Modem data transfer — Which transmission method do dial-up modems most commonly use for character data?

Difficulty: Easy

Correct Answer: Asynchronous

Explanation:


Introduction / Context:
Legacy dial-up modems historically carried serial data over analog phone lines. Knowing whether transmission is synchronous or asynchronous clarifies line coding, framing, and UART behavior.



Given Data / Assumptions:

  • Standard external/internal modems attached to COM ports (RS-232).
  • Typical terminal/serial communication with start/stop bits.
  • PPP is a link-layer protocol encapsulated over the serial link.


Concept / Approach:

Asynchronous serial communication frames each character with start/stop bits. This matches how UARTs and COM ports operate for dial-up access. While some specialized modems support synchronous modes, general PC use is asynchronous.



Step-by-Step Solution:

Identify the physical layer method used by COM port modems: async serial with start/stop bits.Recognize PPP runs over this link but is a protocol, not the electrical transmission method.Select “Asynchronous” as the correct transmission method.


Verification / Alternative check:

Serial port settings (8N1, baud) and UART operation confirm asynchronous framing is standard for dial-up PC setups.



Why Other Options Are Wrong:

Synchronous is less common for PC dial-up; “Timed interval” is not a standard; PPP describes link framing and negotiation layered over the serial method.



Common Pitfalls:

Confusing physical transmission characteristics with higher-layer protocols; assuming PPP implies synchronous signaling.



Final Answer:

Asynchronous.

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