Model, simulate and analyze application throughput and latencies in a tactical vehicular radio network comprising of multiple vehicles each transmitting situational awareness, video and data files.
Case 1: One vehicle transmitting:
Case2: Two vehicles transmitting:
Throughput vs time plot (FTP Application)
Throughput vs time plot (Application - FTP_1)
Throughput vs time plot (Application - FTP_2)
Throughput vs time plot (Video Application)
Throughput vs time plot (Application - Video_1)
Throughput vs time plot (Application - Video_2)
Throughput vs Time Plot (Link)
Throughput vs Time Plot (Link)
The start time for FILE_RECEE is 5 s. We see in the Throughput vs Time plot for FILE_RECEE application, that at the 5th second, the file transmission starts and gets a throughput of approximately 1.1 Mbps. The file size is 562500 B or 4.5 Megabits, and with a throughput of 1.1 Mbps it takes about 4.5s for this file to be transferred. Therefore, the throughput drops at around 9.5 s since the file transfer is completed by then. The file inter arrival time is 20s, and this means the next (2nd) file is generated and transmitted at 25th second (the first file starts at 5s and to this add the inter-file time of 20s). At the 25th second again the file transmission starts, takes around 4.5s for transmission, and is completed around 29.5s. The same cycle continues.
Now let us turn to the Throughput vs Time Plot for the Video application. This application starts at 0 s and sees a throughput of about 600 Kbps, which is equal to the generation rate. This means the network has sufficient capacity to transmit the video. Note that when the FTP application starts at 5 s, it gets priority for transmission and the video throughput drops to 0. The FTP application transmission is completed at 10s. At this point the video transmission resumes. The throughput goes up to about 1.2 Mbps since the buffered video frames are being sent. At around 18th second all the buffered frames have been sent and the video rate drops back to 600 Kbps which is equal to the generation rate.
The link which has a capacity of 1.5 Mbps initially sees about 600 Kbps of occupancy. Once the FTP application starts, the link occupancy goes up to 1.2 Mbps. After the file is transferred in the FTP application, the link occupancy starts reducing since it is only video that is now being transmitted.
We compare the results of two vehicles transmitting simultaneously (case 2), with a single vehicle transmission (case 1). At time 0s, both vehicles start their video transmissions. In case 1 the approximate video transmission throughput was 600 Kbps. In case 2, each video gets a video transmission throughput of 600 Kbps initially, or a total system throughput of 1.2 Kbps. This is possible since the Phy capacity of the link is 1.5 Mbps.
Now the FILE_RECEE_1 starts at the 5th second, while the FILE_RECEE_2 starts at the 10th second. Each of these applications send a file of size 4.5 Megabits every 20s which equals an average FTP generation rate of 225 Kbps. As soon as FILE_RECEE_1 starts in Vehicle 1, it is given priority over the video transmission from the same vehicle. Therefore, the throughput of Video_1 drops immediately. Note that in case 1, FTP application saw an instantaneous throughput of about 1.1 Mbps whereas in case 2, it is only able to get a throughput of ~ 710 Kbps. This is because some of the slots on the link is being allocated to Video_2 application. Therefore the 4.5 Mbit file takes ~ 6.3s to transfer, as opposed to 4.5s in case 1. This in turn means that the video_1 application will start getting transmitted (after being buffered) only from around 11.3 s (5s plus 6.3s). In case 1 the video started getting transmitted from 9.5s itself.
Next, let us study the application throughputs for the transmissions from the second vehicle. Similar to the first vehicle video starts at 0s and sees a throughput of 600 Kbps till the 5th second. At this time FTP_1 starts, and this leads to a slight drop in Video_2, since the DTDMA protocol starts assigning more slots to FTP_1 application. FTP_2 starts from the 10th second. At this point in time, FTP_2 gets higher priority over Video_2 (since both are being transmitted from the same vehicle). Therefore, the throughput of video_2 drops to zero. Similar to FTP_1, FTP_2 also see a throughput of ~ 710 Kbps, and hence it also takes ~ 6.3s to transfer each file. Once this file is transmitted, Video_2 again starts seeing a throughput of ~ 750 Kbps.
The Video application throughput of 750 Mbps in between the file transfers. This is because of video packets getting buffered. Video is being continuously generated at 600 Kbps, but there is packet buffering during the time when the FTP application is being sent. This cycle repeats.
As compared to case 1, where the link was not fully occupied, in case 2, we see the link if fully utilized and the link throughput is at 1.5 Mbps. In fact, the combined generation rate of the 2 FTP and 2 Video application is greater than 1.5 Mbps. It is for this reason that the situational awareness application sees a latency of 9.3 s as against 8.78 ms!