Video and UDP Workflows

This demonstration sends video from BS to UE through OpenISAC. ffmpeg reads and encodes the video file, the BS and UE programs carry the data over the radio link, and ffplay receives it and plays the picture and sound.

Data path

1. On the BS computer, ffmpeg sends video data to local UDP port 50000, where the BS program receives it.
2. BS transmits through its USRP. The UE program receives the signal through the other USRP and recovers the video data.
3. On the UE computer, the UE program sends that data to local UDP port 50001. ffplay listens on this port and plays the video.

These ports are examples. Check network_output.udp_input_port in BS.yaml and network_output.udp_output_port in UE.yaml. When the player runs on the UE computer, set network_output.udp_output_ip in UE.yaml to 127.0.0.1.

1. Start BS and UE

Follow First OTA Run to start both backends. Confirm that UE has synchronized with BS before sending video.

The commands below use the settings from a 100 MS/s X310 test: a 60 Mb/s video target and a 70 Mb/s UDP sending limit. For a first test or a configuration with a lower sample rate, start by changing all four 60M values to 3M and bitrate=70000000 to bitrate=4000000. Increase them gradually after playback works.

2. Send video on the BS computer

Open another terminal on the BS computer. Put video.mp4 in the current directory, or replace video.mp4 in the command below with the actual video file path.

If the BS computer supports NVIDIA NVENC hardware encoding, run:

ffmpeg -re -stream_loop -1 -i video.mp4 \
  -map 0:v:0 -map '0:a:0?' \
  -c:v h264_nvenc -preset p4 -tune ll -rc cbr \
  -b:v 60M -minrate:v 60M -maxrate:v 60M -bufsize:v 60M \
  -g 30 -bf 0 -pix_fmt yuv420p -c:a aac -b:a 128k \
  -f mpegts -mpegts_flags +resend_headers -muxdelay 0 -muxpreload 0 \
  "udp://127.0.0.1:50000?pkt_size=1316&bitrate=70000000&burst_bits=10528&buffer_size=4194304"

The command loops the file, packages the video and any audio as MPEG-TS, and sends the data over UDP to the local BS program. 127.0.0.1 means this computer; 50000 is the BS video-data input port.

For CPU software encoding, replace -c:v h264_nvenc -preset p4 -tune ll -rc cbr with -c:v libx264 -preset veryfast -tune zerolatency. Check the speed reported by ffmpeg: encoding should sustain 1x.

3. Play video on the UE computer

Open another terminal on the UE computer and run:

ffplay -fflags nobuffer -flags low_delay -framedrop -sync ext \
  "udp://127.0.0.1:50001?fifo_size=65536&overrun_nonfatal=1&buffer_size=16777216"

ffplay receives the video data sent by the UE program to local port 50001, decodes the compressed video into pictures, and plays the audio. Keep ffmpeg on BS, both backends, and ffplay on UE running.

If the player runs on another computer, set network_output.udp_output_ip in UE.yaml to that computer’s IP. On the player computer, run the same ffplay command with 127.0.0.1 replaced by 0.0.0.0 to listen on all local network interfaces. Match the port to network_output.udp_output_port as well.

Why limit UDP bursts too?

After encoding a frame, ffmpeg may send many UDP packets together. Large keyframes make this more noticeable. These short bursts can put pressure on backend queues even when the average rate is moderate.

Set both bitrate and burst_bits in the output URL to spread packet transmissions over time. Quote the URL so the shell passes the parameters after & to ffmpeg too.

Setting	What it controls
-b:v 60M	A 60 Mb/s target for video encoding. Audio and packaging need additional bandwidth.
-minrate:v 60M, -maxrate:v 60M	The video rate range used by the encoder’s rate control.
-bufsize:v 60M	A 60 Mbit encoder rate-control buffer.
bitrate=70000000	UDP sending paced at up to about 70 Mb/s; output is lower when less data is available.
pkt_size=1316	Up to 1316 data bytes per UDP packet: seven 188-byte MPEG-TS packets.
burst_bits=10528	About one UDP packet’s worth of data per burst: 1316 × 8 = 10528 bits. Adjust it with packet size.
buffer_size=4194304	A request for a 4 MiB UDP send buffer, subject to OS settings.

Leave room for video, audio, and packaging when choosing the UDP limit, while keeping it within the radio link’s sustainable data throughput. Too low a limit causes backlog; too high a limit allows larger bursts. Validate the example’s 60/70 Mb/s settings with your own device and configuration.

-re reads the file at playback speed, the encoder options control how much video data is produced, and the UDP URL controls how it is sent. Use them together. A 100 MS/s RF sample rate is only one factor in video capacity; modulation, LDPC, CP, pilots, and frame overhead also matter.

Check problems in transmission order

1. Video sender: confirm that the file opens, ffmpeg reports no errors, and speed stays at 1x.
2. Radio link: confirm continuous UE synchronization and check header validation, LDPC, and RX overflow logs.
3. UDP transfer: check addresses and ports, packet-drop counters, and queue buildup. Calculate received throughput as received bytes × 8 ÷ elapsed time.
4. Player: inspect video decode errors, player frame drops, and audio/video time difference separately. -framedrop lets the player skip frames that cannot be displayed in time to reduce playback delay; that count differs from radio packet loss.

For complete option definitions, see the FFmpeg UDP protocol documentation.