AX.25 2.2 packets are either “commands” or responses“. Commands can have a “P” poll bit set, and responses will echo that back as an “F” final bit.

• SABM (Set Asynchronous Balanced Mode) - Creates a new AX.25 2.0 connection.
• SABME (Set Asynchronous Balanced Mode Extended - Creates a new AX.25 2.2 connection.
• RR (Receive Ready) - I'm ready to receive more data.
• RNR (Receive Not Ready) - don't send me more data right now.
• REJ (REJect) - I'm missing some data, please retransmit everything from this point.
• SREJ (Selective REJect) - I'm missing some data, please retransmit just this packet.
• DISC (DISConnect) - Please disconnect.
• FRMR (FRaMe Reject) - You previous frame was invalid and I didn't like it.
• I (Information) - the actual data.
• UI (Unnumbered Information) - Information sent outside of a connection.
• DM (Disconnect Mode) - You asked me to disconnect, and I did.
• XID (eXchange IDentification) - This is what features I supported.
• TEST (Test) - “ping?”
• UA (Unnumbered Acknowledgement) - I successfully received and accepted your previous packet.

Example flow:

1. M7QQA>M7QQB <SABM C P> - M7QQA would like to open a connection to M7QQB.
2. M7QQB>M7QQA <UA R F> - M7QQB confirms connection to M7QQA.
3. M7QQB>M7QQA <I C P R0 S0> “Welcome to M7QQB.” - M7QQB sends a connection banner to M7QQA.
4. M7QQA>M7QQB <RR R F R1> - M7QQA has received all messages before sequence 1, and is ready to receive more.

1. The first field is the frame type.
2. The second field is “C” for control, or “R” for response.
3. The third field is “P”/“F” for poll/final, or omitted if not set.
4. R# means “I have received all packets before #.”
5. S# means “This is packet sequence number #.”

bpqterminal_frames.pdf

decifering_packet_frame_headers.pdf