NO CARRIER (modem)
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NO CARRIER (capitalized) is a text message transmitted by a modem to a computer to indicate that it is not, or no longer, connected to a remote party.
NO CARRIER is a response message that is defined in the Hayes command set. As virtually every computer dial-up modem ever built uses a command set derived from the Hayes command set, the NO CARRIER message is applicable to nearly every dial-up modem in existence. The message is often displayed to the individual computer user who uses a terminal program to access online information services such as BBSes. With the advent of the Internet and dial-up networking wizards that automate the configuration of dial-up connections, this message is often insulated from the view of the user, except perhaps in a "modem status" display box.
[edit] Practical meaning
The NO CARRIER message is issued by a modem for any of the following reasons:
- A dial (ATD) or answer (ATA) command did not result in a successful connection to another modem, and the reason wasn't that the line was BUSY (a separately defined message).
- A dial or answer command was aborted while in progress, either because the computer received a keypress to abort it, or a hangup (drop in DTR signal).
- A previously established data connection has ended (either at the attached computer's command, or as a result of being disconnected from the remote end), and the modem has now gone from the data mode to the command mode.
[edit] Literal meaning
The NO CARRIER message is a literal reference to the audio carrier signal, which historically was an audible tone whose pitch was frequency modulated to encode the individual ones and zeros of the data being transmitted across the telephone line. As modems have progressed to pack more bits per second over a telephone line, the specific implementation may involve some other encoding other than the frequency-modulated audio carrier (particularly on digital modems that connect via ISDN or cellular networks), the message NO CARRIER remains consistent for the sake of compatibility. NO CARRIER means a data session has permanently ended.
