Digital modes
FT8, APRS, packet, Winlink, and digital voice
Packet BBS: Store-and-Forward Mail Over Radio
How packet bulletin board systems store and forward mail: personal mailboxes, hierarchical addresses read left to right, FBB forwarding proposals and BIDs, and what § 97.219 says about forwarded messages.
APRS: Packet Radio for Real-Time Tactical Awareness
How APRS uses AX.25 UI frames for positions, objects, weather and messages; how WIDEn-N digipeating and the ALOHA circle limit a shared 144.39 MHz channel; and what IGates do.
What Packet Radio Is Used For: A Map of the Applications
An overview of what runs on amateur packet radio, from APRS and BBS mail to chat, Winlink, file transfer, telemetry and IP, sorted by how each uses AX.25, plus the digipeater, node and TARPN networks underneath.
Packet Radio: What It Is and What It Isn't
How amateur packet radio actually works: AX.25 frames, 1200 and 9600 bit/s modems, TNCs and KISS, shared-channel access, digipeaters, and the current FCC Part 97 limits.