Modem Control
The Hayes command set, for instance, defines a single escape sequence, +++. (In order not to interpret +++ which may be a part of data as the escape sequence the sender stops communication for one second before and after the +++) .When the modem encounters this in a stream of data, it switches from its normal mode of operation which simply sends any characters to the phone, to a command mode in which the following data is assumed to be a part of the command language. You can switch back to the online mode by sending the O command.
The Hayes command set is modal, switching from command mode to online mode. This is not appropriate in the case where the commands and data will switch back and forth rapidly. An example of a non-modal escape sequence control language is the VT100, which used a series of commands prefixed by the Control Sequence Introducer, escape-[.
Read more about this topic: Escape Sequence
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—Marcel Proust (1871–1922)