## 2 Related Work
However, allocating a short awake interval to numerous devices will result in multiple collisions, which, in the best case, will allow only a few of them to transmit in a cycle. It is important to emphasize that the multi-access reservation scheme suggested in this paper can be easily applied to synchronous MAC protocols, such as DW-MAC [34], in which multiple devices can reserve transmission simultaneously, which will be scheduled during the sleep interval. Adopting the traditional transmitter initiation approach, in which the transmitter transmits a long preamble capturing the channel prior to the data transmission, waiting for its intended receiver to wake up (e.g., [35]) or breaking this long preamble to a sequence of short preambles, thus allowing the intended receiver to notify that it is awake [36; 37], works fine when the traffic is sparse. However, in highly loaded regions or in dense networks in which many devices occasionally need to transmit at the same time, not only can the channel utilization be very low--kept busy with preambles for long time intervals when waiting for the intended receiver (e.g., the sink) to wake up--but also, after a device has captured the channel, the reporting devices waiting to transmit will need to defer their reports for several cycles before they can capture the channel. Another asynchronous duty-cycle MAC approach is the receiver-initiated paradigm, in which rather than the transmitter initiating the data exchange, the receiver, whenever it is ready to receive data, invites the data exchange (e.g., [9; 38; 39; 40; 41; 42]). Specifically, in the original Receiver-Initiated-MAC protocol (RI-MAC) [9], whenever the receiver wakes up and is ready to receive data, it transmits a predefined preamble to initiate data transmissions to itself (a more detailed description of RI-MAC is given in Section 3.1). Several enhancements and modifications to RI-MAC have been suggested in the literature. For example, RIVER-MAC [38] suggests two enhancements to RI-MAC. The first aims at reducing the idle listening interval--instead of a sender idly listening, waiting for a beacon from its intended receiver, a sender strobes Clear Channel Assessments (CCAs) until it detects activity. The second enhancement aims at preventing other receivers from taking control of the channel when an already active receiver invokes the collision resolution mechanism, and is specifically in a silent backoff interval. This is achieved by having the active receiver transmit a train of regular beacons, rather than remaining silent, during the backoff intervals of the collision resolution process. The first enhancement is unrelated to the enhancement that we suggest in this study (MAR-RiMAC), and can be integrated into it. The latter, which deals with the contention mechanism and combats the possible collisions during the backoff idle periods, is redundant for MAR-RiMAC, which practically eliminates the idle time intervals during contention. PW-MAC [43] and AP-MAC [39] aim at reducing the time that a transmitter is awake and waiting for its receiver to wake up by learning its receiver's expected wake-up time. Thus, a device wishing to transmit data, instead of staying awake waiting for its designated receiver to wake up, wakes up just prior to its intended receiver's wake-up instance. Predicting the receiver wake-up instance is again orthogonal to the MAR-RiMAC enhancement, and can also be applied to MAR-RiMAC, reducing the duty cycle of the transmitting devices (see the remark about the transmitter waking up just prior to its intended receiver on p. 24). DURI-MAC [40] is another asynchronous-receiver-initiated duty-cycled MAC protocol that, similarly to previous studies (e.g., [44]) utilizes two channels for communication: one for control and the other for data. While receiving data on the data channel, DURI-MAC suggests sending a busy signal on the control channel to avoid hidden terminals interfering with transmission. The scope of this paper is a single channel.