[aprssig] APRS-IS Database :: Query for static dataset
steve at dimse.com
Sun Jul 19 15:11:19 CDT 2015
On Jul 19, 2015, at 11:08 AM, Kenneth Finnegan via aprssig <aprssig at tapr.org> wrote:
> Each APRS-IS maintains an internal table of packets it has seen in the last 30 seconds for dedup. After 30 seconds, the IS server throws them away.
This is implementation dependent. The original APRS Hub, APRServ, used hashes. A 32 bit hash was generated for an incoming payload, and compared to a list of the hashes from recent packets. Much faster than a string comparison with a very low risk of tossing out a non-dup. Remember this was on 1997 hardware!
Another correction to some of the other responses. Most hubs do keep more than this 30 second list for a number of uses. For example many answer on 10153/20153 with a 30 minute dump of activity followed by live data. This dates from the beginning when APRServ was written primarily to serve javAPRS pages, but many found it useful for client programs too. With this feature one could immediately see all recent activity instead of waiting for each station to beacon and the map to slowly fill.
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