## 3 Protocol Framework
### Basic Framework  
Figure 1 illustrates the overall structure of the MAC protocol which we proposed in this paper. The superframe is composed of a Beacon frame and multiple subframes, and each subframe is combined with an Active frame and a Sleep frame. Furthermore, each Active frame at every subframe is composed of several periods; Sync, Transmission Period (TP), and Contention Period (CP) as shown in Figure 2. TP in the Active frame is composed of both d-TP and u-TP; d-TP denoted for downlink directional transmission period from a PC to an individual node, and u-TP denoted for uplink transmission.  
The beacon packet is generated from PC and it is disseminated to all nodes in Beacon frame with relatively long term interval denoted as \(T_{B}\). As discussed in the previous paragraph, one beacon frame is followed by multiple frames in each \(T_{B}\) as shown in Figure 1. Moreover, \(T_{S}\), which denotes the time duration of each subframe, has relatively short term value compared with \(T_{B}\). Each subframe is composed of an Active frame and a Sleep frame in which the RF transceiver should keep the wake-up condition and the sleep condition. Hence, \(T_{B}\) could be considered as the duty cycle of the transceiver. Moreover, in Sleep frame, the PC and all nodes enter to the condition of deep sleep or standby status for energy conservation. Hence, all transmissions are basically absent during Sleep frame, but we allowed for an exception in the case of emergency data transmission.  
To synchronize every frame between the coordinator and each correspondent node, the PC should periodically transmit the beacon packet containing both \(T_{B}\) and \(T_{S}\). Every node belonging to the same coordinator can be aware of the start time to be awake depending on \(T_{B}\) and reception time of the packet, and each node can also estimate the reception time of the next beacon packet from the same PC. Consequently, if a node does not have any packet to send, it can omit all forwarding frames and can just sleep until the next Beacon frame.  
On the other hand, Sync period is for disseminating Sync packet to awaken nodes from PC, which contains each time offset values for d-TP, u-TP, CP, and Sleep frame. The Sync packet is always sent from a PC at every \(T_{B}\), and we design that each subframe can be alternatively applied to each node. Each corresponding node should decide to participate in the current subframe or not. The rest of a subframe is CP, and we adapt the exceptional case of allowing contentions based on CSMA/CA from nodes on this period, in which this period supports the registration of a new node joining in.  
Under the given timeframes above, all transmissions except in CP should be critically controlled by the PC. By the way, emergency data transmission must be transmitted without any authentication by PC. IEEE 802.15.6 protocol allows the emergency data transmission by out of coordinator, but it allowed in only EAP and RAP. However, emergency data related to the biomedical signal must be immediately sent regardless of the timeframe condition. For this reason, we also designed a mechanism to minimize the latency of emergency data transmission. In the proposed scheme, the emergency data transmission can be allowed in overall time frames (i.e., d-TP, u-TP, CP. And even Sleep frame) with prioritized channel preemption.  
As we described previously, the PC relatively has sufficient energy resource than nodes so that the main goal of the proposed MAC protocol is designing an asymmetrical energy balance model between PC and nodes. Meanwhile, it is fundamentally required that the energy consumption point should be loaded into a PC as possible for conserving the energy of nodes side. The nodes should wake up on the only essentially required moment.