## 1 Introduction  
The increase in average lifespan and the pressure on health budgets in many developed countries have resulted to be important catalysts for the development of innovative and cost-effective health care solutions. Human assistive and wearable technologies such as Wireless Body Area Networks (WBANs) are emerging as an important part of the daily life for ambient assistive living. As an example it was recently reported that the UK's National Health Service (NHS) could save up to seven billions pounds per year by using innovative technologies to deliver quality healthcare to the chronically ill with fewer hospital visits and admissions [1]. In this regards the recent Consumer Electronics Show (CES'14 [2]) was a great showcase for wearable technologies. A number of products from companies such as Vuzix, Samsung, LG, Sony, Razer, _etc._, were introduced for applications such as medicine and fitness, entertainment, augmented reality, and many more. It is predicted that wearable technologies could be a key booster for the consumer electronics industry. According to the wearable technology database [3], there are currently 41 (medical), 77 (fitness) 117 (lifestyle) and 26 (entertainment) devices already available on the market. Typical envisioned applications range from the medical field (e.g., vital sign monitoring, automated drug delivery, _etc._), to entertainment, lifestyle, gaming and ambient intelligence. However, with regards to applications such as disaster, rescue and critical missions, workers safety in harsh environments (e.g., oil and gas fields, refineries, petro chemical and mining industries) as well as roadside and building workers, wearable WBAN technology can also play a vital role to not only save human lives but also to protect critical and valuable assets. In this paper, we will emphasize this new application paradigm for WBANs. WBANs are a variant of Wireless Sensor Networks (WSNs) which consist of a few tiny sensors implanted inside the body or located on the body to typically observe physiological signals emanating from different body organs, body motions as well as the surrounding environment. The network is designed in such a way that the coordinating device communicates with implanted and on-body sensors as well as with the access point which further transmits the collected information to a remote monitoring station. This is a typical scenario where only one body constitutes a WBAN, which is controlled by a more powerful coordinator (in comparison to other nodes in WBAN), and hence communicates with the distant off-body networks. We term this system a '_closed-system'_ because one WBAN cannot communicate directly with other WBANs, whereas, in the context of rescue and critical mission-based applications, '_closed-system'_ WBANs are not enough because body-to-body communications is a key requirement to enhance the communication and reliability of the system. Therefore, in this paper, we classify WBANs as Intra-WBAN (or On-Body), Inter-WBAN (or Body-to-Body) and Beyond-WBAN (or Off-Body) networks, as shown in Figure 1.  
Low-power devices attached on the body are constrained by minimum energy requirements due to their limited battery life and should be small enough in size to be easily wearable. The management of such a heterogeneous network (both from the different devices as well as from the highly dynamic traffic points-of-view) adds another dimension to the design issues [4]. Moreover, for the given applications, harsh environment, Quality-of-Service (QoS) and reliability of successful communication further add to the stringent constraints of the overall system. Therefore, these new applications require a thorough and comprehensive application-oriented and related issues-based review of the existing state-of-the-art, including standards and protocol stacks. In addition, it is necessary to analyze what are the key limitations of the existing solutions and finally to understand the future potential opportunities and challenges in this context.