*Question E2D07:
What digital protocol is used by APRS?*  

*Correct Answer is  C: AX.25*  

### AI Response: (may contain errors)  

APRS, or Automatic Packet Reporting System, is a digital communication protocol used primarily for real-time location tracking and messaging in amateur radio. The correct answer to the question is:

**C: AX.25**

### Explanation of the Correct Answer:
- **AX.25** is a data link layer protocol derived from the X.25 protocol suite. It is specifically designed for amateur radio packet communications. APRS utilizes AX.25 to encapsulate data packets, allowing for the transmission of location information, messages, and telemetry data over radio frequencies.

### Why the Other Choices are Incorrect:
- **A: PACTOR**
  - PACTOR is a digital communication protocol that combines aspects of packet radio and modem technology. It is primarily used for high-speed data transmission and is not the protocol used by APRS.

- **B: QAM**
  - QAM, or Quadrature Amplitude Modulation, is a modulation scheme used to transmit data over radio waves. It is not a protocol but rather a method of encoding data onto a carrier wave. APRS does not use QAM as its communication protocol.

- **D: AMTOR**
  - AMTOR (Amateur Teleprinting Over Radio) is a protocol used for sending text messages over radio. While it is a digital mode, it is not the protocol used by APRS, which relies on AX.25 for its packet structure.

### Summary:
- APRS uses **AX.25** for packet communication, making it suitable for real-time data transmission in amateur radio.
- Other options like PACTOR, QAM, and AMTOR serve different purposes and are not applicable to APRS.

This understanding of protocols will help you in recognizing the specific applications of each in amateur radio communications.