# An Investigation of CTP Use for Wireless Structural Health Monitoring of Infrastructure
## 1 Introduction
The main reason for selecting the IEEE 802.15.4 technology is its low power, low cost, and the fact that they are battery-based, which can last for a very long time. Energy efficiency in the network is of utmost importance and the main motivation is to enhance the QoS. Often direct communication is prohibited due to problems in the connection; hence, using relay nodes to forward the data in a cooperative manner might be better. This standard along with the Zigbee enhancement is operating in the Indriya testbed nodes. Moreover, the example application provided in Contiki, allows the operation of CTP with the IEEE 802.15.4 and Zigbee. The Indriya testbed contains telosB nodes that operate with the CC2420 communication protocol, which works with the IEEE 802.15.4 standard. Hence, it is available for these devices and the application simply evaluates the best parent of each node to transmit data to, using the ETX as a metric. Cooperative communication is the solution to promote energy efficiency of a network, as well as maximise its efficiency [36]. The reader can see an example of cooperative  
Figure 1: (**a**) Wireless node components. (**b**) Cooperative communication example. communication in Figure 1b, where the yellow line represents an expensive transmission with respect to a link-layer metric. The multi-hop nature of the WSN transmissions allows the utilisation of middle nodes to act as relays (Node B), when the problem of packet reception occurs, which drains the battery of the nodes with retransmissions and packet drops. In fact, the main idea behind communication is relaying, at each hop to transmit data from one node to another. Thus, by sharing resources between nodes, the transmission quality is enhanced. As we can see in the survey [37] and references therein, cooperative communication has been included in various layers of the wireless stack. In particular, there is a plethora of works that target cooperative communication in the physical and Medium Access Control (MAC) layers. Here, we are interested more in the upper layers (network layer) and especially cross-layer communication that targets them, in order to increase the performance of a network. Cooperative routing takes advantage of the cooperative transmission of the physical layer. It is a cross-layer approach that combines the physical and the network layers to transmit packets through establishing cooperative links. This cross-layer design approach makes the performance of the routing protocols in wireless networks better. The main objective of cooperative routing is the efficiency in optimising specific Quality of Service (QoS) or energy efficiency metrics. Cooperative routing algorithms have been proposed to target energy efficiency, throughput, packet delivery ratio, outage probability, and collisions of the wireless medium, as we can see in [37] and its references. Cooperative approaches can be found in [38, 39, 40, 41, 42, 43, 44]. In this paper, the CTP is examined, in order to indicate whether it can be cooperative by nature. The CTP tests are initial in the form of an intuitive cooperative example that is left for future work. The objective was to show an extension of CTP, which we believe is of vast importance. The CTP-like protocol is provided in the examples of Contiki and the wireless nodes support the IEEE 802.15.4 standard and the Zigbee enhancement. Note that the CTP is one of the robust protocols in throughput and data transmission [15], since it enables retransmission of the packets and it takes into account the link symmetry, which is the packet reception ratio and the packet acknowledgment ratio. In this way, bidirectional communication is ensured. The retransmissions alleviate the performance since the packet is not dropped with the first loss or non-transmission of acknowledgment. We exploit its parent selection and forwarding mechanism to show that it promotes cooperative communication with a few adjustments.