# On the Feasibility of Wireless Multimedia Sensor Networks over IEEE 802.15.5 Mesh Topologies
## 5 Performance Evaluation
### Latency  
Latency has been computed as the average time between the generation of data messages and their delivery to the destination node (_i.e._, data sink). This includes the time dedicated by all the intermediate nodes belonging to the source-destination path for accomplishing tasks such as the \begin{table}
\begin{tabular}{c c} \hline \hline
**Parameter** & **Value** \\ \hline Channel Type & Channel/WirelessChannel \\ Radio-propagation model & Propagation/TwoRayGround \\ Physical layer & Phy/WirelessPhy/802\_15\_4 \\ Medium Access Layer (MAC) & Mac/802\_15\_4 \\ Antenna model & Antenna/OmniAntenna \\ Frequency & 2.4 GHz \\ CS Threshold & 2.13643 \(\times\) 10\({}^{-7}\) \\ RX Threshold & 2.13643 \(\times\) 10\({}^{-7}\) \\ Coverage range & 30 m \\ Distance between two consecutive nodes & 25 m \\ Routing protocol & TDLS \\ Energy-saving mechanism & SES: Reservation-based method \\ Simulation time limit & 1500 s (25 min) \\ Traffic & Akiyo, Foreman and Mobile QCIF (176 \(\times\) 144 resolution) \\ Video generation rate/service rate & 53 kbps bitrate/1 packet per _wakeup interval_ \\ \hline \hline \end{tabular}
\end{table} 
Table 6: Simulation parameters. Figure 7: Simulation scenarios. data processing, temporary storage in the node's memory or transmission/reception procedures, among others. The latency for all the packets that have successfully reached the destination is shown in Figure 8. It can be observed that the latency shows a considerable variation for different values of WO. It should be noted that while the generation rate remains constant at 53 kbps, only one packet is transmitted per _wakeup interval_, whose length is defined by this parameter. Consequently, as WO (and subsequently the _wakeup interval_) increases, the capacity of the network to accommodate data traffic decreases and packets are stored in the queue of the first node waiting for transmission, therefore increasing the latency measured. Some simulations were also performed transmitting two packets per _wakeup interval_, but the obtained performance was really poor. For example, for a 6 \(\times\) 6 configuration, an increase in the latency of around 50% was measured. In this case, increasing the amount of data sent led to a higher contention for medium access, which worsened the performance of the network. Under these circumstances, for any given WO-AO configuration, the latency increases linearly with respect to the number of transmitted packets (see Figure 9) because of the packet service rate of the network (one packet per WI). The figure shows the evolution of the latency calculated after the arrival of each packet for a 6 \(\times\) 6 network with a 5-3 (WO-AO) configuration. It has to be noted that latency values of zero in the figure represent packet loss. In Figure 8 there is a case that requires further explanation, specifically for AO = 1, where the latency measured is smaller. However, these low values are obtained because of high contention for medium access and the resulting packet loss. These figures must be analyzed in conjunction with the Message Delivery Ratio figures. For AO = 1, it can be observed that packet loss increases considerably since the _active duration_ is too short to successfully reserve all transmission slots. In particular, as the congestion of the network grows with time, most of the packets that are successfully received correspond to the first packets sent, which have a lower latency. On the contrary, packets sent subsequently, which have a higher latency as shown in Figure 9, have a higher loss probability. Since  
Figure 8: Latency metric for the different scenarios. the displayed latency is averaged over all the successfully received packets, this results in a deceptively short latency. Besides, it can be observed that, for a fixed value of WO, the latency remains almost constant in the interval \(3\leqslant\text{AO}<\text{WO}\). Thus, a further increase of the _active duration_ will not have noticeable effects on the latency.