## 1 Introduction  
The increasingly ubiquitous Internet of Things (IoT) penetrates more and more into the domains of our daily lives. It spans a large variety of applications, including healthcare, transportation, home automation, wearable technology, and appliances with remote monitoring capabilities (e.g., [1, 2, 3]). Sensors are vital enablers of the IoT, allowing a plethora of automated devices to monitor and relay collected data to a sink for further analysis and actions that need to be taken according to the collected data. Certain events can trigger additional data acquisition, either requested by the sink or automatically generated by the device, which also needs to be relayed to the sink. Accordingly, a typical IoT device incorporates two basic operations--sensing and wireless connectivity. Exploiting cheap devices enables, on the one hand, the incorporation the technology in more and more applications, yet on the other hand, it requires the development of innovative and efficient protocols and algorithms in the aspects of both sensing and communication. One of the most prevalent means for device energy conservation is duty cycling, in which devices periodically alternate between being active and sleeping. When active, a device is able to transmit or receive data, whereas when sleeping, the device completely turns off its radio to save energy (e.g., [4]). Accordingly, one of the major challenges when operating under duty cycling is the coordination between the devices that are sharing the wireless channel and that are trying to relay the collected data to the sink. In particular, since sensors are meant to be asleep most of the time, a great challenge is to schedule a rendezvous between the transmitter and the receiver. Numerous works have suggested Medium Access Control (MAC) protocols for IoT and Wireless Sensor Networks (WSNs) under various traffic patterns and performance demands (e.g., [5; 6; 7; 8]). These protocols can be categorized under various classification types, such as schedule access vs. random access, synchronous vs. asynchronous protocols, etc. (a short discussion is given in Section 2). One such seminal approach was suggested by the Receiver-Initiated-MAC protocol (RI-MAC) protocol [9], in which instead of the transmitter initiating the data transaction, the receiver initiates it by sending a short preamble informing potential transmitters that it is awake and ready to receive data (an outline of the RI-MAC protocol is provided in Section 3.1). Obviously, under receiver-initiated protocols, such as RI-MAC, when multiple devices are waiting to transmit data to the same receiver, their wake-up preamble triggers simultaneous transmissions to it (resulting in inevitable collision). However, note that irrespective of the MAC protocol utilized (transmitter- or receiver-initiated), when the sink or a relay needs to collect data from many devices, the bottleneck is the wireless channel to the receiver, as many devices are trying to communicate with the same entity. Accordingly, when the surrounding devices are backlogged (e.g., in the occurrence of an event that needs to be reported), the reported data are prone to long delays and to repeated unsuccessful transmissions due to multiple devices attempting transmission simultaneously. The repeated collision problem is aggravated in the presence of hidden terminals. In this respect, contention and collision resolution mechanisms are needed to resolve this collision storm. In this paper, we devise an amendment to the RI-MAC protocol that combats the aforementioned collision storm (The mechanism suggested herein can be applied to other MAC protocols. Nonetheless, for simplicity, and in order to isolate the mechanism from unnecessary noise, we will concentrate on RI-MAC). Our enhancement, termed Multi-Access Reservation RI-MAC (MAR-RiMAC), suggests a transmission reservation procedure that relies on multiple devices accessing the channel simultaneously.