## 1 Introduction  
Medical Internet of Things (IoT) [1] enables a collection of medical devices and applications to be connected through the Internet, and has revolutionized the conventional concept of healthcare. Wearable applications of IoT in the medical field has spawned the era of smart healthcare [2, 3], which enables constant medical supervision under free living conditions and thus upgrades the existing medical infrastructure. A three-tier architecture [2] (as shown in Figure 1)--based proactive healthcare approach could enhance the quality of living in different ways [3] by providing continuous medical assistance at a reasonable cost. Such a health-monitoring system subject to an energy constraint network, namely Wireless Body Area Network (WBAN) [2], a network of small-size, ultra-low-power, wearable or implantable biosensor nodes powered by batteries placed in, on, or around the human body to measure vital physiological parameters together with a WBAN coordinator (that could be a smart handheld device as well), acts as a sink to store and process health data locally. Tier 1 focuses on network formation, whereas in tier 2, the sink communicates via WLAN or GPRS technology to a remote medical server residing at tier 3 for analysis of health data by medical personnel and to provide necessary actions. In tier 1 of WBAN, different applications could use a suitable physical and MAC layer as defined by IEEE standards. These include IEEE 802.15.4 (Zigbee), IEEE 802.15.1 (Bluetooth) and IEEE 802.15.6 (mainly used for implantable nodes). To prolong the lifetime [4] of the network to achieve long-lasting benefits of WBAN applications [2; 5] energy-efficient network layer [6; 7] is essential. The low-power-consumption (typically, power output rating lower than 0 dBm) feature of BAN transceivers limits transmission distances to around 2 m, depending on power output and environmental characteristics [8]. Hence, relay nodes are often placed in the network to acquire the benefits of short-range multi-hop communication [4; 7; 9] with low transmission power over long-distance high-power direct data delivery. However, the latest version of the IEEE standard proposed for WBAN in February 2012 suggests two-hop communication [2]. Although a proprietary system could use more than two hops, in such cases, interoperability could be a significant challenge. Research works reported in [2; 10] identified that data transmission is the prime source of energy expenditure within WBAN and hence strategies should be made to obtain a balance between reliable data delivery with minimal energy depletion. Previous research works reported in [11; 12; 13; 14] addressed the key factors related to the system state such as energy level of nodes, event occurrence, energy-harvesting capacity, data importance etc. to carry out analytical formulation of transmission strategies. Mathematical models, particularly Markov Decision Process (MDP) [15], is found to be exploited in state-of-the-art works reported in [12; 13] to predict the optimal sequence of actions to be followed to achieve the desired goal. In addition, the type of nodes used in WBAN are either implanted or wearable and thus data communication takes place through tissue and/or air medium accordingly [3]. Hence, channel conditions impose an impact on reliable data delivery. Low power transmission in adverse channel conditions could result in data loss. Besides, electromagnetic radiation absorbed by human tissue (measured in terms of Specific Absorption Rate (SAR), [2]) generated due to network activities of bio sensor nodes may result in several health hazards [16] if it reaches beyond the regulatory limit [2]. However, the heat generation is proportional to the energy depletion rate of the participatory nodes in the network which in turn subjects to the effective transmission power of data communication. Thus, obtaining optimal transmission power for intra-BAN communication plays a key role in effective network design.