# 2 CA-R2: A Hybrid MAC Protocol for Machine-Type Communications
## 2 Related Work
According to the number of active devices that have been detected, the gateway can determine the optimal length (\(M\), measured in time slots) of the CAP. Then, each active device randomly selects one of the \(M\) slots and sends a request message to contend for a data slot to be used in the CFP. If two or more devices select the same time slot in the CAP, a collision occurs. In such a case, they have to wait for a new frame to retry transmission. Once the request messages have been received, the gateway allocates data slots to the successful devices. This information is announced in the NP. During the CFP, the devices that have been allocated data slots can transmit data packets using the TDMA policy. In spite of the increased complexity of this framework, extensive simulations reported in [23] demonstrated that it achieves superior performance when compared to solutions like s-ALOHA and reservation-based MAC protocols, such as TR-MAC [24]. However, since offline training is required, managing such a network becomes more challenging. In addition, although the value of \(M\) is optimally calculated in each cycle, the random selection of slots by the contending devices offers no guarantee of a low collision rate during the contention phase. In the hybrid MAC protocol suggested by [PERSON] et al. in [25] each frame consists of four periods: notification period (NP), contention period (CP), announcement period (AP), and transmission period (TP). At the beginning of each frame, in the NP, all devices receive a broadcast message from the base station (BS) which is intended to indicate the start of the CP. During the CP, devices that have a data packet to transmit contend by sending a transmission request (REQ) message to the BS to reserve a transmission slot using the s-ALOHA mechanism. If no collision occurs, the BS will reserve a valid time slot in the TP for the device whose REQ was successful. An ACK message is returned by the BS to notify this event. After the CP finishes, the AP starts. In it, the BS sends a message to all successful devices with the number of their reserved time slots. After that, these devices transmit their packets during the TP using TDMA. [PERSON] et al. report that their proposal shows superior performance in terms of system throughput, average packet delay, success access ratio, and reservation ratio in comparison with some other hybrid schemes (i.e., NP-CSMA/TDMA, \(p\)-persistent CSMA/TDMA, and s-ALOHA/TDMA). A hybrid MAC protocol suggested by [PERSON] et al. in [26] classifies the devices into different classes depending on their priority level. It also divides time into cycles, where each one of them consists of three periods: a notification period (NP), a contention period (COP), and a transmission period (TOP). Each cycle is initiated with an NP, where a base station announces the start of the COP to all devices regardless of its class. Throughout the COP stage, the nodes with data to transmit use \(p\)-persistent CSMA to send transmission requests to the BS. Here, the devices belonging to a class with higher priority will have a higher probability of successful contention than devices belonging to a class with a lower priority. Successful nodes are granted a slot to transmit data using TDMA in the TOP. This architecture turns out to achieve higher throughput, lower average delay, and lower power consumption when compared with TDMA and \(p\)-persistent CSMA [26]. Other proposals were introduced by [PERSON] et al. in [27; 28]. In [27], they introduce a hybrid multiclass MAC protocol for wireless body area networks (WBANs) whose framework consists of the following four stages: notification phase (NP), contention phase (CP), announcement phase (AP), and transmission phase (TP). At the beginning of a frame, in the NP, all devices in the network receive a message from the access point notifying them about the beginning of the CP.