The following document describes the different types of frame used in AX25 level 2, and their meanings:
With MONITOR ON, and MCOM ON a typical frame display might be thus:
G8AAA>G9ZZZ <frame-type [(C/R) (P/F)] [Sn] [Rn] > : : : : : : : : : : : : : Receive sequence no. : : : : : Send sequence no. : : : : Poll/final bit. : : : Command/Response bit. : : Frame type code. : Destination callsign. Originating callsign.
Apart from the frame type code, not all fields are always displayed.
Example: G8PZT>G8UZV <I C S1 R7> so see you later, 73.
For a detailed description of the Command/Response and Poll/Final bits, the reader is referred to the AX25 Link Layer Protocol Manual, available from RSGB. They are beyond the scope of this guide, and will be ignored in the following description.
There are 2 basic types of frame:
Supervisory frames fall into the following categories:
| Mnemonic | Meaning | Description |
| SABM (C) | "Set Asynchronous Balanced Mode" | This frame initiates a connection between two un-connected stations. It is often called a "connect request" and displayed as <C> |
| DISC (D) | "Disconnect Request" | Initiates a disconnection. |
| UA | "Un-numbered Acknowledgement" | Acknowledges a <SABM> or <DISC> frame, and confirms the establishment or the severance of a connection. |
| DM | "Disconnected Mode" | Sent in response to a <SABM> frame if for some reason the TNC cannot accept a connection (e.g. CONOK OFF). Generates a "G9ZZZ Busy" message on the originating terminal. Also sent in response to any frame other than <SABM> when the TNC is not in a connected state. |
| RR | "Receive Ready" | Probably the most common supervisory frame. It is followed by a digit between 0 and 7 called the "Receive sequence number", N(r), and it's function is to acknowledge receipt of all I frames up to, but not including, N(r). It thus indicates the sequence number of the next expected I frame. |
| RNR | "Receiver Not Ready" | Indicates to the sender that the receiving TNC is temporarily unable to accept further I frames. This frame also has a N(r), such that all frames up to, but not including N(r) are acknowledged. |
| REJ | "Reject" | Also followed by a receive sequence no. This frame is used to request re-transmission of frames starting at N(r), if, for example, one frame of a multi-frame transmission is received garbled. |
| FRMR | "Frame Reject" | Sent when an abnormal condition occurs, and indicates that the TNC cannot successfully process the frame, even if it were re-sent. An information field is added to the frame, giving details of the error. |
There are 2 types of information frame, Numbered and Un-numbered: | ||
| I | "Information" (numbered) | This is the type of frame that carries the actual information in a QSO. It contains both send and receive sequence numbers ( N(s) and N(r) ), which ensure that the user data arrives in the correct order. These sequence numbers are always in the range 0 to 7,and increment by one with each frame sent (N(s)) or received (N(r)), rolling over to 0 again after 7. As there can never be more than 7 frames in a single transmission, there is no possibility of frames being lost. |
| UI | "Un-numbered Information" | As the name implies, these frames have no sequence numbers, and are only used in un-connected (Unproto) mode. They are used to send beacons and CQ calls, and are not acknowledged. |
So a typical short QSO might look like this:
| G8PZT>G8UZV <C> | (G8PZT requests connection to G8UZV) |
| G8UZV>G8PZT <UA> | (G8UZV responds, stations are connected) |
| G8PZT>G8UZV <I S0 R0> Hello Jenny... | (First information frame to G8UZV) |
| G8UZV>G8PZT <RR1> | (G8UZV acks frame 0 and awaits frame 1) |
| (a short while later) | |
| G8UZV>G8PZT <I S0 R1> Sorry, can't stop at the moment. | (then sends her response) |
| G8PZT>G8UZV <RR1> | (G8PZT acks frame 0 and awaits frame 1) |
| G8PZT>G8UZV <I S1 R1> OK Jenny, call you later, 73 | (G8PZT sends second info frame) |
| G8UZV>G8PZT <RR2> | (which is acked by G8UZV) |
| G8PZT>G8UZV <D> | (G8PZT requests disconnection) |
| G8UZV>G8PZT <UA> | (G8UZV acks request. Connection severed) |
G8PZT, Fri 27 Sep 1991