The telemetry is transmitted by specific electronics hardware on the vehicle. The protocol is binary and kept as short as possible, since longer phrases may interfere with the transmission of the video signal (these both contend for the same link). Shorter phrases also allow higher update rates and with relatively cheap equipment it’s easily possible to achieve rates of 30Hz with around 80 bytes of data per phrase.
The protocol is specified: protocol. This phrase must be transferred using UDP multicast to the configured telemetry IP and port number.