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I am new to the world of amateur radio and I am carrying out a university project concerning the communication between satellites and ground stations. My question is:

What is the packet size of the AX.25 protocol ?? Or, if there is no constant size, what is the minimum or maximum packet size ??

I've already done some research but haven't found anything relevant.

Thanks for the help

Kevin Reid AG6YO
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Federico
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  • http://www.ax25.net hosts the AX.25 specification. There are different AX.25 frames with defined fields, each with specified bit sizes. If that's what you mean by "size". –  Apr 04 '22 at 18:21
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    I'd like to add that unless it's an amateur satellite, raw AX.25 would be rather surprising for a satellite link. It's really not a *good* protocol, at all! It defines a variable-length data link layer protocol (as you noticed!), but the packet format it defines allows no room for forward error correction. That's not a design flaw that can be worked around – it means no error correction capabilities for the data link information, and that means any AX.25 system *can never* come close to the robustness or data rate that would be possible over the same physical link. – Marcus Müller Apr 06 '22 at 09:20
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    So, AX.25 was designed as if the amateur radio channel was noiseless, or by someone who's not read the basics on noisy channel communications, or by someone before Shannon (1948). It's a terrible design, and it's been bogging down technical progress in the amateur radio space for far too long now. (yes, I'm not very fond of it, in case you wonder.) If you take up the immense cost of shooting a satellite to space, you *will* be using a better data link layer protocol. – Marcus Müller Apr 06 '22 at 09:22
  • (fx.25 adds a FEC "appendix", but it's not really great for its purpose, and now you're transmitting – and encoding – a checksum that's way too heavy for something that has FEC. AX.25 is really a standard that took the X.25 standard for wired communications and acted as if the amateur radio channel was like a basically noise-free wire, and this should have killed it when it was invented, 40 years ago. Yet, the worst provisoria last the longest )-: ) – Marcus Müller Apr 06 '22 at 09:47
  • If I understand it correctly, you already know the minimum and the maximum values, but it had a practical value as well. The number of bytes in a frame had to be well beyond 256. Maybe 244 or so, I will look after it if you wish. It was a commonly used default for the normal, Layer 3 users because Layer 4 nodes added some overhead when they were forwarding the users' AX25 packets. I don't know will you use such network elements or not. But there was no problem with longer packets except they had to be broken into two messages between the nodes that has been done automatically. – ha3flt Jan 09 '23 at 21:54
  • No doubt, AX25 is very obsolete now, but concerning the ham radio, it was a good choice being the simplest, shortest, and easily decodable by eyes in an era when every bit counted. It just hurted if sthg was a bit longer than absolutely needed. Other available protocols like TCP/IP were simply unusable even with 1200 bps (huge overhead!). In the early years without internet, forwarding messages and bulletins through SW bands between continents in SSB/AFSK worked fairly well despite of the (much less) QRN, and the higher and higher traffic was a bigger problem on FM/AFSK than anything else. :-) – ha3flt Jan 09 '23 at 22:04

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Per AX.25 Link Access Protocol for Amateur Packet Radio Version 2.2 Revision: July 1998:

3.9 Invalid Frames

A frame is considered by the link layer to be an invalid frame if it:

• consists of less than 136 bits (including the opening and closing flags)

and

The I Field Length Receive parameter field (PI=6) allows the sending TNC to notify the receiving TNC of the maximum size of an Information field (N1) it will handle without error. A transmitting TNC may not exceed this size, but may send smaller frames. If this field is not present, the current values are retained. The default is 256 octets (2048 bits).

That is, the overall size of the information sent in the frame (and, thus, the maximum octets a frame can be when combined with the fixed sized fields for that type) is negotiated by the peers, but has known defaults.