## 1 Introduction  
The plethora of designs and applications of wireless sensor networks (WSNs) has caused them to prevail in various domains and sectors. WSNs played a significant role in paving the way for the emergence of the Internet of Things (IoT), an abundance of intelligent sensor nodes that are mounted on a variety of objects and connected to the Internet to report abundant information related to the performance/operation of those objects. As IoT applications emerge in each aspect of our day-to-day life, their performance relies on stringent requirements and specifications. The wireless body area network (WBAN) emerges as a critical IoT application. Research into WSNs and IoT introduces WBANs as a significant building block in the health system of any nation, intended to improve health services. A WBAN is mounted on the body of a patient to monitor or collect data on the patient's vital signs like their heartbeat,blood pressure, temperature, respiration, electromyography (EMG), electrocardiogram (ECG), and electroencephalogram (EEG) [1]. By monitoring these signs, the WBAN helps physicians to stay up to date with respect to the patient's health status, create patient history, allow for remote assessment of the patient, receive real-time data about the patient, and notify paramedics in cases of emergency. WBANs can significantly reduce the pressure on health systems by reducing the number of people admitted to hospitals. Also, WBANs can help patients become more mobile and stay at home rather than being confined to hospitals. This latter fact even has a positive impact on the patient's mental health, which speeds up his/her recovery. Healthcare is a vital sector that is projected to have a significant impact on the economy worldwide. According to [2], healthcare applications and related IoT-based services are expected to create an annual growth of USD 1.1-USD 2.5 trillion in the global economy by 2025. The authors also report that as dominant IoT applications, healthcare applications are projected to have a market share of 41% by 2025. For the WBAN to achieve its set goals, it requires quality of service (QoS) guarantees. These guarantees include, but are not limited to, high packet delivery ratios, reduced delays, high throughputs, and energy conservation. These parameters are highly affected by the medium access control (MAC) protocol used by the WBAN. In other words, the design of the MAC protocol should recognize these QoS requirements in order for it to be adopted in the deployment of WBANs (see [1,3,4] for more details on suitable MAC protocols for WBANs). As a result, MAC protocols for WBANs have received significant attention in the literature; ref. [3] provides a recent comprehensive survey of the state-of-the-art MAC protocols for WBANs. Being an example of WSNs, WBANs can be operated by deploying the IEEE 802.15.4 standard, i.e., the low-rate wireless personal area network standard that is highly suited for power-constrained, low-transmission-rate nodes like sensor nodes in WSNs [5]. The IEEE 802.15.4 (in the rest of the paper, we use the terms \"802.15.4\" or \"the standard\" interchangeably with \"IEEE 802.15.4.\") standard defines the specifications of both the physical layer and the MAC sub-layer of WSNs. The performance of the WSN greatly depends on how the MAC protocol parameters are tuned or defined. Improving the design of the standard MAC has been the objective of plenty of studies over the last two decades. Several survey studies that review the achievements in this field are available, like [6, 7, 8]. Research efforts in this direction introduced the IEEE 802.15.6 standard, an IEEE standard for WBANs. However, 802.15.4 is still a potential candidate for WBANs (see the excellent surveys [1, 8, 9, 10] that list 802.15.4 as a well-studied technology for deployment in WBANs). In fact, 802.15.4 is a potential technology for IoT applications in general, and WBANs more specifically [10].