Some satellites use AX.25, some use special packet protocols developed for satellite communications. A few transmit AX.25 packets over FM transmitters, but most use SSB transmissions. While receiving packets, the TNC automatically decodes, checks for errors, and displays the received messages. In addition, any packet TNC can be used a packet relay station, sometimes called a digipeater. This allows for greater range by stringing several packet stations together.
Packet radio provides error free communications because of built in error detection schemes.
If a packet is received, it is checked for errors and will be displayed only if it is correct. This means that you can leave your packet station on constantly. Other users can connect to you at any time they wish to see if you are home. Another advantage of packet over other modes is the ability for many users to be able to simultaneously use the same frequency channel simultaneously. A TNC contains a modem to decode the audio signals into digital signals.