Take a few minutes and watch this very well put together video of getting going with Winlink Express and using a software modem called WinMOR, which will work over any soundcard system with HF, e.g. SignalinkUSB or built in soundcard like some modern ICOM radios use.
https://www.youtube.com/watch?v=WRv9DbqnnDg
Results of the experiments with different protocols by Pablo VA3HDL and me. We used P2P mode to send messages to each other. Neither of us had a VARA software licence which provides much higher speed transmission. The licence is $69 US. VA3HDL used his FT991A while VE3NRT used an Elecraft K3.
AX.25 Packet, using Direwolf (VE3NRT) and Sound Modem (VA3HDL).
Worked well at 1200bps on both 2m FM and 10m SSB. We could not get it working at 9600bps. VA3HDL has an FT991A which has a USB interface without serial port PTT. While the slow speed version of Sound Modem has CAT control, the high speed version doesn't. Direwolf, which supports both low speed and high speed modes in 1 program, does not have CAT control on Windows, although it does on LInux. VE3NRT was able to transmit 9600 bps messages using Direwolf.
When VA3HDL put his 2m antenna on his tower he was able to send messages into the Winlink system through the gateway run by VA3MCT.
VARA-HF
Tried on 80M SSB. Worked fine. VE3NRT tried the mode for standard Winlink messages through several gateways up to 1000 km away. The "slow" mode in VARA send a short message with all headers in about 2 minutes.
VARA-FM
Tried on 2m which worked well and similar to AX25 1200bps packet.
ARDOP
This mode, designed for HF, was tried on 80M and worked at about the same speed as VARA-HF.
Conclusions
All the above modes, with the exception of Direwolf, are quite easy to set up. Sound Modem is favoured by the Winlink community although Linux users may prefer to use Direwolf as it works on Linux and is also open source, unlke the others. ARDOP is also open source but the performance is not fantastic.
VARA is the highest performance software modem on both FM and HF but requires a USD 69 licence. It is, however, very popular and there are lots of gateways available. Pactor 4 is higher performance but is more then 10x the price of the VARA licence.
Further testing will require VE3NRT to erect a 2m antenna where it can be heard at a distance. This will hopefully be done soon.
Chris VE3NRT