Command And Data Modes (modem)
Command and Data modes refer to the two modes in which a computer modem may operate. These modes are defined in the Hayes command set, which is the de facto standard for all modems. These modes exist because there is only one channel of communication between the modem and the computer, which must carry both the computer's commands to the modem, as well as the data that the modem is enlisted to transmit to the remote party over the telephone line.
When a modem is in command mode, any characters sent to it are interpreted as commands for the modem to execute, per the Hayes command set. A command is preceded by the letters 'AT', which stand for 'Attention'. For example, if a modem receives 'ATDT5551212' while in the command mode, it interprets that as an instruction to dial the numbers 5551212 on the telephone, using touch-tone dialing. While in command mode, the modem may send responses back to the computer indicating the outcome of the command. For example, the modem may respond with the word "BUSY" in response to the ATDT command, if it hears a busy signal after dialing and is configured to listen for busy signals.
Any communication in command mode (in both directions) is terminated by a carriage return.
When a modem is in data mode, any characters sent to the modem are intended to be transmitted to the remote party. The modem enters data mode immediately after it makes a connection. For example, if ATDT5551212 resulted in a phone call that was answered by another computer modem, the modem would report the word "CONNECT" and then switch to data mode. Any further characters received over the serial link are deemed to be from the remote party, and any characters sent are transmitted to the remote party.
When a voice-capable modem is in "voice data" mode, any data sent to the modem is interpreted as audio data to be played over the phone line, rather than character bytes to be transmitted digitally to the other party.
Read more about Command And Data Modes (modem): Switching Between Modes, Practical Contemporary Use, Data Link Escape (DLE) Messages
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