## 4 The WBANs Communication Stack
### Routing Protocols
#### 4.3.4 Mission-Critical Routing Protocols  
Mission-critical applications over wireless sensor networks (WSNs) need to support fast, reliable, and fault tolerance in its routing protocol [99]. In this context, the Secure, Fast Rebuilding, and Energy Efficient (SFREE) Routing Protocol for Mission-Critical Application over WSNs is presented in [99]. The presented protocol is compared to LEACH and Hierarchical Periodic, Event-driven and Query-based (HPEQ) protocols, and it is reported that the proposed protocol provides reliability with reducing processing time and energy dissipation through cluster based authentication mechanism and delayed propagation of management messages. It is shown through simulations that (10 to 15) % of overall energy dissipation is reduced along with 40% reduction of cluster rebuilding time. However, the protocol is designed for static topology and the results will deviate under network mobility. Tree structures can also used in mission critical applications, for example, the authors in [100] propose a paper on minimizing end-to-end delay using a multipath routing (MEEDMR) protocol. The multiple paths and alternate paths are discovered by using tree based search algorithms from source to \begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline \multirow{2}{*}{**Protocols**} & \multirow{2}{*}{**Objective**} & \multicolumn{6}{c}{**Performance Metrics**} \\ \cline{3-8}  & & **Network** & **Network** & **Network** & **Security** & **Delay** & **PDR** & \begin{tabular}{c} **Energy** \\ **Consumption** \\ \end{tabular} \\ \hline \multirow{2}{*}{HIT [97]} & \multirow{2}{*}{\begin{tabular}{c} To maximize lifetime \\ by reducing direct \\ transmission to sink \\ \end{tabular} } & \multirow{2}{*}{Large} & \multirow{2}{*}{High} & \multirow{2}{*}{Yes} & \multirow{2}{*}{Low} & \multirow{2}{*}{Medium} & \multirow{2}{*}{Low} \\ \cline{1-1} \cline{6-6}  & & & & & & & \\ \hline \multirow{2}{*}{\begin{tabular}{c} AnyBody \\ [94] \\ \end{tabular} } & \multirow{2}{*}{\begin{tabular}{c} To achieve high packet \\ delivery ratio and low \\ delay \\ \end{tabular} } & \multirow{2}{*}{Large} & \multirow{2}{*}{High} & \multirow{2}{*}{No} & \multirow{2}{*}{N/A} & \multirow{2}{*}{\
\begin{tabular}{c} Very \\ High \\ \end{tabular} } & \multirow{2}{*}{High} \\ \hline PEGASIS &
\begin{tabular}{c} To reduce the energy \\ consumption in \\ data-gathering networks \\ \end{tabular} & \multirow{2}{*}{High} & \multirow{2}{*}{No} & \multirow{2}{*}{N/A} & \multirow{2}{*}{Medium} & \multirow{2}{*}{Low} \\ \cline{1-1} \cline{6-6}  & & & & & & \\ \hline \hline \end{tabular}
\end{table} 
Table 12: Comparison of Clustered-based Routing Protocols. destination considering packet loss and packet delay. To provide reliable transmission, the data packets are distributed throughout multiple paths. Whenever the end-to-end reliability falls below a certain value, the route maintenance is performed. In [101] its authors introduced a link-state routing protocol for multi-channel multi-interface wireless networks with an objective to minimize the broadcast overheads. For this purpose cluster-heads have been used and the proposed multi-channel link-state routing (MCLSR) protocol is implemented on a test bed which is based on Linux 2.4.6 Kernel, where two radio interfaces, are embedded in the kernel, as kernel modules. Authors have compared performance of their proposed protocol with an ad-hoc on demand distance vector routing reactive protocol [102], which is also tailored for multi-channel multi interface networks. The measurements on our test bed show that the proposed link-state routing protocol provides transient route discovery time and lower packet drop rate, which is important in mission-critical networking  
The authors in [103] specifically focus on interoperability between heterogeneous networks. They address the routing problem in scenarios where nodes that belong to one organization (or network) could transport traffic to nodes from another organization (or network).