## 1 Introduction
Specifically, in our modification, rather than sending the packet immediately after the receiver's (sink's) wakeup announcement (which results in collision when more than one device attempts transmission), each backlogged device sends a short Request to Transmit (RtR) beacon, which incorporates its identity and notifies the sink that it is backlogged. The sink identifies the backlogged devices waiting to transmit to it and polls them one by one, thus avoiding the collision. The RtR beacons benefit from two basic advantages: First, they are very short messages, and practically all they convey is the identity of their senders. Second, they can be transmitted simultaneously such that the receiver can identify the senders (the number of simultaneously transmitted RtRs supported by the enhancement is configurable and influences the length of the RtRs). We suggest two different mechanisms for the RtR realization. The first realization relies on Correlatable Symbol Sequences (CSSs), which are predefined binary codewords (sequences) that are detected by the receiver via correlators. Accordingly, each device is associated with a unique sequence that conveys its identity and is associated with a correlator; hence, it can be identified by the sink (see Section 3.3). The second realization also relies on a unique sequence associated with each potential transmitter, yet this time, rather than correlation, the decoding of the received signal relies on an energy detection mechanism (see Section 3.3). Since in both mechanisms, the number of simultaneously transmitting devices that can be identified by the sink is limited by the length of the pre-configured sequences, we devise a collision resolution mechanism for the case of the transmitting devices exceeding this limit. Our collision resolution mechanism partitions the colliding devices into smaller groups that can be supported by the multiple-access (simultaneous transmission) mechanism. The contributions of this work are as follows:* We devise MAR-RiMAC, an RI-MAC protocol amendment to address the collision storm problem. Our amendment is a reservation-based mechanism that allows the devices to send their reservations concurrently. Based on the received reservations, the sink pools the backlogged devices consecutively. * We suggest two mechanisms to support the concurrent reservation transmission--one that is based on correlatable symbol sequences, and the other is based on energy detection. * In the case that the number of concurrently transmitting devices exceeds the maximal supported concurrent transmissions, we utilize a collision resolution mechanism that distributively partitions the colliding groups into smaller subgroups, which can be supported by the concurrent reservation transmission mechanism. * We implement both MAR-RiMAC and RI-MAC on the OMNeT++ simulator, and rigorously examine and compare the performance of both protocols under various traffic loads and topologies. We show that the suggested amendment improves performance in the case of high contention and especially in the presence of hidden nodes, yet preserves similar performance in light-traffic scenarios. The rest of this paper is organized as follows. In Section 2, we discuss related works in data aggregation in general and duty-cycle MAC protocols for sensor networks in particular. Section 3.1 provides a brief overview of the RI-MAC protocol on which our enhancement relies. Section 3.2 describes the design of MAR-RiMAC, the suggested protocol enhancement. Section 3.3 presents two mechanisms with low communication overhead that MAR-RiMAC leverages, which enables the reception of multiple reservations simultaneously. Sections 3.4-3.6 discuss several properties and extensions for the protocol. Section 4 presents a thorough evaluation of MAR-RiMAC in comparison to RI-MAC via simulation results (OMNeT++ network simulator). Finally, Section 5 concludes the paper.