Introduction
    AGWPE Overview
    More about AGWPE
1. Interface
    Getting Started
    Kits and Pre-assembled
    Receive Audio Cable
    Transmit Audio Cable
    PTT (TX Control) Cable
    2 Radio Modification
2. AGWPE Set Up
    Download and Install
    Basic AGWPE Setup
    2 Radio Setup
    2 Card Setup
3. Sound Card Setup
    Basic Settings
    Additional Settings
    Tuning Aid
4. Windows™ Setup
    TCP/IP Settings
    Update Windows
5. Problems?
    Program Behavior
    Receiving
    Transmitting
    Connections
6. Using AGWPE
    AGWPE on a Network
    Baud Rates & Modes
    Remote Control
    TCP/IP Over Radio
    Tips and Tricks
    Traffic Parameters
7. Compatible Programs:
    Setup Help
    UI-View
    WinAPRS
    Winpack
    Others
  
   

Deciphering Packet Headers

AGWPE follows the AX.25 protocol that is the standard for ham radio packet, however, AGWPE does not use the standard TNC2 packet frame display status protocol, but a more comprehensive custom protocol developed by George SV2AGW.

If your terminal program (e.g. AGWterm) is set to display packet frame headers, you will see these status indicators and can follow what AGWPE is doing and how it is interacting with another station. This can sometimes be helpful in diagnosing connection problems.

Listed below are the explanations for each type of packet frame header.

Note that within the packets, there may be these additional indicators:

PID is the "protocol identifier". PID=F0 means the frame contains text -- "F0". If the frame contained TCP/IP data, the ID would be "CC". Other identifiers are used for NET/ROM , Flexnet, Apple Talk, etc.

P and F mean Poll and Final. The P and F "bits" are used to indicate when a reply or ack (acknowledgement) is desired. For example, when the sending station includes a P bit in the packet header, it means "respond immediately to this packet". If the P bit is missing, the destination station will wait for additional packets until one appears with the P bit and then acknowledge all of the packets at once. The sending station controls how many packets AGWPE will send without a P bit by setting the MAXframe parameter. For example, if MAXFrame is set to 5, four (4) packets will be sent without the P bit and the fifth will contain the P bit.  The destination station will respond to the fifth packet with an ack that includes the F bit to indicate the last number of the packet frame sequence which is received.

Note that all packets frames sent are identified by number, the first number being 0 (zero) and the last being 7. After 7, the numbering sequence restarts at 0 (zero). The numbering system is used to track packets, so that each receiving station can compare how many packets were sent to it and how many it has received. If a packet is missing, the receiving station will request a re-transmission of numbered packets.

Examples of packet frame headers:

<UI pid=F0 Len=32> - an unconnected frame (e.g. beacon) sent to no station in particular but to everyone, i.e. . The PID is "F0" since it contains simple ASCII text. (UI frames are used in APRS.) Len=32 means the packet contains 32 characters.


<SABM P> - a request from station A to station B for a connection. The P bit is used to request an immediate reply. (Set Asynchronous Balanced Mode)


<UA F > - the reply from station B to station A accepting the connection request. It is also used to accept a disconnection <DISC> request.  The F is used to tell station A that all packets were received successfully. (If station B could not accept the request, it would send a DM frame;  see below.)


<DISC P> - a request from either station to disconnect from the other. 


<DM F > - a refusal to accept the original connection request, perhaps because another connection in in progress.


<I P R3 S0 pid=F0 Len=28 > - this in an information ( I ) packet containing data (text, etc). The P requests an immediate acknowledgment. If the P were absent, then the receiving station would delay its acknowledgement until it received a frame with a P in it.  R3 indicates that the station last received the other station's packet #2 and is ready to receive  #3. S0 indicates that this packet being sent is # 0 (zero). Len=28 means the packet contains 28 characters.


<RR P/F R1 >  - this is a "ready to receive' (RR) packet simply acknowledging receipt of packet #0 and ready to receiver #1.


<REJ P/F R1 > - this is  reject (REJ) packet meaning that the packet just received was out of sequence or a duplicate; ready to receive packet #1 instead.

 [The REJ can also be sent by a TNC to indicate its buffer is full and it is not ready to receive (NRN)]. In addition, some  AX.25 implementations, e.g. BPQ-DOS node, use a REJ packet instead of P;  immediately after the last packet in a sequence (maxframe), a REJ is also sent to force an acknowledgement by the receiving station.

Another frame that is rarely seen is a FRMR (Frame Reject). It would be sent if the packet received had an invalid control field, an illegal data field, a data field that was too long, or other problem. In general, you will not see such a frame unless you are writing your own AX.25 code and testing it.