APRS Digipeater Algorithm

Amateur radio repeaters retransmit signals from other stations to increase their range.

Analog voice repeaters listen on one frequency and simultaneously retransmit the same signal on a different frequency.

APRS digital repeaters (digipeaters) use a “store and forward” approach.  A packet is received, examined, then possibly modified and retransmitted.   Usually it is retransmitted on the same radio channel but it is also possible for a multi-port digipeater to link multiple radio channels.  Packets received on one channel can be retransmitted on different channels.  

The original APRS Protocol Reference 1.0.1 (2000) didn't say much about how a digipeater should work.   Perhaps it was because everyone was using 1980s style TNCs at the time.  Someone wishing to implement a digipeater had only poorly documented inconsistent precedent from legacy TNCs, long before the WIDEn-N paradigm was created.  As a result, we find many implementations that don’t behave exactly the same.  Some are just plain wrong.  

Recommended background reading:  Understanding APRS Packets.

Digipeater behavior is now covered in the APRS Protocol Reference 1.2 (2024) and described in more detail here.