It is possible to configure the following settings:
1] Baud rate: 9600 bps | Channel spacing: 12.5 kHz
2] Baud rate: 19200 bps | Channel Spacing: 25 kHz
Option 1 got a better sensivity of 3 dB. In theory, the distance of the 12.5 kHz setting is 1.43 greater than the 25 kHz setting. With 12.5 kHz bandwidth, a distance of 15 km can be achieved in line of sight.
Over the air, the sent data get compressed. On the serial line 10 bits are used on one character (start bit, 8 data bits, stop bit), but over the air, only 8 data bits are sent.
In transmission, serial 38400 bps is faster than 19200 bps via air, but 8 data bits are sent, so the ratio of serial and air speed is smaller. The rest of the bits, are written to the transmit buffer.
If the packet is very large, there is a risk that the buffer will overflow, which must use the handshake RTS flow control.
In the receive side modem, you get the data flow from the radio, which is slower than the serial port. The receiver tries to keep the data flow constant, but eventually a data break appears in the data flow.
This happens more easily when FEC is on (radio slower). To avoid this, the CRC16 error check keeps the packets constant.