Whatīs Packet Radio after all ?
Packet Radio is a digital comunications mode used by hams to send and receive files, texts and messages all over the world, itīs based on the AX25 serial protocol with some minor changes because the original protocol was build based on a full duplex mode, usualy is made in a simplex mode except for sattelite transmissions.
The AX25 is baesd on the Balanced Link Access Procedure ( BLAP ) of the CCITT X.25 standard. BLAP in turn conforms to the HDLC standard.
The most common chip used in VHF comunictions is the old TCM3105, it is a Bell 202 standard modem.