## 4 The WBANs Communication Stack
### Routing Protocols
\begin{table}
\begin{tabular}{c c c c} \hline \hline \multicolumn{2}{c}{**MAC Protocols**} & \multicolumn{2}{c}{**Key Characteristics**} & \multicolumn{2}{c}{**Performance Metrics**} \\ \hline \multirow{4}{*}{802.15.4-based} & \multirow{2}{*}{Change [50]} & Three access schemes are considered & \multirow{2}{*}{\begin{tabular}{c} Better throughput and reduced delay \\ but higher power consumption. \\ \end{tabular} } \\  & & with multiple traffic and coexistence & & Better throughput and reduced delay \\  & & capabilities. & & but higher power consumption. \\ \cline{2-4}  & \multirow{2}{*}{[PERSON] [51]} & To optimize wake-up schedule of & \multirow{2}{*}{\begin{tabular}{c} Improved energy efficiency and delay \\ with reference to standard 802.15.4. \\ \end{tabular} } \\  & & 802.15.4 non-beacon mode. & & \\ \hline \multirow{4}{*}{Low Duty Cycle} & \multirow{2}{*}{[PERSON] [57]} & Multichannel and centrally controlled & \multirow{2}{*}{\begin{tabular}{c} It requires high synchronization \\ overheads and not suitable for dynamic \\ traffic because of higher packet lost. \\ \end{tabular} } \\  & & (master-slave) synchronization for & & \\  & fixed traffic. & & traffic because of higher packet lost. \\ \cline{2-4}  & \multirow{2}{*}{[PERSON] [58]} & Collision avoidance having low & \multirow{2}{*}{\begin{tabular}{c} Energy efficient with 4.5\% duty cycle \\ synchronization overheads. \\ \end{tabular} } \\  & & & & but higher latency. \\ \cline{2-4}  & \multirow{2}{*}{[PERSON] [4]} & It uses traffic-status-register-bank to & \multirow{2}{*}{\begin{tabular}{c} Traffic-adaptive energy and latency \\ efficient, scalable and flexible but \\ sensitive to abrupt and burst traffic. \\ \end{tabular} } \\  & & keep the track of traffic variations & & Traffic-adaptive energy and latency \\  & & through an evolutionary adaptive & & efficient, scalable and flexible but \\  & & algorithm. & & sensitive to abrupt and burst traffic. \\ \hline \multirow{4}{*}{WBAN-Specific} & \multirow{4}{*}{\begin{tabular}{c} Headbeat [55] \\ time synchronization. \\ \end{tabular} } & Uses heartbeat rhythm instead of & \multirow{2}{*}{\begin{tabular}{c} Low synchronization overheads and \\ optimized collisions energy efficient, \\ but higher latency, lower throughput \\ and non-adaptive to traffic variations. \\ \end{tabular} } \\  & & & & but higher latency, lower throughput \\ \cline{2-4}  & \multirow{2}{*}{MedMAC [56]} & Maximize energy efficiency through & \multirow{2}{*}{\begin{tabular}{c} Energy and latency efficient for \\ low-to-medium traffic patterns. \\ \end{tabular} } \\  & & dynamic adjustments for QoS & & \\  & requirements & & \\ \cline{2-4}  & BodyMAC [54] & Introduced efficient downlink and & \multirow{2}{*}{\
\begin{tabular}{c} Optimizes energy efficiency by \\ reducing idle listening, collisions and \\ overheads \\ \end{tabular} } \\  & & uplink with effective and flexible & & reducing idle listening, collisions and \\  & & utilization of bandwidth & overheads \\ \hline \multirow{4}{*}{802.15.6-based} & \multirow{4}{*}{802.15.6 MAC} & It provides higher throughput and & \multirow{4}{*}{
\begin{tabular}{c} High throughput and data rates, better \\ coexistence and security techniques. \\ \end{tabular} } \\  & & security, greater flexibility (both & \\ TDMA and CSMA/CA as well as & & & coexistence and security techniques. \\ \cline{1-1} \cline{3-3}  & & hybrid medium access) and & & \\ \cline{1-1} \cline{3-3}  & & applications specific control & \\ \hline \hline \end{tabular}
\end{table} 
Table 9: Summary on existing major WBANs MAC protocols. Figure 17: Classifications of Routing Protocols. are severe constraints for invasive WBANs) are much relaxed in the given context. However, as explained earlier in MAC protocols design attributes (_i.e._, Section 4.2.2), it is the quality-of-service and reliability of successful transmission and reception, are most important for routing protocols as well. Below we will present the classification and state-of-the-art routing protocols for WBAN.