Mavlink mode

By default WFB-NG encapsulates one source UDP packet to one WiFi packet. But mavlink packets are very small (usually less than 100 bytes) and send them in separate packets produces too much overhead. To avoid it use TX with -m mavlink_agg_in_ms switch to enable mavlink mode. It will accumulate UDP packets while size < MAX_PAYLOAD_SIZE and mavlink_agg_in_ms is not expired.