# A Survey of Routing Protocols in WBAN for Healthcare Applications
## 3 Classification of Routing Protocols for WBAN
### Cross-Layer Routing
#### 3.3.6 Comparison and Analysis  
As shown in Table 4, these protocols are optimized across two or three protocol layers. In terms of reliability metrics, CLDO [26], [PERSON] et al. [28], and AAT [29] protocols have advantages. CLDO achieves reliability transmission through complex theoretical analysis, but complex computation in the initial stage of the protocol, inevitably brings additional delay and fails to achieve a performance balance. PCLRP [25], CLRS [27], and [PERSON] et al. [28] all consider the priority of data, and both PCLRP [25] and [PERSON] et al. [28] use back-off time mechanism in the MAC layer, which guarantees a low delay and a reliable transmission of high priority data. The difference is that [PERSON] et al. [28] designs an adaptive power control in the physical layer, taking into account the reliability of low priority data transmission, reducing the delay of data transmission, so it is slightly better than PCLRP, in reliability and delay. The disadvantage of CLRS [27] is that when the shadow effect occurs, the whole transmission stops, until the shadow disappears, which inevitably causes a great delay.  
Adaptive power control is a hot topic in current research. Both [PERSON] et al. [28] and AAT mention adaptive power control. AAT is based on the estimation of channel state, while [PERSON] et al. [28] is based on the prediction of link quality. Each method is worth learning, its common point is to achieve the purpose of reliable transmission and energy efficiency by power control. However, the disadvantage of [PERSON] et al. [28] is that the selected relay node is always used until the energy of the node is lower than a certain value or the channel quality deteriorates, which inevitably leads to the early death of the node and affect the lifetime of the whole network. Based on the above analysis, AAT protocol is more balanced in reliability, delay and efficiency, and is more perfect if data priority is taken into account.  
Cross-layer method plays an important role in the overall optimization of network. WBAN needs not only the superiority of a single performance, but also the imperfection of all performance, it can achieve ideal results by trade-offs among multiple performances.  
\begin{table}
\begin{tabular}{c c c c c c c} \hline \hline
**Protocol** & **Cross Layer** & **Characteristic** & **Priority** & **Reliability** & **Delay** & **Energy Efficiency** \\ \hline PCLRP & MAC and & Slot partitioning and & & & & \\
2016 & network & routing customization & \(\surd\) & medium & high & medium \\ \hline \multirow{2}{*}{CLDO (2017)} & PHY, MAC & Finding the best & & & & \\  & and network & Power, relay and & & & & \\  & & packet size & & & & \\ \hline \multirow{2}{*}{CLRS (2018)} & PHY and & Improvement of & & & & \\  & MAC & retransmitting & & & & \\  & & superframe & & & & \\ \hline \multirow{3}{*}{[28] (2018)} & PHY, MAC & & Link quality & & & \\  & PHY, MAC & prediction and & & & & \\  & and network & adaptive power & & & & \\  & & control & & & & \\ \hline \multirow{3}{*}{AAT (2018)} & PHY, MAC & & Channel state & & & \\  & PHY, MAC & estimation and & & & & \\  & and network & adaptive power & & & & \\  & & control & & & & \\ \hline \hline \end{tabular}
\end{table}
Table 4: Comparison of Cross-Layer Routing Protocols.  
Figure 11: Superframe structure of the ATT protocol.