## 1 Introduction  
Wireless body sensor networks (WBSNs) or body area networks (BANs) constitute an emerging and promising technology that will change people's healthcare experiences radically [1,2]. Unlike traditional healthcare systems, WBSNs will release patients from long hospital stays, thus reducing medical labor and infrastructural costs. In general, WBSNs will be cost effective, and they can continuously monitor physiological signals of patients, which will be very helpful, especially for the aging population [3]. Moreover, the use of WBSNs may enable ubiquitous healthcare and could lead to proactive and even remote diagnosis of diseases in an early stage. These systems provide uninterrupted health monitoring services, allowing patients to perform everyday activities, which lead to the enhancement of the quality of life [4]. However, wearable health monitoring technology is still young, and some challenging issues, such as quality of service (QoS), security and privacy, as well as social issues need to be resolved before this technology can be used widely. QoS is one of the main concerns for this technology. Generally, in network systems, such as WBSNs, QoS is viewed from two perspectives: the network and user/applications [5]. In the user/application perspective, QoS refers to an assurance of a set of requirements/services that are expected of the system by the users or applications. From the network perspective, QoS refers to a set of service qualities that the network offers to a user or application in terms of network QoS parameters, such as delay, reliability, energy efficiency, _etc._, during data delivery. As medical wearable systems deal with real-time and life-critical applications, they require a strict guarantee of QoS in both perspectives. To support QoS in WBSNs, the QoS requirements of these systems in user and network perspectives should be identified and addressed accordingly [6, 7, 8, 9, 10]. Addressing QoS in WBSNs is a difficult task. Some studies identified only reliability, real-time data delivery and network lifetime as QoS requirements of any wireless emergency system, such as WBSNs [2,11]. However, the QoS requirements in WBSNs are complex, and these systems may involve additional requirements in both perspectives of QoS [6, 7, 8, 9, 10]. Moreover, along with the resource constraints, these systems suffer from environmental and users' contexts in most applications, which pose additional challenges to support QoS [8]. For example, in hospital environments, very low signal-to-noise ratio (SNR) values are expected. Moreover, in and on body dynamic path loss [12] and patient's mobility increase the probability of packet loss in these systems. All of these issues constrain the solution space for QoS support in WBSNs and need to be considered in designing a QoS mechanism and framework (mostly media access control or MAC protocol centered) for these systems [6,8,10]. Existing works are available in the literature [13, 14, 15, 16, 17] that deal with QoS support in WBSNs. In these works, one or more of the necessary QoS requirements in health monitoring systems are missing. For example, reliable data delivery is covered in the proposed framework by [PERSON] _et al._[17], but this requirement is missing in the work by [PERSON] _et al._[15]. Energy-saving MAC by [PERSON] _et al._[15] addresses real-time data delivery, but the work in [17] does not address this QoS requirement. Moreover, these studies do not consider environmental issues in designing their frameworks, and they may not be applicable in providing QoS support for real WBSNs [6,8,10]. A set of mechanisms (e.g., error recovery, clustering, power control) is available that can deal with QoS issues in WBSNs. Error recovery mechanisms are a very strong candidate to support QoS in WBSNs, as they can avoid retransmissions and improve QoS in terms of reliability, energy efficiency and real-time data delivery [11,18,19,20]. WBSNs need QoS mechanisms that can act as a function of (adaptive) network channel conditions [10,18].