## 2 Related Work
Obviously, using part of the bandwidth to transmit a busy signal has its advantages, yet reduces the transmission rate in the data channel (narrower bandwidth). Furthermore, note that even though DURI-MAC can reduce the collisions between a data transmission after a successful handshake and a wake-up beacon by a hidden device, it cannot resolve the collision problem discussed above. In particular, the control channel enhancement suggested by DURI-MAC will not resolve collisions when multiple transmitters are waiting for the same receiver to wake up, and moves to the data channel to transmit their data after the wake-up beacon. The control channel utilization can be integrated into MAR-RiMAC, thus resolving the same kinds of collisions between data and wake-up beacons as DURI-MAC. Another asynchronous-receiver-initiated duty-cycled MAC protocol is A-MAC ([41]), which reduces the time in which a device needs to determine whether to stay awake or go to sleep after probing the channel. Since a device probes the channel numerous times a day, even a small reduction can lead to high cumulative savings in wake-up time. A-MAC further achieves more accurate decisions with respect to the staying-awake decision, reducing both false positives and false negatives. The scheme suggested by A-MAC can be applied to MAR-RiMAC. Generally, MAC protocols in which multiple nodes try to initiate a transmission simultaneously are prone to collisions and performance degradation. Although, the receiver-initiation-based approach works extremely well when, at each time instance, different sensors need to communicate with other sensors, when multiple devices need to communicate with the same receiver simultaneously (e.g., for event-driven reports to a specific sink or relay), it can perform poorly due to repeated collisions between devices trying to transmit to the same node. For example, imagine a receiver-initiated MAC protocol (e.g., RI-MAC [9]) in which a sink receives reports for numerous directly connected devices. In such cases, whenever the sink transmits the predefined signal (beacon) in order to initiate report transmissions, there is a high probability that multiple devices are waiting to transmit their reports and will start transmission simultaneously, resulting in collision. Even though receiver-initiated MAC protocols, such as RI-MAC, have devised collision resolution mechanisms to resolve such collisions (some of which are discussed above), when the number of contenders is large, resolution of such collisions with traditional mechanisms, such as the traditional backoff mechanism (e.g., RI-MAC), can be prolonged due to repeated collisions. Some protocols address the superfluous energy consumption of a device staying awake waiting for its intended receiver to wake up (both in the transmitter- and receiver-initiated paradigms) by relying on each device (receiver) to have a predetermined wake-up schedule that is known to its intended transmitters so that they can adjust their wake-up time for transmission accordingly (e.g., [39; 43; 45; 46]). Obviously, even though such an enhancement can reduce the duty cycle of both approaches considerably, it will not resolve the collision storm problem under the receiver-initiated paradigm discussed earlier, since all the devices waiting to transmit will wake up and respond (transmit) to the same inviting beacon simultaneously. In this paper, in order to address the aforementioned collision storm, we suggest a highly efficient multi-access reservation enhancement, in which multiple nodes can simultaneously notify the intended receiver of their desire to transmit to it. The suggested mechanism can be easily integrated with many of the aforementioned protocols. However, for simplicity, and since the suggested enhancement is highly suited for receiver-initiated MAC protocols, we will concentrate on RI-MAC ([9]).