APRS-AX.25.en

Finnish version is at APRS-AX.25 page

APRS uses variation of AX.25-link-layer protocol, which leaves several of AX.25 protocol details ignored.

Most important differences:

• Three topmost bits on Source and Destination address fields SSID bytes are never validated. Practically any bit values can be used, but recommended version is to use 011 or 111 for both fields.
• VIA address fields (digipeater fields) can be up to 8, AX.25 v2.2 says that there can be up to two.
• The topmost bit on SSID bytes of VIA address fields is "Has been digipeated", and the two reserved ones should be "11", bit again nobody validates those two reserved bits!