As previously reported (see the article here), I identified a bug when using alphanumeric SSIDs. Upon further investigation and consultation, however, I have confirmed my original understanding, when I first deployed SSID aliases, that packets gated to RF should be encapsulated using the 3rd party format.
While the technical details I shared in the initial article are correct, they don’t account for the technical specifications of APRS-IS. These are the specifications that dictate how APRS-IS protocol should be deployed, including in IGates.
Third Party Packets
The APRS-IS specifications specify that a packet being gated from the Internet must use the third party format. This format, as described in the IGate Details page, under paths, shows that the IGate should include a third party header in compliance with Chapter 17 of the APRS specifications.
In this format, the originating callsign of the packet is the callsign of the IGate, but the originating callsign of the message becomes part of the transported data, therefore it can support two-character alphanumeric SSID.
Why is there still a problem?
It turns out that not all IGate software conforms with the APRS-IS specifications. Three different non-compliant behaviors have been observed.
In some cases, packets have been gated to RF, but with the SSID removed from the originating station removed. Further analysis of raw packets is required, but it is likely that these packets are being gated without using the third party format resulting in the removal of the source of the message is used as the originating callsign of the packet.
Another observed case is that the packets are not gated to RF at all. In one specific example, the gateway is using the correct third party format when gating messages to RF, but does not gate packets from stations with alphanumeric SSID.
I am still debating how to address this issue, and attempting to work with IGate providers to resolve, but the SSID feature may be returned to its original operation since it was never really a bug with SMSGTE in the first place.