[aprssig] APRStt software?
bruninga at usna.edu
Tue May 19 17:00:00 CDT 2009
> If I remember the APRStt spec correctly,
> it actually requires a DB of callsign
> shortcuts that might be quite a challenge...
That was abandoned in this new rendition of APRStt. In this version, users always send their full callsign first (with overlay character). After that, they can use only their 3 digit suffix (plus overlay character)
> Has anyone actually figured out what the
> rate of hash collisions is likely to be?
> Looking at the stations around here, I see
> that a bunch of them would have the same
> first 3 digits...
I dont think it is a problem for many reasons:
1) APRStt is probably not going to be used every day all the time by the other 95% of Ham radio operators. If we can only get 4% of ham radio users to use APRS (now), I doubt we could even hope to double that (another 4%). So just because there are possible identical suffix digits, the chances for conflict in real time at the same time within the same hour are very small.
2) That is why we worked in the OVERLAY byte. There are 36 possible overlay bytes. So even if two people have the same SUFFIX digits, they can distinguish by choice of overlay.
3) The suffix digits are only need to be unique within the last 30 minutes (or so...) of use of the full call. IE, a user has to send his full call the first time he uses APRStt. THen he can use his suffix digits within some time-out period.
4) If there is a match of suffix-digits to more than one call, then APRStt NAKs it by immediated response and asks the user to use his full call. Or he can change his overlay and try again.
5) The difference between a full call and a suffix-digits call is only a second or so, and so it can ALWAYS be used at any instant there is a conflict.... it just takes a little more air time.
6) MOST applications of APRStt will be at special events, were tactical calls will probably be used for brevity. In that case, probably stations will get sequencial numbers like 100, 101, 102, and so on. And each one of them can have one of 36 overlay bytes.
Anyway, APRStt has been greatlly reduced and simplified form the original APRSdos implementation. And we now have a better idea of the kinds of applications which will benefit.
And in my opinion, the predominant use will be Special Events where lots of planning can find ways for volunteers with non-APRS HT's can add benefit to the small minority of those with APRS HT's. THus including more hams in the experience.
Hope that helps
Bob, (in Poland)
After this email, Im gonna see if my WB4APR-7 shows up here...
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