# 2 CA-R2: A Hybrid MAC Protocol for Machine-Type Communications
## 2 Related Work  
In the literature, there are several pieces of work whose purpose is to introduce hybrid MAC mechanisms for M2M communications. This section provides an overview of some relevant works on this subject, but we focus on protocols whose operation is similar to our proposal, that is, protocols that are divided into at least the two previously mentioned stages: contention and transmission. Note that the elements of a description should not be used to describe other works since there is no guarantee that they mean the same concept. In [19], [PERSON] et al. propose a hybrid MAC protocol for M2M networks. In the proposal, there is a contention stage, called contention only period (COP), and a data transmission stage, called transmission only period (TOP). During the COP, which is based on \(p\)-persistent CSMA, the devices compete for transmission slots. Before the TOP takes place, the IDs of successfully contending devices and scheduling information are transmitted by a base station in an announcement period (AP). Only successful devices can transmit data during the TOP using TDMA. Analytical and simulation results shown in [19] demonstrate the advantages of this hybrid MAC protocol when compared to ALOHA, s-ALOHA, and TDMA. However, significant control and computational overhead may arise, as the base station must transmit synchronization frames, the IDs of successful devices, and scheduling information during the AP. The framework implemented in the hybrid MAC protocol proposed by [PERSON] et al. in [20] combines CSMA, DCF, and TDMA. The proposal consists of four different stages: a notification period (NP), a contention period (CP), an announcement period (AP), and a transmission period (TP). First, the access point broadcasts a notification to all devices during NP to start the CP. During the CP, the devices that have data to transmit send transmission requests to the access point using the CSMA technique. Then, during the AP, the access point signals the end of the contention period and the beginning of the TP to the nodes whose transmission requests where successful. Finally, during the TP, the devices transmit data using the DCF mode of IEEE 802.11 within TDMA slots to cope with communication failures due to the lack of clock synchronization. In spite of the evident signaling overhead, the authors report that the proposed MAC protocol achievesbetter performance than slotted-ALOHA and TDMA in terms of aggregate throughput and average transmission delay. In the hybrid MAC protocol proposed for M2M networks by [PERSON] et al. [21], each frame consists of three main parts: contention only period (COP), notification only period (NOP), and transmission only period (TOP). During the COP, the devices contend for transmission time slots using the non-persistent CSMA (NP-CSMA) mechanism. In the NOP, the base station announces the time slot reservations to be used in the TOP to transmit data packets. The method used for transmission is TDMA. It is interesting to note that if a successfully contending device cannot be granted a time slot during the TOP, it can send its data within the transmission slot of another device subject to the restriction that a target Signal-to-Interference and Noise Ratio (SINR) is not exceeded. In this way, the mechanism increases the number of transmitted packets. The authors report that the proposed protocol achieves higher throughput, lower average delay time, and a lower packet collision ratio when compared with NP-CSMA and s-ALOHA. [PERSON] et al. [22] proposed a scalable MAC framework assisted by machine learning. In the work, each cycle, called a superframe, consists of four parts: a rendezvous period (RP), a contention access period (CAP), a notification period (NP), and a contention-free period (CFP). A particular characteristic of this proposal is that by using machine learning techniques, in the RP a gateway dynamically detects the number of active devices from the signals that they emit.