The delay becomes an important metric in the WSN design when a delay sensitive traffic is introduced. In WSNs, observations should be sent in real or near-real time manner as in SHM [2]. As WSN frequently reports data, minimizing delay is a major goal for WSN designer. The delay experienced at the source node is due to the following components: First, the sensing delay is the time taken to finish the data acquisition; second, the processing delay at the transmitter and the receiver sides; third, the transmission delay to deliver the obtained data to the next hop. Fourth, the queuing delay in the receiver side. Finally, the propagation delay is the time that the data uses to travel across the physical media and depends on the characteristics of the link.
For wireless media, the propagation delay depends on the distance between the source and the destination, the refraction index and the propagation speed. However, it is negligible compared to the other components as the distance encountered in WSNs between nodes is frequently short. Here, the transmission and the queuing delay are only considered as the most dominating components in the delay. The intermediate nodes experience delay only when they forward the data to the next hop.