## 2 Related Work  
MAC protocols have a vital role for the efficient functionality of WBASNs. The efficient MAC layer protocols for WBASNs have been a hot research topic for past few years. Aiming at provision of specified QoS in terms of delay, throughput, reliability and energy many efforts have been made to develop a new improved MAC protocol based on existing standards i.e., IEEE 802.15.4 and IEEE 802.15.6. Based on the literature, we summarize a comparative analysis of the protocols in term of their QoS support in Table 2, where D represents delay, R reliability, E energy efficiency, T throughput, C collision handling and S scalability. IEEE 802.15.6 is known as the WBAN standard and theoretically claims to provide a rich communication suite that is suitable for different kind of WBAN applications [31]. However, it only elaborates the operational details to interoperate the WBAN devices and leave the implementation on users per their application needs. In this context, the MAC scheme does not reflect a complete MAC protocol [32]. There are some other hybrid approaches which incorporate the contention mechanism over reservation mechanisms to improve the performance of critical-applications [33]. \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Protocol** & **Standard** & **Access Scheme** & **Theme** & **Focused QoS** \\ \hline \multirow{4}{*}{DQBAN [12]} & \multirow{4}{*}{IEEE 802.15.4} & \multirow{4}{*}{CSMA/CA} & To enhance IEEE 802.15.4, two queues & \multirow{4}{*}{R, C} \\  & & & are introduced for successful channel & \\  & & & & access and data transmissions & \\  & & & (collision resolution queue and & \\  & & & data transmission queue). & \\ \hline \multirow{2}{*}{D2 MAC [11]} & \multirow{2}{*}{IEEE 802.15.4} & \multirow{2}{*}{CSMA/CA} & Based on adaptive backoff time & \multirow{2}{*}{D} \\  & & & by using fuzzy logic. & \\ \hline \multirow{4}{*}{U-MAC [8]} & \multirow{2}{*}{IEEE 802.15.4} & \multirow{2}{*}{TDMA} & Urgency MAC provides priority for & \multirow{4}{*}{D} \\  & & & critical data for patient monitoring. & \\ \cline{4-5}  & & & U-MAC do not use option & \\ \cline{4-5}  & & & of retransmissions. & \\ \hline \multirow{4}{*}{HUA-MAC [10]} & \multirow{2}{*}{IEEE 802.15.4} & \multirow{2}{*}{TDMA} & Hybrid unified MAC only uses & \multirow{2}{*}{D, R} \\  & & & contention free period (CFP) for & \\ \cline{4-5}  & & & important data transmission, & \\ \cline{4-5}  & & & the normal traffic uses CAP. & \\ \hline \multirow{2}{*}{Channel-MAC [9]} & \multirow{2}{*}{IEEE 802.15.4} & \multirow{2}{*}{TDMA} & A single radio multi-channel TDMA & \multirow{2}{*}{R} \\  & & & MAC protocol to provide high reliability. & \\ \hline \multirow{2}{*}{EELDC [21]} & \multirow{2}{*}{IEEE 802.15.4} & \multirow{2}{*}{TDMA} & Energy efficient low duty cycle to & \multirow{2}{*}{E} \\  & & & provides low duty cycle values to & \\ \cline{4-5}  & & & increase the lifetime of the nodes. & \\ \hline \multirow{4}{*}{CA-MAC [20]} & \multirow{2}{*}{IEEE 802.15.4} & \multirow{2}{*}{TDMA} & Context aware MAC make nodes & \multirow{2}{*}{E} \\  & & & enable about energy of there and other & \\ \cline{4-5}  & & & nodes and help to increase the & \\ \cline{4-5}  & & & lifetime of the network. & \\ \hline \multirow{4}{*}{BDD [24]} & \multirow{4}{*}{IEEE 802.15.4} & \multirow{2}{*}{TDMA} & Battery dynamics driven protocol & \multirow{4}{*}{E} \\  & & & considers the current power conditions & \\ \cline{4-5}  & & & of the battery and useful for & \\ \cline{4-5}  & & & better energy consumption. & \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Comparative analysis of the proposed protocols. Table 2 lists the literature of the related protocols with the information regarding QoS support. To improve the IEEE 802.15.4, an enhanced version of IEEE 802.15.4 in 2012 and named IEEE 802.15.4e were published. This enhancement mainly introduces three behavioral modes to improve the channel access mechanisms of the existing IEEE 802.15.4 standard [37; 38].