed [5; 6; 7; 8; 9; 10]. However, these protocols assume that the sensor nodes are always active and do not consider the duty-cycle mechanism for energy efficiency. Hence, several other protocols [11; 12; 13; 14; 15; 16; 17] have been proposed, where opportunistic routing is combined with duty-cycled MAC in order to prevent unnecessary energy consumption. These protocols were significant developments in the opportunistic routing paradigm for asynchronous WSNs, and successfully balanced network load with extended lifetime efficiently. Figure 1 presents the sustained developments in terms of the number of publications every year related to OR with ADCM over WSNs. From the site, scholar.google.com, we have searched the related papers on the topic of OR with ADCM with the key words and expressions \"opportunistic routing\" + asynchronous + duty + cycle + mac + wireless + sensor + network. We counted only the number of published studies since 2005, because the first duty cycle MAC, S-MAC, was published in 2002, and the first opportunistic routing protocol, ExOR, was published in 2005. Table 1 shows an at-a-glance summary of survey articles on OR and MAC protocols related to our study. To gain a better idea of the taxonomy of opportunistic routing protocols, we refer readers to a previous article [18]. In the mentioned article [18], the authors have explained in detail the types of opportunistic routing protocols and their characteristics. The authors have also provided a comparative analysis that can provide a clear view of the main features of the protocols and their contributions. However, the authors in [18] have only focused on overall opportunistic routing protocols based on their structure and working process, without detailing energy consumption factors. Because of their useful characteristics, studies have been conducted to evaluate the performance of opportunistic routing protocols in other communication networks as well [19]. Moreover, to appreciate the importance of combining routing with duty-cycled MAC protocols in order to reduce the energy consumption of the sensor node, readers can refer to the survey by [PERSON] and [PERSON] [20]. In [20], the authors have described different types of scheduling methods for duty-cycling in the MAC layer and routing schemes, including opportunistic routing. They analyzed protocols and presented their respective pros and cons to show their effectiveness with regard to energy efficiency.