Question E2D07:
What digital protocol is used by APRS?  

Choices:  
A: PACTOR  
B: QAM  
C: AX.25  
D: AMTOR    

Correct Answer is  C: AX.25  

To answer: APRS uses AX.25.

Why C (AX.25) is correct:
- APRS transmits position/status data using AX.25 packet radio in unnumbered information (UI) frames.
- Typical settings:
  - VHF: $R = 1200$ bps AFSK (Bell 202 tones: 1200/2200 Hz) on 144.390 MHz in North America.
  - HF: $R = 300$ bps AFSK.

Why the other choices are incorrect:
- A: PACTOR — An HF ARQ protocol for robust file/message transfer; not used for APRS beacons.
- B: QAM — A modulation method, not a network/protocol layer; APRS does not use QAM.
- D: AMTOR — Older HF text/teletype (SITOR) protocol; not used by APRS.

Useful rules of thumb:
- APRS = AX.25 UI frames.
- Data rates to remember: $1200$ bps (VHF) and $300$ bps (HF) using AFSK.