## 6 Comparison Analysis and Research Challenges  
### Comparison Analysis  
The using of multipath routing in distributing the energy load among available neighbor nodes is a very useful characteristic of opportunistic routing protocols. On the other hand, reducing the synchronization overhead in the MAC protocol is one of the most efficient ways to reduce energy consumption in duty-cycled sensor nodes. Hence, combining these two characteristics, the ORW was the first invention in this field. However, even though this protocol was better than the conventional one, it has yielded issues related to energy consumption, discussed in Section 5, while working with the asynchronous duty-cycled WSN. Table 2 presents a short review of the milestone protocols proposed to date in the field of asynchronous WSNs. Table 2 illustrates which issues are addressed by these protocols and how they address the corresponding problems. The advantages and disadvantages of using these protocols are also presented in Table 2, so that, based on application requirements, they can bechosen whenever needed. However, from ORW to LORA, all of the opportunistic routing protocols described here are designed on top of a low-power listening Box-MAC protocol. This MAC protocol is an improved version of B-MAC and X-MAC, which increases the performance of the low-power listening asynchronous duty-cycled sensor nodes. Only LEOR has proposed its own MAC mechanism for communication. With its self-assigned MAC protocol, with multi-channel OR, it improves energy efficiency by 52% and 60% compared to MORR, in sparse and dense networks, respectively [17]. In Section 6, we have also provided a brief review of the development of asynchronous duty-cycle MAC protocols, where we have discussed the MAC protocols. ORW worked on a first-come-first-serve basis. The node that wakes up first can forward the packet, which will lead to multiple receiver and redundant packet transmission problems, because there is a high probability that more than one node will wake up at the same time and will listen to the transmission. ORD outperforms ORW because it performs additional calculations in order to shorten the competition of candidate nodes. In sparse networks, compared to ORW, the network lifetime is increased twofold, whereas in dense networks, it has been shown to increase by 76% [12]. However, ORD increases the frequency of the duty-cycle and holds packets for a long time, which affects both energy consumption and latency. ORR improved ORW and ORD by using a different routing metric called the Forwarder Score, which is evaluated based on the residual energy of neighbor nodes and the hop count to the sink node. ORR performs better in dense networks. In sparse and dense networks, the energy consumption in each node decreases by 5% and 72%, respectively [13]. As a result, it boosts the lifetime of the entire network tremendously. However, it has high calculation and overhead costs. In order to deal with these issues, MORR was later proposed. Compared with ORR, MORR decreases energy consumption by 85% and 83%, in sparse and dense networks, respectively [15]. Nevertheless, the authors did not consider the length of the duty-cycle, and instead used a longer active period, which is a major source of energy usage. In addition, choosing a small neighborhood with a minimum hop count will make the system choose the same nodes repeatedly, which will affect the load-balancing between nodes and create energy-holes in the network. Recently, LORA [16] was introduced in the literature, where it presented a unique routing metric, which involves the multiplication of four distributions. Until recently, this protocol was the most efficient, and outperformed previously existing protocols for asynchronous WSNs. Simulations have so far been performed using a sparse network, the results of which have shown that LORA can reduce energy consumption by 30% and 35%, compared to ORW and ORR, respectively [16]. However, it also has significant issues that need to be addressed by future studies.