# 2 CA-R2: A Hybrid MAC Protocol for Machine-Type Communications
## 1 Introduction  
Some recent studies [1] predict that by 2050, most of the population in the world will live in urban environments; therefore, cities will have to efficiently manage their resources and provide services more effectively. Due to such transition toward smart cities, the Internet is evolving to become a ubiquitous network of interconnected devices that will allow the monitoring and control of multiple environmental variables in real time. This trend, where the vast majority of Internet users are machines and not humans, signals the arrival of the Internet of Things (IoT). Such an exponential growth will also bring about the need to provide wireless network access to a large number of autonomous devices. In fact, a recent study conducted by IoT Analytics [2] predicts that by 2030, there will be 41.1 billion active IoT device gateways, or access points, worldwide. This situation has motivated the evolution of diverse technologies, mainly cellular communications and wireless local area networks, with the consideration that a large number of connected devices may need network access services at one time. For this vision of the future to be achieved, adequate support for M2M communications is indispensable. However, the available studies provide evidence to support the fact that the characteristics of traffic generated in access networks by IoT applications are different from those present in traditional Human-Type Communications (HTC) [3; 4]. For example, in M2M communications, data are predominantly transferred in the uplink direction, whereas in HTC it is otherwise. Other characteristics that can be identified in M2M communications are the following: First, in many IoT applications, the effective payload of data packets is small [5]. Second, these packets can be generated not only at random instants but also at regular time intervals. A networking technology that intends to serve M2M communication devices must implement strategies that take into account the characteristics of this type of traffic. However, existing protocols, which were developed for HTC, are inefficient under such conditions [6]. MAC protocols are critical components of a network architecture intended to be used in IoT applications. In a typical scenario, these protocols must be designed considering that a large number of IoT devices may simultaneously attempt to transmit small packets over low-data-rate links. Therefore, MAC algorithms must be scalable, that is, when the number of simultaneous access requests grows, a number of desirable performance features should be preserved (e.g., high throughput, low latency, and fairness). However, widely used MAC protocols, which were conceived to support human-to-human or human-to-machine interactions, have limitations in this regard. In addition, IoT devices are typically both powered by a battery and limited in terms of processing power; therefore, such protocols should also be simpler to implement and have lower computational costs compared to protocols used in other applications. For all these reasons, in recent years, we have seen the appearance of several MAC proposals intended to be used in M2M communications to improve performance or incorporate additional features. At this point, it is worth revisiting some basics. MAC protocols can be broadly classified as contention-based, reservation-based, and hybrid. Contention-based MAC protocols are the simplest in terms of configuration and implementation. Among these protocols, we have the following well-known examples [7; 8]: ALOHA, slotted ALOHA (s-ALOHA), and Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA). An implementation of the CSMA/CA protocol is the basis of the Distributed Coordination Function (DCF), which is the fundamental access method of the popular IEEE 802.11 standard [9] for wireless local area networks (WLANs). A disadvantage of contention-based MAC protocols is that as the number of devices in the network grows, the collision probability also increases.