## 5 End-to-End Reliable Transmission Mechanisms for HCWSNs
### Network Coding Techniques for Improved Reliability  
Efforts from the literature to use network coding as a means to improve reliability in wireless networks are briefly reviewed in this subsection. Application of the network coding concept to WSN is relatively new, hence, not much work currently exist in the literature. A network coding based protocol from the literature that is specifically designed for WSNs is AdapCode [33]. AdapCode is a reliable data dissemination protocol that uses adaptive network coding to reduce broadcast traffic in the process of code distribution updates in WSNs. Specifically, AdapCode achieves high reliability by adaptively changing the network coding scheme according to perceived link quality. The analysis in [33] shows that the capability for and the process of network coding for a WSN protocol is not too resource taxing or memory draining for WSN nodes because network coding only imposes a 3 byte header overhead, linearly independent packets are easily retrieved, and Gaussian elimination only  
\begin{table}
\begin{tabular}{|p{56.9 pt}|p{56.9 pt}|p{56.9 pt}|p{56.9 pt}|} \hline
**Criteria** & **PSFQ [34]** & **GARUDA [35]** & **ART [36]** \\ \hline Reliability (in terms of PDR) & (+) PDR increases with increasing hops at high error rates (30\%) & (+) Targets 100\% reliability within a sub region, to cover the sensing field, and to a probabilistic subset & (+) Targets 100\% query and event reliability using end-to-end ACK/NACK \\ Energy Efficiency & (+) Conserves power by using hop error recovery & (+) Avoids NACK implosion and high energy consumption by invoking WFP pulses & (+) Balances available energy amongst nodes using energy-aware node classification algorithm \\ Timeliness (Latency) & (+) Fetch Quickly scheme allows for acceptable latency when error rate is very high (70\%) & (+) Uses a locally designated server and out-of-sequence forwarding to reduce latency & (+) Reduced delay due to classified E-Nodes minimizing amount of data sent and using event-based reliability to avoid ACK implosion \\  & (–) Pump Slowly scheme degrades latency performance when error rate is around 30\% & (+) Scalable with respect to network size, packet loss rate, reliability semantics, and message characteristics & (+) Effectively manages increased node density since only E-nodes are used to address congestion \\ Scalability & (+) Aggregation of error, minimum retransmissions, and transmissions requests allows network to handle more data packets & (+) Scalable with respect to network size, packet loss rate, reliability semantics, and message characteristics & (+) Effectively manages increased node density since only E-nodes are used to address congestion \\ \hline \end{tabular}
\end{table}
Table 4: Summary of reliable transport protocols for suitability in HCWSN.  
requires 1 kilobyte of memory. Unfortunately, AdapCode may not be fully suitable for the health care applications considered in this paper because it does not support multicast and also does not adapt to network dynamics induced by mobility. Nevertheless, AdapCode has shown that the application of network coding in WSNs can be beneficial towards improved performance in HCWSNs. Thus, future integration of network coding into a multicast routing protocol should be explored.