Modem control using the Hayes/AT command set: Which AT command is used to hang up (disconnect) the modem connection?

Introduction / Context:
Before broadband and USB modems, dial-up devices were controlled by the Hayes AT command set. Understanding basic AT commands remains useful for embedded modems, terminal servers, and telecom labs where legacy procedures persist.

Given Data / Assumptions:

• The question refers to the widely adopted Hayes “AT” command syntax.
• The goal is to terminate an active call and force the modem on-hook.
• Standard commands like ATD (dial), ATH (hang up), ATZ (reset) are assumed known.

Concept / Approach:

The Hayes set uses mnemonic letters following the “AT” prefix. “H” stands for hook control. ATH places the modem on-hook (disconnects), while ATH1 can force off-hook. ATD triggers dialing, and ATZ resets the modem to defaults and re-executes stored profile. ATR is not a standard hang-up command; it is typically used for reading settings on some devices but is not the hang-up control.

Step-by-Step Solution:

Verification / Alternative check:

Any terminal session to a classic modem confirms ATH returns “OK” and drops carrier. Documentation from Hayes-compatible devices lists ATH as the disconnect command.

Why Other Options Are Wrong:

• ATZ: resets the modem; may also drop the call but is not the proper hang-up command.
• ATD: initiates dialing, opposite of disconnect.
• ATR: not the hang-up function; behavior varies by vendor.
• None of the above: incorrect because ATH is exactly right.

Common Pitfalls:

Issuing commands while still in data mode; forgetting the escape sequence “+++” and guard time; assuming ATZ is the safest disconnect (it can alter settings if profiles change).

Final Answer:

ATH