# 2 CA-R2: A Hybrid MAC Protocol for Machine-Type Communications
## 2 Related Work
In the CP, only the devices that are ready to transmit contend for transmission opportunities by using the s-ALOHA scheme, and the successfully contending devices send their packets by employing a TDMA scheme in the TP. As a particularity of this protocol, the devices are grouped into two classes, where class 1 devices generate critical data that require high reliability and low delay, while class 2 devices generate packets that are less critical. Based on the class of each device and its data packet size, the coordinator (i.e., the access point) will take the necessary decisions to allocate resources during the TP. From the simulation results, it is concluded that the proposed protocol performs better than a protocol based on the combination of NP-CSMA and TDMA in terms of the system sum throughput and average packet delay. However, because s-ALOHA is used as the contention mechanism, the applications of this protocol seem to be more appropriate to long-range networks with low data rates. Otherwise, congestion scenarios may occur, leading to low scalability. Furthermore, [PERSON] et al. proposed another approach in [28]. This one is based on a hybrid MAC frame for WBAN networks that consists of the following two stages: a contention period, called the \"CSMA/CA period\", and a transmission period, called the \"TDMA period\". At the beginning of the CSMA/CA period, the access point (AP) sends a message to all devices in the network. From that moment on, the devices that have data to transmit send a request-to-transmit (REQ-T) message at random but based on the size of their own contention window. If two or more devices send a REQ-T simultaneously, a collision will occur. Devices that do not have packets to transmit will enter into a sleep state to save power. To end the CSMA/CA period, the AP transmits a general feedback message (OACK) to all devices in order to inform which REQ-T messages were successful instead of sending individual acknowledgement messages each time it successfully receives a REQ-T message. Furthermore, the OACK message contains the transmission order so that a specific time slot for data transmission is assigned to each device with a successful REQ-T. During the TDMA period, data packet transmission is performed depending on the order established in the CSMA/CA period. In case of a successful transmission of a data packet, the AP returns an ACK message. Otherwise, this last message is not sent. In this way, the device whose packet was not delivered sends a retransmission intent message with which the AP will assign a new slot. The results reported in [28] show that the use of a single OACK leads to shorter delays when compared to the conventional ACK used in most of the literature. Furthermore, they concluded that their proposal achieves better performance than the HyMAC [29] and CPMAC [30] protocols. In spite of these promising results, and probably due to its intended application, the proposal was tested with a maximum of 15 devices attempting to transmit simultaneously. Thus, its scaling properties remain unknown. In the hybrid MAC protocol proposed by [PERSON] et al. in [PERSON] et al. (2018), each superframe is composed of five periods: beacon period (BP), contention period (CP), assignment period (AP), energy period (EP), and variable upload period (vUP). During the BP, the coordinator node broadcasts a synchronization packet containing essential parameters, such as the total number of minislots, a minislot duration and the maximum data packet length. In the CP, devices with data to transmit divide their payload into smaller packets (if necessary) and compete for channel access using a CSMA/CA-based algorithm. Each device is allowed to transmit a single packet that includes information about how many packets remain to be sent. Then, during the AP, the coordinator node broadcasts an assignment list with the reserved slots for each device that successfully transmitted during the CP. This assignment is based on the remaining number of packets indicated by each device.