## 5 End-to-End Reliable Transmission Mechanisms for HCWSNs
### Reliable Transport Layer Protocols
These event messages are typically the first messages that are sent out by an E-Node sensor when a new event is detected (e.g., a new value is sensed). When the sink node receives the event message with the event notification bit set, it responds with an ACK to the E-Node. If the ACK is not received by the E-Node within a certain timeout period, the distributed congestion control algorithm is used to send _congestion alarm_ messages to non-essential nodes. The congestion alarm messages provide instruction that the traffic of N-Nodes should be reduced (_i.e._, temporarily stop the sending of sensed data). When an ACK is finally received by the E-Node, a _congestion safe_ message is broadcast to announce that normal operation of the network can resume. The simulation results have shown that under a 100% reliable PDR objective, the traffic load in the network can be drastically reduced [36]. Furthermore, ART performs better than other message-level reliability schemes in terms of latency and packet loss [36]. The performance evaluations for PSFQ, GARUDA, and ART have used different experiments to measure the effectiveness of the reliable transport protocols [34, 35, 36]. Table 4 provides a summary of techniques used the aforementioned protocols to address reliability, energy efficiency, latency, and scalability. In PSFQ, PDR and latency are measured as a function of the network size (number of hops). PSFQ achieves 100% delivery up to ten hops away from the source node when the error rate is 50%, and more than 90% delivery for paths 13 or more hops away [34]. As the number of hops across the network increases from 1 to 5, the average latency increases linearly from 175 ms to approximately 325 ms [34]. In the experiments for GARUDA, latency and energy consumption are measured as a function of node density (_i.e._, scalability experiment). For multiple-packet delivery, the latency and energy consumption scale well with an increase in node density. For example, as the node density increases from 200 to 800, the latency only increases from 5 s to 13 s and the energy consumption increases from 5 J to 9 J [35]. In ART, simulation results are provided for network lifetime under light and heavy traffic conditions with varying sensor node update intervals. For a network of 100 nodes that periodically update every 30 s, the network lifetime is approximately 225 s, irrespective of the traffic level [35]. Obviously, for a prototype HCWSN, this value is not favorable because the first battery operated sensor will have to be replaced in less than 4 min. However, simulating ART under different conditions may reveal an extended life time. The simulation results for ART also show that under heavy traffic load conditions where the node density is 200, the end-to-end delay remains less than 5 s and the PDR is greater than 90% [36]. The solid performance of ART is due to the advantage gained by having designated E-Nodes deal with retransmissions that reduces the amount of data in the network. In HCWSNs, it is imperative to provide reliable data transfer both upstream and downstream. The ability for a doctor to query a patient is equally as important as a patient's ability to have his or her measurements sent to the doctor or a processing server. Therefore, a reliable transport protocol that features an ability to ensure reliable upstream and downstream data transfer while also supporting multicast communication is most ideal for integration in a HCWSN. While GARUDA and PSFQsupport multicast communication, they are limited in the context of supporting HCWSNs because reliability is only provided downstream. On the other hand, ART also supports multicast (and unicast) transmissions while providing reliability for upstream events and downstream queries. The results of the performance evaluation show that the distributed congestion control mechanism effectively mitigates congestion in the network. Furthermore, when the network node density is less than 100, the end-to-end delay and PDR are at an acceptable level (<5 s and >90%, respectively [21]).