NO CARRIER (capitalized) is a word code transmitted from a

modem The Democratic Movement (, ; MoDem ) is a centre to centre-right political party in France, whose main ideological trends are liberalism and Christian democracy, and that is characterised by a strong pro-Europeanist stance. MoDem was establis ...

to its attached device (typically a

computer A computer is a machine that can be Computer programming, programmed to automatically Execution (computing), carry out sequences of arithmetic or logical operations (''computation''). Modern digital electronic computers can perform generic set ...

), indicating the modem is not (or no longer) connected to a remote system. NO CARRIER is a response message that is defined in the

Hayes command set The Hayes command set (also known as the AT command set) is a specific command language originally developed by Dale Heatherington and Dennis Hayes for the Hayes Smartmodem in 1981. The command set consists of a series of short text string ...

. Due to the popularity of Hayes modems during the heyday of

dial-up Dial-up Internet access is a form of Internet access that uses the facilities of the public switched telephone network (PSTN) to establish a connection to an Internet service provider (ISP) by dialing a telephone number on a conventional telepho ...

connectivity, most other modem manufacturers supported the Hayes command set. For this reason, the NO CARRIER message was ubiquitously understood to mean that one was no longer connected to a remote system.

Carrier tone

A carrier tone is an audio

carrier signal In telecommunications, a carrier wave, carrier signal, or just carrier, is a periodic waveform (usually sinusoidal) that conveys information through a process called ''modulation''. One or more of the wave's properties, such as amplitude or frequ ...

used by two modems to suppress

echo cancellation Echo suppression and echo cancellation are methods used in telephony to improve voice quality by preventing echo from being created or removing it after it is already present. In addition to improving subjective audio quality, echo suppression i ...

and establish a baseline frequency for communication. When the answering modem detects a

ringtone A ringtone is the sound made by a telephone to indicate an incoming telephone call. Originally referring to the sound of electromechanical striking of bells or gongs, the term refers to any sound by any device alerting of an incoming call. On p ...

on the phone line, it picks up that line and starts transmitting a carrier tone. If it does not receive data from the calling modem within a set amount of time, it disconnects the line. The calling modem waits for the tone after it dials the phone line before it initiates data transmission. If it does not receive a carrier tone within a set amount of time, it will disconnect the phone line and issues the ''NO CARRIER'' message. The actual data is transmitted from the answering modem to the calling modem via

modulation Signal modulation is the process of varying one or more properties of a periodic waveform in electronics and telecommunication for the purpose of transmitting information. The process encodes information in form of the modulation or message ...

of the carrier.

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. The abort can be triggered by the computer receiving a keypress to abort or the computer dropping the

Data Terminal Ready Data Terminal Ready (DTR) is a control signal in RS-232 serial communications, transmitted from data terminal equipment (DTE), such as a computer, to data communications equipment (DCE), for example a modem, to indicate that the terminal is ready ...

(DTR) signal to hang up. *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.

Current use

As modems have progressed to pack more bits per second over a telephone line, the specific implementation involves modulating on other than the audio-frequency carrier (particularly on digital modems that connect via

ISDN Integrated Services Digital Network (ISDN) is a set of communication standards for simultaneous digital transmission of voice, video, data, and other network services over the digitalised circuits of the public switched telephone network. ...

or cellular networks), the message NO CARRIER remains consistent for the sake of compatibility.

Linux Linux ( ) is a family of open source Unix-like operating systems based on the Linux kernel, an kernel (operating system), operating system kernel first released on September 17, 1991, by Linus Torvalds. Linux is typically package manager, pac ...

's network stack uses the NO CARRIER status for a network interface that is turned on ("up") but cannot be connected because the

physical layer In the seven-layer OSI model of computer networking, the physical layer or layer 1 is the first and lowest layer: the layer most closely associated with the physical connection between devices. The physical layer provides an electrical, mechani ...

is not operating properly, e.g. because an

Ethernet Ethernet ( ) is a family of wired computer networking technologies commonly used in local area networks (LAN), metropolitan area networks (MAN) and wide area networks (WAN). It was commercially introduced in 1980 and first standardized in 198 ...

cable is not plugged in.

As Internet slang

The NO CARRIER message can be used at the end of a sentence for humorous purposes in Internet messages and forum posts, signifying that the person typing the message was suddenly cut off. This joke may mean that the typist was writing something excessively boring or illogical, or something the authorities (usually either the government or the site moderators) would supposedly want to suppress. Alternatively, it may imply they were simply a victim of bad luck. The NO CARRIER message is usually preceded by a string of garbage characters, as a parody of what used to frequently happen to users of dialup BBSes whose telephones suddenly dropped signal in the middle of a message. () This joke anachronistically continues to be seen on the Internet despite the waning use of modems in favor of

broadband In telecommunications, broadband or high speed is the wide-bandwidth (signal processing), bandwidth data transmission that exploits signals at a wide spread of frequencies or several different simultaneous frequencies, and is used in fast Inter ...

connectivity.

References

{{reflist Modems Computer errors Computer networking English phrases