Thanks for the long and detailed reply.
On Wed, Jan 15, 2025 at 11:08 PM, Thomas Leibold wrote:
Based on your callsign you appear to be in the UK. I'm not sure why you emphasized that one of the two locations is a commercial building but in some (most?) countries it is not legal to use Amateur Radio for commercial purposes. Obviously an Amateur Radio station can be located at a business as long as it is only operated by licensed hams for legitimate (non-commercial) communication purposes.
I own the WW2 bunker.? It is a commercial building.? BUT the radio is not used commercially.? The location of the bunker gives me a longer reach along the coast-line that from my house.? The hope is one day I may be able to contact another packet-user over 2m.
(the -0 SSID) is the same as having no SSID at all (you cannot use CALLSIGN and CALLSIGN-0 to distinguish between two stations, they are the same!).
Ta, and for the gotchas ....
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In both APRS and Winlink you find documentation that associates specific SSIDs as an identifier for the station type. In some regions similar agreements have been made regarding the use of SSIDs as station type identifiers.
I guess that is what I am asking about.? I never seem to be able to guess if -7 is a BBS or a CHAT or nothing-at-all.
There does not seem to be a standard in the UK???? Or am I wrong?
None of those guidelines agree with each other and you should feel free to ignore those unless your usage applies to one of those cases (APRS has anyway developed a different system to identify the type of a station with far greater granularity than the original 16 offered by SSID).
Another use of SSID is to use them as a substitute for the concept of a network port since that is missing from the AX.25 protocol. For a single packet station to offer more than one service, each such service has to be associated with a different SSID. For example a station may use SSID -1 for the PBBS and SSID -5 for a NetROM Node.
I am a TCP/IP networking background, so that helps my understanding.
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There are two basic types of packet communication. Unconnected mode is comparable with UDP in IP networks and since there are no acknowledgements or other responses in this mode, two stations operating with the same callsign is generally harmless (but may not be legal since it probably violates rules about station identification).
Understood.
Connected mode, which is comparable with TCP in IP networking, consists of establishing a connection between two stations. If you have two packet stations A1 and A2 configured with the same callsign and use station A1 to connect to a third-party station B then the response from B will likely be heard by A1 and A2. A1 will be happy to hear that the connection was accepted and start the actual transfer while unattended station A2 will respond with an error response (disconnect message) since it is not aware of an active connection between A2 and B. This can (and usually does) interfere with the communication between A1 and B. Using SSIDs to make the callsigns unique for A1 and A2 is a way to avoid this specific problem.
Understood.
All of the above deals with choosing a specific SSID on purpose. There is however one aspect about SSIDs where different SSIDs get created automatically without the direct influence of the packet operator and much about it that you read online or hear from other hams is wrong (including many sources that really ought to know better)!
There are situations where packet traffic initiated by you is handled by a 3rd party on your behalf. The most common scenario is digipeating where you have other stations retransmit a message that you created. Depending on the distance between you and the intended recipient(s), there may be multiple stations involved in retransmitting (digipeating) your message. Up to 8 digipeaters may be specified by the original sender and the original station will most likely hear at least the first of those retransmissions and possibly more. To ensure that the retransmission uses a distinct id while still remaining to be associated with the original transmitters callsign, the digipeater subtracts 1 from the message before retransmitting it. For a station that started with the basic callsign (no SSID suffix), the first digipeater would send the message as if it came from SSID -15 (no SSID is equal to -0 and subtracting 1 from 0 in 4-bit unsigned arithmetic results in 15).
Thank you - this makes a lot of sense - I had not seen that in anything I have read.? So does that mean if I send out to a digipeater from SSID-5 that SSID-4 (and perhaps -3 (etc) depending on topology) needs to be unused?
??????????????????????????????????????????????????????????????????????????????????????????????????? The second digipeater sees an incoming message with SSID -15 and retransmits with SSID -14. So far so good (and most online sources agree). However when your station is operating with a different SSID, let's say -7 then the first digipeater will use -6 and the second digipeater will use -5 (not the -15 and -14 many expect to see in that case).
You have answered my question!? I respond to email as I read it, I dont real it all first.? Wittgenstein and his tractatus caught me!
This rule about digipeating also applies to Kantronics Ka-Nodes (which are an improvement on basic digipeating to give it NetROM like behavior). Even when using NetROM nodes, while there is no SSID modification for messages travelling through a NetROM network, there will still be a minus 1 subtraction for the SSID if the final destination is not itself a NetROM node.
Both connected mode and unconnected mode packets can be digipeated.
I need to read more.? This is nice stuff.? Some of which I think I used to know ;-(
If you are sure that you will never use digipeaters or other forms of 3rd party retransmissions then you could use sequential numbers for your SSIDs. However I would still recommend to play it safe and use -0 and -8 (for a separation of 8, which is the maximum number of digipeater hops) for two stations on the same frequency. If you later add more stations use SSIDs -4 and -12 (for a separation of 4 which is often the practical limit of digipeating).
Binary chop it.? Understood.
There are additional potential pitfalls (I don't think they apply to Direwolf, but it helps to be aware them).
Hardware TNCs in particular can be configured to accept any SSID on incoming packets (wildcard SSID) which must be turned off if you want to use SSIDs to distinguish more than 1 packet station on the same frequency (or the wrong station will respond as well).
I have a few old TNCs - Kantonics, PAKRATT etc.? I will remember to check for that.
Software/firmware bugs related to the use of SSIDs may complicate your life (invalid assumptions of software developers considering only some of the use cases for SSIDs).
Yep, software development is hard.? We should all just use chatGPT!!
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Sir, thank you for your time and patience in that explanation.? I now know a lot more than I did, and better go read yet more web-sites.
iainmaoileoin