## 3 Multiple-Access Reservation Receiver Initiation MAC (MAR-RiMAC)
### Multiple-Access Transmission Reservation Enhancement--MAR-RiMAC  
As previously explained, when multiple devices are trying to transmit to the same device, as typically occurs in data-gathering applications, the performance of RI-MAC can suffer a significant performance degradation due to the multiple collisions between devices that are trying to transmit data, especially when some of the transmitting devices are hidden from one another. An illustration is depicted in Figure 1b. Our enhancement is aimed at eliminating the collisions and the aggravating consequences of these collisions, e.g., wasted air-time due to failed transmission attempts and the cost of initiating a collision resolution procedure. Accordingly, our design relies on a reservation mechanism in which even when multiple devices are trying to reserve the channel for transmission simultaneously, their intended receiver (sink) will be able to identify them and poll them consecutively. Specifically, our design relies on allocating each device a unique sequence, such that these sequences enable the receiver to identify the transmitter even during overlapping transmissions. We start by describing an overview of the protocol, assuming the existence of the mechanism that enables the receiver to decode multiple reservations transmitted simultaneously. We postpone the discussion regarding the mechanism itself to the following subsection, in which we will present two different mechanisms that can attain the required outcome.  
The protocol is based on RI-MAC; a node with pending data (report) to send stays awake, silently waiting for its intended receiver to wake up. Each device wakes up based on its own schedule and, after verifying that the channel is idle (i.e., performing a Clear Channel Assessment (CCA check)), transmits an RtR beacon. Nonetheless, in contrast to RI-MAC, devices with pending data send a preassigned \"Request to Transmit\" beacon (RtT) to this awakening device (receiver). These RtTs are uniquely assigned to the devices,  
Figure 1: Overview of the Receiver-Initiated Medium Access Control (MAC) (RI-MAC) protocol. A backlogged device waits silently for its intended receiver beacon to start its data transmission. A non-backlogged device wakes up periodically, broadcasts its wakening notification, and, if no device transmits to it, goes back to sleep. (**a**) Under low load. (**b**) Under high load, when multiple devices attempt transmission simultaneously, a collision resolution procedure is invoked.  
and can hence identify the transmitting devices distinctively. The main advantage of these RtTs is that they can be recognized by the intended receiver, even during simultaneous transmissions (the description of the RtTs is given in the following subsection). The receiving device identifies the backlogged devices that are waiting to transmit to it based on the received RtTs, and sequentially pools them for transmission. Similarly to RI-MAC, the pooling request also serves as acknowledgment of the previously received report; accordingly, the pooling request is done by sending an ordinary RtR with the pooled device ID on it (note that adding an additional address to the RtR is supported by the RI-MAC beacon frame format). An illustration of the protocol is given in Figure 2.  
Next, we discuss the RtT mechanism.