## 5 Evolution of Opportunistic Routing in Asynchronous WSN
### Evolution of Asynchronous Duty-Cycled MAC Protocols
#### 5.1.5 RI-MAC  
The RI-MAC protocol [32] is a receiver-initiated MAC protocol, which is one of the most widely used protocols compatible with asynchronous duty-cycled WSNs. It has the potential to handle contending flows and thus burst traffic more efficiently and effectively. A sender using the RI-MAC protocol does not occupy the medium until the intended receiver is ready to receive the data. The receiver-initiated transmission makes RI-MAC more efficient in detecting collisions and recovering lost DATA frames. RI-MAC significantly reduces the amount of time a pair of nodes needs to occupy the medium for before they reach a rendezvous time for data exchange, compared to the preamble transmission in B-MAC and X-MAC. This short amount of medium-occupation time enables more contending nodes to exchange DATA frames with their intended receivers, which helps to increase the capacity of the network and thus the potential throughput. More importantly, this increase is adaptive; it lets a beacon serve as both an acknowledgment to previously received DATA, and as a request for the initiation of the next DATA transmission.  
In RI-MAC, medium access control, among senders that want to transmit DATA frames to the same receiver, is mainly handled by the receiver. This design choice makes RI-MAC more efficient in detecting collisions and recovering lost DATA frames than B-MAC and X-MAC when the senders are hidden from each other, which is a common situation in multi-hop sensor networks. Together with the lower cost related to detecting collisions and recovering lost DATA frames, RI-MAC achieves higher power efficiency, especially when the network load increases. Following this concept, other recognized protocols have been proposed in the literature [38; 39; 40; 41; 42; 43; 44], which are all receiver-initiated MACs. An effective use of this protocol can be found in [PERSON] et al., 2019 [45]. Moreover, in order to evaluate the performance of this protocol, many survey studies [46; 47; 48; 49; 50] have been performed.  
In order to understand the actual data transmission procedure, we have provided a visual representation in Figure 9. Suppose that node A has a packet to send. It wakes up and silently waits for the receiver's beacon. After obtaining the beacon, it will send data. Now, there are two senders, A and C, both of whom are waiting for [PERSON] to wake up and are unaware of each other. Upon receiving [PERSON]'s beacon, they both send data at the same time and collision occurs. Now [PERSON] will send a beacon that will contain an extended back-off window size. Sender nodes will then choose a random back-off time to wait, and send the data again. In this way, RI-MAC handles its collisions.  
Figure 8: Packet transmission in BoX-MAC-1 (**left**) and BoX-MAC-2 (**right**) [31].  
Figure 9: Packet transmission in RI-MAC [32].  
However, when a sender has a frame to send, it immediately wakes up to wait for the receiver, leading to a longer sender duty cycle because of its idle listening for the receiver to wake up. The key idea of these receiver-initiated MAC protocols is very effective and powerful in terms of energy consumption. However, these protocols have a critical disadvantage when there are multiple senders and one receiver. In the case of multiple senders that are ready to send data frames, collisions may occur with very high probability because senders wake up at the same time as the receiver initiating. Moreover, only a single hop network was considered.