## 5 End-to-End Reliable Transmission Mechanisms for HCWSNs
### Reliable Transport Layer Protocols
In this process, the WFP pulse, which is a short period signal that is twice the regular transmission power, is propagated to all nodes in the network to notify them that the first packet is ensuing. Each node continues the WFP pulsing until it has received the first packet. If it does not receive the first packet within a certain time, a NACK is sent to its single hop neighbor to request for retransmission. During the delivery of the first packet, a single packet flood approach is used to dynamically assign a subset of nodes to function as loss recovery servers. These nodes are termed core nodes and their distributed designation represents an approximation of the minimum dominating set (MDS) of the network sub-graph to which reliable message delivery and the recovery of lost packets may be required. When the core nodes have been designated, GARUDA employs a two-stage loss recovery process that first deals with core nodes recovering from lost packets, then non-core nodes recovering from lost packets. The loss recovery process for core nodes is conducted in unison with the default message forwarding process, and all requests and retransmissions for lost packets are performed as unicast transmissions to the nearest upstream core that has the packet stored in its cache. In the loss recovery process for non-core nodes, the non-core nodes wait for an indication from the core nodes that they have received all the packets in a message. When this indication is received, the non-core node first checks its core node table for the core node that has the closest hop distance to it. The non-core node then initiates the recovery process through the nearest code node. With regards to the defined requirements for HCWsNs, GARUDA addresses the need for a reliable transport protocol that supports a multicast paradigm where a sink node can reliably deliver data to a group of sensors belonging to the same multicast group. The performance evaluation shows that GARUDA exhibits significantly lower latencies when compared to other ACK and NACK based transport protocols, and also exhibits significantly better energy consumption efficiency as the number of sent data packets is increased [35]. GARUDA offers amenable features that could well benefit its adoption in a health care based WSN. However, there is no explicit mechanism to account for different mobility, which could result in a degradation of performance. _ART_[36]: The Asymmetric and Reliable Transport (ART) mechanism was proposed to address bidirectional end-to-end reliability in wireless sensor networks. In WSNs, bidirectional reliability relates to reliably transmitting detected event messages to the sink and reliably transmitting query messages to the sensors. To accomplish this feat, ART utilizes an energy-aware classification algorithm that groups sensors into essential nodes (E-Nodes). The essential nodes are chosen to provide sensing coverage of a designated area. The remaining nodes are called non-essential nodes (N-Nodes). The energy-aware classification algorithm is based on a greedy-weighted algorithm that determines the remaining battery power on each node. This information is used to greatly reduce the probability of a low-power node becoming an essential node. As a result, the available energy in the network can be more balanced and thus the network lifetime can be prolonged. To facilitate reliable event and query transfer, ART uses asymmetric lightweight ACK and NACK mechanisms, respectively, between the sink and essential nodes. The queries sent by the sink to the sensors are sequentially ordered. Therefore, if a sensor detects a gap in the received queries or if the queries are received out of order, it is assumed a loss has occurred. To recover the lost message, the sensor sends a NACK to the sink node. For reliable event transfer, the E-Nodes only require an acknowledgement from the sink node for event messages that have an event notification bit set.