## 3 Proposed Design
### Problem Formulation Using Traffic Patterns  
We select IEEE 802.15.4 for the experiments and implementation due to its simple hybrid channel access mechanisms. IEEE 802.15.4 operates under a superframe which can be divided in active and inactive modes. The active mode consists of two basic periods, including CAP and CFP. CAP consists of time slots which can be acquired through slotted CSMA/CA, whereas CFP provides GTS slots and works in TDMA manner. Both periods work in an energy efficient manner by using synchronization mechanisms [1; 2]. CAP follows several steps in the contention process. In the first step, the slotted algorithm for CSMA/CA initializes the values of the number of backoff attempts (NB), contention windows (CW) and backoff exponent (BE). The node waits according to its selected backoff period and then performs clear channel access (CCA). In the 1 st set of CCA attempts, if the channel is found idle then CW is decremented from 2 to 1 and the node performs the 2 nd CCA. If the channel is idle in the 2 nd CCA and the value of CW decremented from 1 to 0 then CCA is assumed to be successful and the data is transmitted else the CCA process starts again. In the case that the channel is sensed busy in both CCA attempts, NB and BE are incremented by one, whereas the value of CW is initiated from 2. Figure 1 shows the scenario of channel access for a single CAP slot. The biomedical sensor devices generate frames according to their sampling interval shown in Table 3 and try to access the channel through contention mechanism of slotted CSMA/CA. When both modes i.e., CAP and CFP are used together then in the CFP mode GTS slots are assigned to important periodic data like ECG, EMG and EEG, whereas other nodes which require event bases or emergency data use the CAP mode. Figure 1: Channel access mapping of bio-medical devices for slotted CSMA/CA in a CAP slot. \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Physiological** & **Data Generation** & **Data Generation** & **Packet Size** & **Required Data** \\
**Parameter** & **Interval** & **(Bits)** & **(Bytes)** & **Rate (Kbps)** \\ \hline ECG & 4 ms & 16 & 17 & 34 \\ EMG & 6 ms & 11 & 16.3 & 19.6 \\ EEG & 4 ms & 11 & 16.3 & 19.6 \\ Pulse oximeter (SpO2) & 10 ms & 12 & 16.4 & 13.2 \\ BP & 10 ms & 12 & 16.4 & 13.2 \\ Respiration & 40 ms & 8 & 16 & 3.2 \\ Blood glucose & 250 s & 0.032 (1 bit) & 15.1 & 0.528 \\ Skin temperature & 60 s & 0.266 & 16.98 & 2.27 \\ Activity & 10 ms & 11 & 16.3 & 3.3 \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Traffic pattern summary. The IEEE 802.15.4 standard supports the maximum frame size up to 127 bytes including 25 bytes of MAC header and 102 bytes of payload. Figure 2 illustrates the frame structure of IEEE 802.15.4. A successful packet transmission of IEEE 802.15.4 with its overhead is illustrated in Figure 3. By analyzing the single successful packet transmission mechanism, overhead for a successful channel access at different stages can be observed. It includes random backoff delay, two times CCA, 802.15.4 header with each frame, SIFS and acknowledgement transmission time [1]. The maximum packet size from these bio-medical sensor devices used in the patient monitoring system is 17 bytes for a single frame including headers as shown for ECG in Figure 4. The size is almost the same for other used devices; therefore, we consider the total packet size for these devices as 17 bytes in our experiments. Each frame will content for the channel to send the data, so a node which wins access to the channel transmits a small packet and for the next packet transmission, it needs to contest for the channel again. The common characteristic of these biomedical sensor devices is that they generate many small packets in a limited time. Moreover, the generated data from these devices require a combined set of QoS including limited delay, specified throughput, reasonable reliability and energy efficiency.