Hi Daniel,
I am not entirely familiar with the X-25 protocol. There is an add-on module to LabVIEW called the Modulation Toolkit and this is what is generally used to modulate and demodulate data, For more information about the Modulation Toolkit, click
here. If you can give some more information about the x25 protocol and your application we might be able to figure something out. Please do take a look at the modulation toolkit, it does include many of the popular modulation schemes such as AM,FM,ASK,QPSK,QAM etc.
Regards
Malay