## 3 Protocol Framework  
Recently, many types of research related to WBAN has been designed to handle the energy efficiency and latency under the human body network environment and they have made many contributions. However, some points to satisfy the significant requirements of WBAN remain as challenging issues and these could be classified into two phenomena below.  
The first point is the immediate transmission for emergency data. Most research, except for the IEEE 802.15.6 standard, focused on the overall energy efficiency and overall latency, but could not make an effort for emergency data transmission because they just allocate separate time slots for this purpose. Moreover, it is not clear that the standard technology of IEEE 802.15.6 efficiently accomplished this issue under EAP and RAP; it just employed the Slotted Aloha and CSMA approach without any prioritization for emergency data. Specifically, all data were just fairly transmitted during these periods regardless of whether it is an emergency or not. The emergency data can be incurred at any time regardless of time slot structure. Therefore, the superframe-based WBAN MAC protocol has to support an immediate transmission method for emergency data within the overall time domain.  
The second fundamental requirement for WBAN is that the energy balance between nodes and coordinator must be differently considered. Most smart devices (e.g., phone, watch, and wristband) can generally play a role of coordinator in WBAN environment because they have more sufficient energy resource than the nodes, and their battery might be rechargeable. WBAN MAC protocols have to consider this issue by asymmetrically allocating the energy consumption point on the both roles.  
As a result, the proposed scheme on this paper was focused on the above two significant issues to satisfy the essential requirement of WBAN environment by handling the emergency data transmissions and designing asymmetric energy balance model between nodes and coordinator.  
IEEE 802.15.6 standard technology employed CSMA-based mechanism when transmitting emergency data from nodes. However, CSMA-based mechanism absolutely depended on Clear Channel Assessment (CCA) which was provided in a radio transceiver, but the reliability of CCA effect cannot be guaranteed because Medical Implant Communication Service (MICS) band, which is a widely used radio band for WBAN, does not permit over the transmission power of \(-85\) dBm. Moreover, CSMA-based protocols have some energy-waste problems (e.g., idle listening, overhearing, protocol overhead, and preamble sampling). With these reasons, we found that CSMA-based protocol is not appropriate for WBAN and decided to hybridize both CSMA and TDMA for WBAN MAC protocol.  
Before describing the proposed protocol in detail, we named the coordinator as Personal Coordinator (PC) and assumed that the communications of node-to-node were not presented because general WBAN nodes report their sensed data only to a coordinator. Note that this paper is mainly aiming to design of both asymmetrical energy-balanced model and QoS for emergency data transmission. Based on the prioritized scheme, the proposed protocol concurrently supports both transmission types of normal and emergency without any operation mode change, contrarily to IEEE 802.15.6.