## 1 Introduction  
In recent years, Wireless Body Area Networks (WBAN) have been attracting the national interest of biomedical informatics by meeting the communication technology requirement. Many related studies have concentrated on fusing medical services and communication technologies to realize unexplored fields of medical service for enhanced quality of life. This development can conserve the cost of medical services and allow wide distribution of medical knowledge to nonmedical personnel.  
In this setup, all information of the users who wear the healthcare equipment should be gathered by micro-sized sensors, which measure and collect body signals, such as electrocardiogram (ECG), electroencephalogram (EEG), or Vital Sign (e.g., heart rate, blood pressure, temperature, pH, respiration, oxygen saturation) [1]. These signals should be delivered to a remote base station for diagnosis and prescription, with harmless personal communication method.  
This situation requires all physical and physiologic data monitored from the human body for the synthetic data fusion and the central preprocess with high efficiency of energy utilization. Another significant requirement is guaranteeing the minimized latency for emergency data transmission. It is clear that the emergency data has to be urgently reported as soon as possible for immediate rescue whenever a node senses dangerous signal of body condition.  
Meanwhile, communication devices in WBAN are generally composed of nodes sensing the body signal and a coordinator analyzing the sensed body signal. Another significant role of the coordinator is harmonizing multiple nodes against collisions, interferences, and unnecessary energy consumptions. The main difference on the specific environment of WBAN is energy resource. While the nodes contacting the body have extremely limited battery capacity due to their light-weighted and micro-sized feature, a coordinator (e.g., mobile phone) has relatively sufficient energy resource because it has abundant hardware resources and rechargeable battery. Hence, the main focus for designing WBAN should consider the energy balance between nodes and coordinator.  
WBAN technology has been investigated for near distance (3 m-10 m) communication within or around the body with conditional radio transmission power which must be ergonomically harmless to human health. Moreover, it is clear that the Medium Access Control (MAC) layer is the most crucial layer in terms of energy efficiency because it handles idle listening, retransmission, carrier sensing, and control of the transmission power. Consequently, WBAN has to meet these restricted requirements and the MAC protocol can become a key technique for addressing these emerging issues.  
Furthermore, it is possible to analyze the general conditions of the WBAN environment by focusing on the MAC layer. First, we can find that most of the transmissions might be incurred in uplink direction, that is, from nodes to coordinator. Second, a coordinator should support the registration for newly participating node at any time. Third, all data can be classified into periodic report data and emergency data to allow the MAC protocol to provide different Quality of Service (QoS). Fourth, when the MAC protocol considers energy efficiency, it has to adaptively and asymmetrically control the energy balance between nodes and coordinator.  
Many WBAN MAC protocols have long been designed and proposed. However, they have not considered the above issues, especially in adapting different energy balance between nodes and a coordinator, while also guaranteeing the minimized delivery delay for transmission of emergency data. Hence, this motivates us to design a new MAC protocol, called aSymMAC, to analyze the above four issues with dealing solutions on time registration, allocating downlink transmission only if needed, handling QoS between periodic report data and emergency data, and designing an asymmetric energy-balanced model for efficient energy distribution belonging to WBAN environment.