# Implementation of a Topology Independent MAC (TiMAC) Policy on a Low-Cost IoT System
## 1 Introduction  
Recent technological advancements in IoT, as well as the emerging new network paradigm as envisioned by 5G [1, 2], have been the motivation for employing numerous different devices in order to support everyday activities. IoT, firstly introduced in 1999 [3], has been widely adapted to a variety of fields, such as environmental monitoring, infrastructure management, and home automation [4], as a concept of making the devices \"smarter\"--thus less dependent on human intervention. By the end of 2020, 25 billion IoT devices are expected to have been employed [5] worldwide. The growth of IoT and the increase in the supported devices lead to the generation of large amounts of data that have to be processed, stored, and transmitted by the different devices. In order for this data to be processed, stored, and transmitted, massive computing resources, storage space, and communication bandwidth are needed. In this sense, many works have focused their attention on tackling the above constraints. For example, computing tasks in IoT devices regarding the generated data can be assigned to cloud servers, since the computing power on the cloud outclasses the capability of the IoT devices [6]. In addition, in the post-cloud era arriving, many applications will also be deployed at the _edge_ of a system [7], so as to bring services closer to the IoT devices by pooling the available computing, storage, and networking resources at the edge of the network [4]. Regarding the devices composing an IoT system, they have to be of low cost and consume as little energy as possible, in order to support implementation on large-scale and multi-hop environments. In addition, sufficient computational capabilities are required for supporting the emerging applications and techniques, such as acting as edge devices in an edge-computing based system, where many processes occur directly on the IoT devices, rather than on the cloud. MAC policies, in IoT and networks overall, have a significant impact on the devices' characteristics described above. For example, MAC policies control the radio transmitter which is the most energy demanding part of an IoT device [8]; thus, the overall energy consumption and lifetime of the devices are largely determined by the MAC policy implemented. Apart from energy conservation (which is the case in many works in the literature [9; 10]), the role of MAC policies is to decide how the devices will access the shared medium, ensuring that only one node may access the channel at a time. Regarding the aforementioned devices' access to the shared medium, in many systems, specific time constraints have to be satisfied. For example, for safety applications in vehicular networks [11], it is expected that various and different devices can communicate and transmit their data under certain time bounds. Combining the above with the requirement for low-cost devices, as mentioned earlier, there is a need for more research on time-constrained wireless communication of low-cost devices. This need has initiated a revisit of Time Division Multiple Access (TDMA) MAC policies that are inherently capable of meeting time constraints when compared to the contention-based MAC policies, such as Carrier Sense Multiple Access (CSMA) [12] policies. Furthermore, in a system under a TDMA policy, there is no need for the nodes to listen to the medium prior to transmitting [13], while at the same time TDMA's deterministic approach guarantees a minimum performance level in terms of throughput. On the other hand, specific challenges arise when TDMA policies are applied, such as the need for continuous synchronization among the devices [14]. The latter drawback's difficulty to tackle lies both on the aforementioned extensive data generated by IoT devices and on the deviation occurring on the different devices' clocks as time passes [15; 16]. It is argued that this problem renders the use of TDMA policies impractical.