# 2 CA-R2: A Hybrid MAC Protocol for Machine-Type Communications
## 1 Introduction
At the same time, another node that all of a sudden joins the contention (with a small contention window) may successfully transmit its packet on its first try. This effect is not desirable in many applications, especially the ones that require high-priority data delivery. Therefore, optimization of the contention algorithm used in hybrid protocols is a relevant matter that deserves attention. An alternative, which provides low access delay when collisions occur in the medium, is the implementation of protocols related to the tree algorithm [6; 14]. An example of this approach is Distributed Queuing (DQ), which was introduced in [15], and a variant was presented in [6]. Another variant of the tree algorithm, known as the two-cell algorithm (2C) [16], shows some level of fairness with the following rule. When a collision occurs, no new nodes are allowed to join the contention until all the ones involved in the previous collision successfully transmit. In this paper, we introduce 2C (adaptive) with resource reservation, or 2 CA-R\({}^{2}\) for short, a hybrid MAC protocol that combines random access to achieve resource reservation and transmission based on time scheduling. The proposal consists of two stages, namely, \"collision resolution interval\" and \"data transmission interval\". The proposed 2 CA-R\({}^{2}\) protocol is intended to support M2M communications in the device domain [17], considering a network within the transmission range of a WLAN or shorter. What distinguishes the proposal from other works is the use of the Adaptive-2C conflict resolution algorithm as the random access mechanism in the contention stage. The Adaptive-2C algorithm was previously introduced and evaluated by our research team in [18]. As it will be described in the following, the properties of this contention algorithm allow us to achieve a seamless integration between the two intervals, i.e., contention and transmission. When the collision resolution interval ends, the stations take turns to transmit their data packets using contention-free slots. The main contribution of this paper can be summarized as the introduction and evaluation of 2 CA-R\({}^{2}\), a hybrid adaptive 2C-based MAC protocol. The evaluation was carried out using both computer simulations and a mathematical model based on a Markov chain. Although this protocol is simple to implement, our results demonstrate that it scales better and achieves outstanding performance when compared to the widely used DCF protocol. The remainder of this paper is structured as follows. In Section 2, we provide an overview of some related works. In Section 3, we describe the base protocols, namely, 2C and Adaptive-2C. In Section 4, we describe the proposed hybrid protocol, that is, 2 CA-R\({}^{2}\). In Section 5, we derive an analytical model intended to estimate the performance of the Adaptive-2C algorithm, which is the basis of the collision resolution interval of the proposal. In Section 6, we describe the performance evaluation experiments that were carried out and discuss the corresponding results. Finally, Section 7 concludes this paper.