# Wireless Remote Control for Underwater Vehicles
## 5 Simulation Results
### Video and Images: Traffic Shaping and Video De-Jittering Buffer  
Packet delay variation (PDV) strongly affects multimedia streams, causing clumping and dispersion. A large PDV is caused by both the fact that a video stream is composed of frames with different size and the presence of other types of traffic injected into the network. In our case, on the one hand, the acoustic HF modem is required to transmit a VLQ video that requires a variable  
Figure 10: Root mean square error (RMSE).  
bit rate and, on the other hand, the optical channel is required to service 2 video streams and image transmissions at the same time. Figure 11 presents the difference between the transmitted (grey line) and the received (red line) instantaneous video bit rate of the VLQ stream: thanks to the fact that the acoustic HF channel is completely dedicated to the transmission of the VLQ stream, the two bit rates are quite similar, with the exclusion of the maximum bit rate peak, that is higher than the maximum throughput achievable with acoustic HF. The average delay of the VLQ stream is 514 ms, while the PDV of this video stream is equal to 112 ms, two orders of magnitude higher than the maximum PDV required for a smooth video stream, that is equal to 3 ms [115]. To mitigate the PDV effect, a de-jittering buffer can be employed at the receiver: indeed, as long as the bandwidth can support the stream and the buffer size is sufficient, buffering only causes a detectable delay before the start of media playback. Specifically, a PDV less than 3 ms can be obtained with a de-jitter buffer that introduces a delay of 1.1 s (Figure 11).  
Figure 12,b presents the difference between the transmitted (grey line) and the received (red line) instantaneous video bit rate of the VHQC and VHQG streams, respectively: it can be observed that, from 31,702 to 31,782 s, there is a drop in the received bit rate: this happens due to the fact that some acquired images are in the process of being transmitted, saturating all the bandwidth of the optical modem. This undesired effect can be mitigated by using traffic shaping, e.g., by limiting the image packets transmission in time, setting a minimum transmission delay between subsequent image traffic, called image delay from here onward, larger than zero. The expectation of performance gain introduced by traffic shaping is confirmed by both the yellow and the green lines, where the image delays have been set equal to 10 ms and 40 ms, respectively. Moreover, in the latter case, the received video bit rate is quite similar to the transmitted bit rate, with the exclusion of the bit rate peaks that are higher than the maximum throughput achievable with the optical modem. Figure 13 shows how the introduction of traffic shaping also reduces the average and the maximum video delay as well as the PDV of both VHQC and VHQG. Indeed, while without traffic shaping the average video delay is 3 s and the PDV is 10 ms, with an image delay of 40 ms the average video delay is less than 100 ms and the PDV is 5 ms for VHQC and 6 ms for VHQG. In order to further lower the PDV, a de-jitter buffer can be employed (Figure 13): to reach the target PDV of 3 ms, a de-jitter buffer that adds a minimum delay of 30 ms can be employed.  
Figure 11: Transmitted and received instantaneous video bit rate calculated on a time window of 1 s (**a**) and packet delay variation (PDV) obtained with different de-jitter buffer configurations (**b**) of the VLQ stream.